WO2012160761A1 - Imaging optics, imaging apparatus and digital device - Google Patents

Abstract

The imaging optics (1) of the present invention comprise, in successive order from the object side, an aperture (15) and positive/negative/positive/negative first through fourth lenses (11-14), both surfaces of the fourth lens (14) being convex surfaces, and the optics satisfying the conditional expression -1000 < (r1 + r4)/(r1 - r4) < -55, where r1 and r4 are the radius of curvature, respectively, of the object-side surface of the first lens (11) and the image-side surface of the second lens (12).

Description

Imaging optical system, imaging apparatus, and digital device

The present invention relates to an imaging optical system, and more particularly to an imaging optical system suitably applied to a solid-state imaging device such as a CCD image sensor or a CMOS image sensor. The present invention relates to an imaging device including the imaging optical system and a digital device equipped with the imaging device.

In recent years, image sensors using solid-state image sensors such as CCD (Charged Coupled Device) type image sensors and CMOS (Complementary Metal Oxide Semiconductor) type image sensors have become more sophisticated and downsized. Digital devices such as mobile phones and personal digital assistants equipped with image pickup devices using various image pickup devices are becoming widespread. In addition, the imaging optical system (imaging lens) for forming (imaging) an optical image of an object on the light receiving surface of the solid-state imaging device, which is mounted on these imaging devices, is further reduced in size and performance. The demand for is increasing. In particular, in recent years, higher resolution has been demanded of the imaging optical system due to the progress of pixel miniaturization in solid-state imaging devices. In such an imaging optical system, a four-element optical system has been proposed because higher performance is possible compared to a two-element or three-element optical system.

Such an imaging optical system is disclosed in, for example, Patent Documents 1 to 4. The imaging optical system disclosed in Patent Document 1 is arranged in order of the first positive lens, the second negative lens, the third positive lens, and the fourth positive lens in order from the object side, and the first positive lens to the second negative lens. The combined focal length of the second negative lens to the fourth positive lens is negative. The photographing lens disclosed in Patent Document 2 has an aperture stop closest to the object side, and thereafter, in order from the object side, a first lens having a positive refractive power, a second lens having a negative refractive power, and a positive lens. A third lens having a refractive power and a fourth lens having a positive refractive power are arranged. The imaging lens disclosed in Patent Document 3 includes an aperture stop, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, in order from the object side. A fourth lens having a negative refractive power is arranged, the second to fourth lenses are made of a resin material, the focal length of the entire lens system is f, and the focal length of the first lens is f1, When the focal length of the second lens is f2, the Abbe number of the second lens at the d-line is νd2, and the Abbe number of the third lens at the d-line is νd3, f / f1 <1.5, -2. 5 <| f / f2 | <−1.5, 15 <νd2−νd3. In addition, the imaging lens disclosed in Patent Document 4 includes, in order from the object side, a first lens having a positive power, a second lens having a negative power, and a first lens having a positive surface with a convex surface on the image side. A case in which three lenses and a fourth lens having a negative power in the vicinity of the optical axis are concave or flat in the vicinity of the optical axis, the overall focal length is f, and the fourth lens has a focal length f4 Furthermore, 0.28 <| f4 / f | <0.60.

Incidentally, since the imaging optical system disclosed in Patent Document 1 is a so-called reverse Ernostar type, its fourth lens is a positive lens. For this reason, compared with the case where the fourth lens is a negative lens as in the so-called telephoto type, the imaging optical system disclosed in Patent Document 1 has a longer back focus because the principal point position of the optical system is on the image side. Therefore, it is a disadvantageous type for downsizing. Furthermore, in the imaging optical system disclosed in Patent Document 1, since the lens having negative refractive power is one of the four first to fourth lenses, it is difficult to correct the Petzval sum, and the image It is difficult to ensure good performance at the periphery.

In this regard, the imaging lens disclosed in Patent Document 2 is a telephoto type, but has a narrow shooting angle of view and insufficient aberration correction. Further, if the total length of the entire imaging lens system is shortened, it becomes difficult for the imaging lens disclosed in Patent Document 2 to cope with the increase in the number of pixels of the imaging device due to performance degradation.

In addition, the imaging lens described in Patent Document 3 has a shape in which the peripheral portion of the fourth lens protrudes greatly in the image plane direction, and is therefore disposed between the fourth lens and the solid-state imaging device. In order to avoid contact with a parallel flat plate such as an optical low-pass filter, an infrared cut filter, or a seal glass of a solid-state image sensor package, or a substrate of the solid-state image sensor, it is necessary to lengthen the back focus. In fact, although the imaging lens disclosed in Patent Document 3 is a telephoto type, the back focus is relatively long, and sufficient miniaturization has not been achieved. In addition, the imaging lens described in Patent Document 3 has insufficient aberration correction to cope with the increase in the number of pixels.

In addition, the imaging lens described in Patent Document 4 can correct aberrations of about F2.8, but it can only handle insufficient brightness in portable terminals where pixels are becoming increasingly thin.

Japanese Patent Laid-Open No. 2004-341013 JP 2002-365530 A JP 2005-292559 A JP 2009-020182 A

The present invention has been made in view of the above-described circumstances, and an object of the present invention is to obtain a four-element imaging optical system that is smaller, has various aberrations corrected better, and is bright at about F2.4. Is to provide. And this invention is providing an imaging device provided with this imaging optical system, and a digital apparatus carrying this imaging device.

The imaging optical system according to the present invention includes, in order from the object side, an aperture 15 and first to fourth lenses that are positive, negative, positive, and negative. The fourth lens has concave surfaces on both sides, the object side surface of the first lens and the second lens. When the curvature radii on the image side surface of the lens are r1 and r4, the conditional expression −1000 <(r1 + r4) / (r1−r4) <− 55 is satisfied. The imaging optical system having such a configuration has a bright four-lens configuration of about F2.4, is smaller, and can correct various aberrations better. And the imaging device and digital apparatus using such an imaging optical system can achieve size reduction and high performance.

The above and other objects, features and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings.

It is a lens sectional view showing the composition typically for explanation of an image pick-up optical system in an embodiment. It is a schematic diagram which shows the definition of the image surface incident angle of a chief ray. It is a block diagram which shows the structure of the digital device in embodiment. It is an external appearance block diagram of the mobile phone with a camera which shows one Embodiment of a digital device. 3 is a cross-sectional view illustrating an arrangement of lens groups in the imaging optical system in Embodiment 1. FIG. 7 is a cross-sectional view illustrating an arrangement of lens groups in an imaging optical system in Embodiment 2. FIG. 6 is a cross-sectional view illustrating an arrangement of lens groups in an imaging optical system in Embodiment 3. FIG. 6 is a cross-sectional view illustrating an arrangement of lens groups in an imaging optical system in Embodiment 4. FIG. 10 is a cross-sectional view illustrating an arrangement of lens groups in an imaging optical system in Embodiment 5. FIG. 10 is a cross-sectional view illustrating an arrangement of lens groups in an imaging optical system in Example 6. FIG. FIG. 3 is an aberration diagram of the imaging optical system in Example 1. FIG. 6 is an aberration diagram of the image pickup optical system in Example 2. FIG. 6 is an aberration diagram of the image pickup optical system in Example 3. FIG. 6 is an aberration diagram of the image pickup optical system in Example 4. 10 is an aberration diagram of the image pickup optical system in Example 5. FIG. FIG. 10 is an aberration diagram of the image pickup optical system according to the sixth embodiment.

Hereinafter, an embodiment according to the present invention will be described with reference to the drawings. In addition, the structure which attached | subjected the same code | symbol in each figure shows that it is the same structure, The description is abbreviate | omitted suitably. Further, the number of lenses in the cemented lens is not expressed as one for the entire cemented lens, but is represented by the number of single lenses constituting the cemented lens.

<Explanation of terms>
The terms used in the following description are defined in this specification as follows.
(A) A refractive index is a refractive index with respect to the wavelength (587.56 nm) of d line | wire.
(B) Abbe number is nd, nF, nC, and Abbe number is νd for d-line, F-line (wavelength 486.13 nm), C-line (wavelength 656.28 nm), respectively.
νd = (nd−1) / (nF−nC)
The Abbe number νd obtained by the definition formula
(C) When the notation “concave”, “convex” or “meniscus” is used for the lens, these represent the lens shape near the optical axis (near the center of the lens).
(D) The notation of refractive power (optical power, reciprocal of focal length) in each single lens constituting the cemented lens is power when both sides of the lens surface of the single lens are air.
(E) Since the resin material used for the composite aspherical lens has only an additional function of the substrate glass material, it is not treated as a single optical member, but is treated as if the substrate glass material has an aspherical surface, and the number of lenses Shall be handled as one sheet. The lens refractive index is also the refractive index of the glass material serving as the substrate. The composite aspherical lens is a lens that is aspherical by applying a thin resin material on a glass material to be a substrate.

<Description of Imaging Optical System of One Embodiment>
FIG. 1 is a lens cross-sectional view schematically illustrating the configuration of an imaging optical system in the embodiment. FIG. 2 is a schematic diagram showing the definition of the image plane incident angle of the chief ray. In the following, the image plane incident angle of the chief ray is the angle (deg, degree) of the chief ray having the maximum field angle among the incident rays to the imaging surface with respect to the vertical line standing on the image plane, as shown in FIG. The image plane incident angle α is the principal ray angle when the exit pupil position is on the object side with respect to the image plane.

In FIG. 1, the imaging optical system 1 forms an optical image of an object (subject) on the light receiving surface of an image sensor 17 that converts an optical image into an electrical signal. The optical system is composed of four lenses of first to fourth lenses 11 to 14 in order. The image sensor 17 is arranged such that its light receiving surface substantially coincides with the image plane of the imaging optical system 1 (image plane = imaging plane).

In the imaging optical system 1, focusing is performed by moving the first to fourth lenses 11 to 14 in the optical axis direction by extending all the balls.

Further, the first lens 11 has a positive refractive power, the second lens 12 has a negative refractive power, the third lens 13 has a positive refractive power, and the fourth lens 14. The both surfaces are concave and have negative refractive power. More specifically, in the example shown in FIG. 1, the first lens 11 is a biconvex positive lens having convex surfaces on both sides, and the second lens 12 is a meniscus negative lens having a convex surface facing the object side. The third lens 13 is a positive meniscus lens convex to the image side, and the fourth lens 14 is a biconcave negative lens. These first to fourth lenses 11 to 15 are aspheric on both surfaces. Further, in the example shown in FIG. 1, the fourth lens 14 has a negative refractive power at the center (near the optical axis), and the negative refractive power becomes weaker toward the end of the effective region, and the optical axis AX is aligned. In the contour line of the lens cross section (the lens cross section including the optical axis AX along the optical axis AX), the contact points IP4 and IP4 are provided when going from the intersection of the optical axes AX to the effective area end.

These first to fourth lenses 11 to 14 may be glass molded lenses, for example, or may be lenses made of a resin material such as plastic. In particular, when mounted on a portable terminal, a lens made of a resin material is preferable from the viewpoint of weight reduction and cost reduction and from the viewpoint of workability. In the example shown in FIG. 1, the first to fourth lenses 11 to 14 are resin material lenses.

Further, the imaging optical system 1 satisfies the following conditional expression (1) when the radius of curvature of the object side surface of the first lens 11 is r1 and the radius of curvature of the image side surface of the second lens 12 is r4. Satisfies.
−1000 <(r1 + r4) / (r1−r4) <− 55 (1)

In the imaging optical system 1, an optical diaphragm 15 such as an aperture diaphragm is disposed on the object side of the first lens 11.

Further, a filter 16 and an image sensor 17 are disposed on the image side of the image pickup optical system 1, that is, on the image side of the fourth lens 14. The filter 16 is a parallel plate-like optical element, and schematically represents various optical filters, a cover glass (seal glass) of the image sensor 17, and the like. An optical filter such as an optical low-pass filter or an infrared cut filter can be appropriately disposed according to the use application, the configuration of the image sensor, the camera, or the like. The image sensor 17 performs photoelectric conversion to image signals of R (red), G (green), and B (blue) components in accordance with the amount of light in the optical image of the subject imaged by the imaging optical system 1, and performs predetermined conversion. This is an element that outputs to an image processing circuit (not shown). As a result, the optical image of the object on the object side is guided to the light receiving surface of the image sensor 17 at a predetermined magnification along the optical axis AX by the imaging optical system 1, and the optical image of the object is captured by the image sensor 17. .

The imaging optical system 1 having such a configuration is composed of four first to fourth lenses 11 to 14, and each of the first to fourth lenses 11 to 14 has the above optical characteristics, and these By arranging the four first to fourth lenses 11 to 14 in order from the object side to the image side, various aberrations can be corrected more favorably with a brightness of about F2.4 and a small size. It becomes.

More specifically, the imaging optical system 1 includes, in order from the object side, a diaphragm 15, a positive lens group Gr1 including first to third lenses 11 to 13, and a negative lens group Gr2 including a negative fourth lens 14. It is a so-called telephoto type to be arranged, and has a lens configuration that is advantageous for shortening the overall length of the imaging optical system (imaging lens) 1.

In addition, in the example shown in FIG. 1, two of the four lens configurations of the first to fourth lenses 11 to 14, in the example shown in FIG. 1, the second and fourth lenses 12, 14 have a diverging action. The number of surfaces can be increased, and the Petzval sum can be easily corrected. As a result, the imaging optical system 1 can ensure good imaging performance up to the periphery of the screen.

Further, by arranging the diaphragm 15 on the object side of the first lens 11, the imaging optical system 1 can ensure the telecentricity of the image-side light beam.

Furthermore, by making the fourth lens 14 into a biconcave shape, the peripheral portion of the fourth lens 14 does not protrude greatly in the image plane direction, and therefore the fourth lens 14 and the image sensor 17 such as a solid-state image sensor, for example. For example, a parallel plate (filter 16 in the example shown in FIG. 1) such as an optical low-pass filter, an infrared cut filter, or a seal glass of the image sensor package, a substrate of the image sensor 17, etc. The back focus can be shortened while avoiding contact with the imaging optical system 1, which is advantageous for shortening the overall length of the imaging optical system 1.

The conditional expression (1) defines the relationship between the radius of curvature of the object side surface of the first lens 11 and the image side surface of the second lens 12, thereby reducing the overall length of the imaging optical system 1 and appropriate aberrations. It is a conditional expression for achieving correction. When the value of conditional expression (1) is less than the upper limit, the radius of curvature of the image side surface of the second lens 12 does not become too strong, the occurrence of higher-order spherical aberration and coma aberration is suppressed, and further, manufacturing errors This is preferable because the influence of the above is reduced and the mass productivity is improved. On the other hand, when the value of conditional expression (1) exceeds the lower limit, it is prevented that the radius of curvature on the image plane side of the second lens 12 becomes too weak with respect to the object side surface of the first lens 11. Since the principal point position can be arranged on the object side, the entire length of the imaging optical system 1 can be shortened, which is preferable.

From such a viewpoint, the conditional expression (1) is preferably the following conditional expression (1A).
−800 <(r1 + r4) / (r1−r4) <− 60 (1A)

In this specification, the term “miniaturization” means that the distance on the optical axis from the lens surface of the lens closest to the object side to the image side focal point in the entire imaging optical system is L, and the diagonal length of the imaging surface (for example, When the diagonal length of the rectangular execution pixel region in a solid-state imaging device or the like is 2Y, it means that L / 2Y <1 is satisfied, and more preferably L / 2Y <0.9 is satisfied. The image side focal point refers to an image point when a parallel light beam parallel to the optical axis is incident on the imaging optical system. Further, when a parallel plate member such as an optical low-pass filter, an infrared cut filter, or a seal glass of a fixed imaging device package is disposed between the most image-side surface and the image-side focal point of the imaging optical system. This parallel plate member calculates the above formula as an air equivalent distance.

Moreover, in this imaging optical system 1, both surfaces of the first lens 11 are convex. Generally, in order to shorten the overall length of the optical system, it is necessary to keep the refractive power (optical power) of the first lens strong. The imaging optical system 1 can share the optical power distribution on both sides by forming the first lens 11 into a biconvex shape. Therefore, the imaging optical system 1 having such a configuration can suppress the occurrence of higher-order spherical aberration and coma aberration by preventing the curvature radius on one side from becoming extremely strong (small).

In the imaging optical system 1, the second lens 12 has a meniscus shape with a convex surface facing the object side. The imaging optical system 1 having such a configuration makes it possible to arrange the principal point position on the object side by forming the second lens 12 in a meniscus shape having a convex surface facing the object side. Shortening of the overall length can be achieved.

In the imaging optical system 1 described above, all of the first to fourth lenses 11 to 14 are resin material lenses made of a resin material.

Recently, for the purpose of downsizing the entire solid-state imaging device, even a solid-state imaging device having the same number of pixels has been developed as a device having a small pixel pitch and consequently a small imaging surface size. In such an imaging optical system for a solid-state imaging device with a small imaging surface size, it is necessary to make the focal length of the entire system relatively short, so that the curvature radius and the outer diameter of each lens are considerably reduced. Therefore, compared to glass lenses manufactured by time-consuming polishing, all lenses are made of plastic lenses manufactured by injection molding, so that even lenses with small radii of curvature and outer diameters are inexpensive. Mass production is possible. Further, since the plastic lens can lower the press temperature, it is possible to suppress the wear of the molding die. As a result, it is possible to reduce costs by reducing the number of replacements and maintenance of the molding die. For this reason, the imaging optical system 1 of this embodiment can implement | achieve predetermined performance comparatively easily, and can aim at cost reduction.

Further, in this imaging optical system 1, when the combined focal length of the first lens and the second lens is f2, and the focal length of the entire imaging optical system 1 is f, the first lens 11 and the second lens 12 satisfies the following conditional expression (2).
1 <f12 / f <1.7 (2)

Conditional expression (2) is a conditional expression for appropriately setting the combined focal length f12 of the first lens 11 and the second lens 12 and achieving more preferable shortening of the entire length of the imaging optical system 1 and correction of aberration. . Therefore, when the value of the conditional expression (2) falls below the upper limit value, the imaging optical system 1 having such a configuration appropriately maintains the positive composite focal length of the first lens 11 and the second lens 12. The total length can be shortened. On the other hand, when the value of the conditional expression (2) exceeds the lower limit value, it is possible to prevent the positive combined focal length of the first lens 11 and the second lens 12 from becoming too short, and a higher order spherical surface. Occurrence of aberration and coma can be suppressed.

From such a viewpoint, the conditional expression (2) is preferably the following conditional expression (2A).
1.15 <f12 / f <1.5 (2A)

In the imaging optical system 1, the fourth lens 14 satisfies the following conditional expression (3) when the thickness of the fourth lens on the optical axis is T4.
0.05 <T4 / f <0.17 (3)

As described above, the image side surface of the fourth lens 14 of the imaging optical system 1 has an aspherical shape that weakens the negative refractive power from the optical axis AX toward the periphery and has a perpendicular contact. For this reason, the imaging optical system 1 having such a configuration is easy to ensure the telecentric characteristics of the image-side light beam. Further, since the image side surface of the third lens 13 does not need to weaken the negative refractive power excessively at the periphery of the lens, the imaging optical system 1 having such a configuration can satisfactorily correct off-axis aberrations. it can.

The conditional expression (3) is a conditional expression for appropriately setting the axial thickness of the fourth lens 14 and appropriately achieving the image plane property of the imaging optical system 1. In general, the fourth lens 14 has a refractive power in the vicinity of the optical axis and a refractive power in the vicinity that are significantly different from those of other lenses, and thus the axial thickness has a great influence on the curvature of field. For this reason, when the value of the conditional expression (3) falls within the above range (satisfies the above range), the imaging optical system 1 having such a configuration has an image plane property of the imaging optical system 1 of the over side. It can be prevented from falling too much to the under side.

From such a viewpoint, the conditional expression (3) is preferably the following conditional expression (3A).
0.08 <T4 / f <0.15 (3A)

In addition, the perpendicular contact is within the effective radius of the lens, and at each point on the contour curve of the lens cross section along the optical axis (the lens cross section including the optical axis along the optical axis) A point on the aspherical surface where the tangent plane of the spherical vertex is a plane perpendicular to the optical axis. The effective area refers to an area set as an area that is optically used as a lens by design.

In the imaging optical system 1, when the radius of curvature of the object side surface of the fourth lens 14 is r7 and the radius of curvature of the image side surface of the fourth lens 14 is r8, the fourth lens 14 has the following ( The conditional expression 4) is satisfied.
0.1 <(r7 + r8) / (r7−r8) <1 (4)

This conditional expression (4) is a conditional expression for setting the surface shape of the fourth lens 14 appropriately and optimizing the back focus. Therefore, in the imaging optical system 1 having such a configuration, when the value of the conditional expression (4) is less than the upper limit value, the peripheral portion of the fourth lens 14 is not greatly projected in the image plane direction. For this reason, the imaging optical system 1 having such a configuration is arranged between the fourth lens 14 and the imaging element 17, and is a parallel flat plate such as an optical low-pass filter, an infrared cut filter, or a sealing glass of an imaging element package, for example. In addition, it is possible to avoid contact of the image sensor 17 with a member such as a substrate. On the other hand, when the value of the conditional expression (4) exceeds the lower limit value, the refractive power of the object side surface of the fourth lens 14 is appropriately maintained to shorten the back focus, thereby imaging with such a configuration. The optical system 1 can shorten the overall length of the imaging optical system 1.

From such a viewpoint, the conditional expression (4) is preferably the following conditional expression (4A).
0.5 <(r7 + r8) / (r7−r8) <1 (4A)

In the imaging optical system 1, when the curvature radius of the image side surface of the second lens 12 is r3, the second lens 12 satisfies the following conditional expression (5).
1.6 <r3 / f <2.2 (5)

This conditional expression (5) is a conditional expression for appropriately setting the radius of curvature of the object side surface of the second lens 12 to achieve the shortening of the entire length of the imaging optical system 1 and appropriate aberration correction. When the value of the conditional expression (5) is below the upper limit value, it is possible to prevent the negative optical power of the second lens 12 from becoming too large. The imaging optical system 1 having such a configuration can capture images. The overall length of the optical system 1 can be shortened. On the other hand, when the value of the conditional expression (5) exceeds the lower limit, the imaging optical system 1 having such a configuration suppresses higher-order spherical aberration and coma aberration generated on the object side surface of the second lens 12. be able to.

From such a viewpoint, the conditional expression (5) is preferably the following conditional expression (5A).
1.75 <r3 / f <2.15 (5A)

In the imaging optical system 1, when the thickness of the third lens 13 on the optical axis is T3, the third lens satisfies the following conditional expression (6).
0.1 <T3 / f <0.6 (6)

This conditional expression (6) is a conditional expression for setting the on-axis thickness T3 of the third lens 13 appropriately to achieve the shortening of the entire length of the imaging optical system 1 and the aberration correction. When the value of the conditional expression (6) exceeds the lower limit value, the imaging optical system 1 having such a configuration can appropriately maintain the focal length f3 of the third lens 13, and the imaging optical system 1 Shortening of the overall length can be achieved. On the other hand, when the value of the conditional expression (6) is less than the upper limit value, the imaging optical system 1 having such a configuration does not make the focal length f3 of the third lens 13 too short. Generation of coma aberration can be suppressed.

From such a viewpoint, the conditional expression (6) is preferably the following conditional expression (6A).
0.25 <T3 / f <0.4 (6)

In the above-described imaging optical system 1, a cam, a stepping motor, or the like may be used for driving the movable first to fourth lenses 11 to 14, or a piezoelectric actuator may be used. Good. In the case of using the piezoelectric actuator, it is possible to drive each group independently while suppressing an increase in the volume and power consumption of the driving device, and the imaging device can be further downsized.

In the above description, the lens is made of a resin material. However, in the above-described imaging optical system 1, a glass lens having an aspherical surface may be used. In this case, the aspheric glass lens may be a glass molded aspheric lens, a ground aspheric glass lens, or a composite aspheric lens (aspheric glass resin formed on a spherical glass lens). Good. Glass molded aspherical lenses are suitable for mass production, and composite aspherical lenses have a high degree of design freedom because there are many types of glass materials that can serve as substrates. In particular, an aspherical lens using a high refractive index material is not easy to mold, so a composite aspherical lens is preferable. In the case of a single-sided aspherical surface, the advantages of the composite aspherical lens can be fully utilized.

In the above-described imaging optical system 1, when a plastic lens is used, it is preferably a lens molded using a material in which particles having a maximum length of 30 nanometers or less are dispersed in plastic (resin material). .

In general, mixing fine particles with a transparent resin material scatters light and reduces the transmittance, making it difficult to use as an optical material. However, the size of the fine particles should be smaller than the wavelength of the transmitted light beam. The light is not substantially scattered. And although a resin material will have a refractive index falling with a temperature rise, an inorganic particle will raise a refractive index with a temperature rise conversely. For this reason, it is possible to make the refractive index change hardly occur with respect to the temperature change by acting so as to cancel each other by utilizing such temperature dependency. More specifically, by dispersing inorganic fine particles having a maximum length of 30 nanometers or less in a resin material as a base material, a resin material with reduced temperature dependency of the refractive index is obtained. For example, fine particles of niobium oxide (Nb ₂ O ₅ ) are dispersed in acrylic. In the imaging optical system 1 described above, a plastic material in which such inorganic particles are dispersed is used for a lens having a relatively large refractive power or all the lenses, so that the temperature of the entire imaging optical system 1 can be changed. Image point position fluctuation can be suppressed to a small level.

Such a lens made of plastic material in which inorganic fine particles are dispersed is preferably molded as follows.

The temperature change n (T) of the refractive index is expressed by the formula Fa by differentiating the refractive index n with respect to the temperature T based on the Lorentz-Lorentz equation.
n (T) = ((n ² +2) × (n ² −1)) / 6n × (−3α + (1 / [R]) × (∂ [R] / ∂T)) (Fa)
Where α is a linear expansion coefficient and [R] is molecular refraction.

In the case of a resin material, in general, the contribution of the refractive index to the temperature dependence is smaller in the second term than in the first term in the formula Fa, and can be almost ignored. For example, in the case of PMMA resin, the linear expansion coefficient α is 7 × 10 ⁻⁵ , and if it is substituted into the formula Fa, it becomes n (T) = − 12 × 10 ⁻⁵ (/ ° C.), which is substantially equal to the actually measured value. Match.

Specifically, the temperature change n (T) of the refractive index, which was conventionally about −12 × 10 ⁻⁵ [/ ° C.], can be suppressed to an absolute value of less than 8 × 10 ⁻⁵ [/ ° C.]. preferable. More preferably, the absolute value is less than 6 × 10 ⁻⁵ [/ ° C.].

Therefore, as such a resin material, a polyolefin resin material, a polycarbonate resin material, or a polyester resin material is preferable. In the polyolefin resin material, the refractive index temperature change n (T) is about −11 × 10 ⁻⁵ (/ ° C.), and in the polycarbonate resin material, the refractive index temperature change n (T) is about −14 × 10 ⁻⁵ (/ ° C.), and in the case of a polyester resin material, the temperature change n (T) of the refractive index is about −13 × 10 ⁻⁵ (/ ° C.).

<Description of digital equipment incorporating imaging optical system>
Next, a digital device in which the above-described imaging optical system 1 is incorporated will be described.

FIG. 3 is a block diagram showing the configuration of the digital device in the embodiment. The digital device 3 includes an imaging unit 30, an image generation unit 31, an image data buffer 32, an image processing unit 33, a driving unit 34, a control unit 35, a storage unit 36, and an I / F unit 37 for the imaging function. Composed. Examples of the digital device 3 include a digital still camera, a video camera, a surveillance camera (monitor camera), a portable terminal such as a mobile phone or a personal digital assistant (PDA), a personal computer, and a mobile computer. Equipment (eg, a mouse, scanner, printer, etc.) may be included. In particular, the imaging optical system 1 of the present embodiment is sufficiently compact when mounted on a mobile terminal such as a mobile phone or a personal digital assistant (PDA), and is preferably mounted on this mobile terminal.

The imaging unit 30 includes an imaging device 21 and an imaging element 17. The imaging device 21 includes an imaging optical system 1 as shown in FIG. 1 that functions as an imaging lens, a lens driving device (not shown), etc., for performing focusing by driving a lens for focusing in the optical axis direction. It is prepared for. Light rays from the subject are imaged on the light receiving surface of the image sensor 17 by the imaging optical system 1 and become an optical image of the subject.

As described above, the imaging device 17 converts the optical image of the subject formed by the imaging optical system 1 into an electrical signal (image signal) of R, G, and B color components, and each of the R, G, and B colors. It outputs to the image generation part 31 as an image signal. The image sensor 17 is controlled by the control unit 35 for imaging operations such as imaging of either a still image or a moving image, or reading (horizontal synchronization, vertical synchronization, transfer) of an output signal of each pixel in the image sensor 17. .

The image generation unit 31 performs amplification processing, digital conversion processing, and the like on the analog output signal from the image sensor 17 and determines an appropriate black level, γ correction, and white balance adjustment (WB adjustment) for the entire image. Then, known image processing such as contour correction and color unevenness correction is performed to generate image data from the image signal. The image data generated by the image generation unit 31 is output to the image data buffer 32.

The image data buffer 32 is a memory that temporarily stores image data and is used as a work area for performing processing described later on the image data by the image processing unit 33. For example, the image data buffer 32 is a volatile storage element. It consists of a certain RAM (Random Access Memory).

The image processing unit 33 is a circuit that performs predetermined image processing such as resolution conversion on the image data in the image data buffer 32.

Further, if necessary, the image processing unit 33 could not be corrected by the imaging optical system 1 such as a known distortion correction process for correcting distortion in the optical image of the subject formed on the light receiving surface of the imaging element 17. It may be configured to correct aberrations. In the distortion correction, an image distorted by aberration is corrected to a natural image having a similar shape similar to a sight seen with the naked eye and having substantially no distortion. With this configuration, even if the optical image of the subject guided to the image sensor 17 by the imaging optical system 1 is distorted, it is possible to generate a natural image with substantially no distortion. Further, in the configuration in which such distortion is corrected by image processing by information processing, in particular, only other aberrations other than distortion aberration need to be considered, so that the degree of freedom in designing the imaging optical system 1 is increased and the design is improved. It becomes easier. In addition, in the configuration in which such distortion is corrected by image processing based on information processing, the aberration burden due to the lens close to the image plane is reduced, so that the exit pupil position can be easily controlled, and the lens shape is easy to process. It can be shaped.

Further, the image processing unit 33 may include a known peripheral illuminance decrease correction process for correcting the peripheral illuminance decrease in the optical image of the subject formed on the light receiving surface of the image sensor 17 as necessary. The peripheral illuminance drop correction (shading correction) is executed by storing correction data for performing the peripheral illuminance drop correction in advance and multiplying the image (pixel) after photographing by the correction data. Since the decrease in ambient illuminance mainly occurs due to the incident angle dependence of the sensitivity in the image sensor 17, the vignetting of the lens, the cosine fourth law, and the like, the correction data has a predetermined value that corrects the decrease in illuminance caused by these factors. Is set. With such a configuration, even if the peripheral illuminance drops in the optical image of the subject guided to the image sensor 17 by the imaging optical system 1, it is possible to generate an image having sufficient illuminance to the periphery. It becomes.

In this embodiment, the shading correction is performed by setting the pitch of the arrangement of the color filters and the on-chip microlens array slightly smaller than the pixel pitch on the imaging surface of the imaging device 17 so as to reduce the shading. It may be done. In such a configuration, by setting the pitch slightly small, a color filter and an on-chip microlens array are arranged on the optical axis side of the imaging optical system 1 for each pixel as it goes to the periphery of the imaging surface in the imaging element 17. Therefore, the obliquely incident light beam can be efficiently guided to the light receiving portion of each pixel. Thereby, shading generated in the image sensor 17 is suppressed to a small level.

The driving unit 34 drives the lens for focusing in the imaging optical system 1 so as to perform desired focusing by operating the lens driving device (not shown) based on a control signal output from the control unit 35. To do.

The control unit 35 includes, for example, a microprocessor and its peripheral circuits, and includes an imaging unit 30, an image generation unit 31, an image data buffer 32, an image processing unit 33, a drive unit 34, a storage unit 36, and an I / F unit. The operation of each part 37 is controlled according to its function. In other words, the imaging device 21 is controlled by the control unit 35 to execute at least one of the still image shooting and the moving image shooting of the subject.

The storage unit 36 is a storage circuit that stores image data generated by still image shooting or moving image shooting of a subject. For example, a ROM (Read Only Memory) that is a nonvolatile storage element, a rewritable nonvolatile memory, or the like. It comprises an EEPROM (Electrically Erasable Programmable Read Only Memory) that is a storage element, a RAM, and the like. That is, the storage unit 36 has a function as a still image memory and a moving image memory.

The I / F unit 37 is an interface that transmits / receives image data to / from an external device, and is an interface that conforms to a standard such as USB or IEEE1394.

The following describes the imaging operation of the digital device 3 having such a configuration.

When shooting a still image, the control unit 35 controls the imaging device 21 to shoot a still image and operates the lens driving device (not shown) of the imaging device 21 via the driving unit 34. , Focusing is performed by paying out all balls. As a result, the focused optical image is periodically and repeatedly formed on the light receiving surface of the image sensor 17, converted into image signals of R, G, and B color components, and then output to the image generation unit 31. . The image signal is temporarily stored in the image data buffer 32, and after image processing is performed by the image processing unit 33, an image based on the image signal is displayed on a display (not shown). The photographer can adjust the main subject so as to be within a desired position on the screen by referring to the display. When a so-called shutter button (not shown) is pressed in this state, image data is stored in the storage unit 36 as a still image memory, and a still image is obtained.

In addition, when performing moving image shooting, the control unit 35 controls the imaging device 21 to perform moving image shooting. After that, as in the case of still image shooting, the photographer refers to the display (not shown) so that the image of the subject obtained through the imaging device 21 is placed at a desired position on the screen. Can be adjusted. When a shutter button (not shown) is pressed, moving image shooting is started. At the time of moving image shooting, the control unit 35 controls the imaging device 21 to shoot a moving image and operates the lens driving device (not shown) of the imaging device 21 via the driving unit 34 to perform focusing. Do. As a result, a focused optical image is periodically and repeatedly formed on the light receiving surface of the image sensor 17, converted into image signals of R, G, and B color components, and then output to the image generation unit 31. . The image signal is temporarily stored in the image data buffer 32, and after image processing is performed by the image processing unit 33, an image based on the image signal is displayed on a display (not shown). Then, when the shutter button (not shown) is pressed again, the moving image shooting is completed. The captured moving image is guided to and stored in the storage unit 36 as a moving image memory.

In such a configuration, the imaging device 21 using the imaging optical system 1 having a four-lens configuration, which has a brightness of about F2.4 and is small and can correct various aberrations better, and A digital device 3 is provided. In particular, since the imaging optical system 1 is reduced in size and performance, it is possible to employ the imaging element 17 having a high pixel while reducing the size (compacting). In particular, since the imaging optical system 1 is small and can be applied to a high-pixel imaging device, the imaging optical system 1 is suitable for a mobile terminal that is increasing in pixel count and functionality. As an example, a case where the imaging device 21 is mounted on a mobile phone will be described below.

FIG. 4 is an external configuration diagram of a camera-equipped mobile phone showing an embodiment of a digital device. 4A shows an operation surface of the mobile phone, and FIG. 4B shows a back surface of the operation surface, that is, a back surface.

In FIG. 4, the mobile phone 5 is provided with an antenna 51 at the top, and on its operation surface, as shown in FIG. 4A, a rectangular display 52, activation of image shooting mode, still image shooting and moving image An image shooting button 53 for switching to shooting, a shutter button 55, and a dial button 56 are provided.

The cellular phone 5 incorporates a circuit for realizing a telephone function using a cellular phone network, and includes the above-described imaging unit 30, image generating unit 31, image data buffer 32, image processing unit 33, and driving unit. 34, the control part 35, and the memory | storage part 36 are incorporated, and the imaging device 21 of the imaging part 30 faces the back.

When the image shooting button 53 is operated, a control signal indicating the operation content is output to the control unit 35, and the control unit 35 performs the activation and execution of the still image shooting mode and the activation and execution of the moving image shooting mode. Execute the action according to the operation content. When the shutter button 55 is operated, a control signal indicating the operation content is output to the control unit 35, and the control unit 35 executes an operation corresponding to the operation content such as still image shooting or moving image shooting. .

<Description of More Specific Embodiment of Imaging Optical System>
Hereinafter, the specific configuration of the imaging optical system 1 as shown in FIG. 1, that is, the imaging optical system 1 provided in the imaging device 21 mounted in the digital device 3 as shown in FIG. However, it will be explained.

5 to 10 are cross-sectional views showing the arrangement of lenses in the image pickup optical system according to the first to sixth embodiments. FIGS. 11 to 16 are aberration diagrams of the imaging optical system in Examples 1 to 6. FIGS.

In the imaging optical systems 1A to 1F of the first to sixth embodiments, as shown in FIGS. 5 to 10, the first to fourth lenses L1 to L4 are sequentially arranged from the object side to the image side, and focusing (focusing) is performed. ), The first to fourth lenses L1 to L4 move together in the optical axis direction AX when all the balls are extended.

More specifically, in the imaging optical systems 1A to 1F of Examples 1 to 6, the first to fourth lenses L1 to L4 are configured as follows in order from the object side to the image side.

The first lens L1 is a biconvex positive lens having positive refractive power, the second lens L2 is a negative meniscus lens having negative refractive power with the convex surface facing the object side, and the third lens L3 is The positive meniscus lens having positive refractive power with the convex surface facing the image side, and the fourth lens L4 is a biconcave negative lens having negative refractive power. These first to fourth lenses L1 to L4 are aspherical on both surfaces and are made of a resin material. Then, the negative refractive power of the image side surface of the fourth lens L4 decreases from the center (optical axis AX) toward the end of the effective area, and the lens cross section along the optical axis AX (light along the optical axis AX). In the contour line of the lens cross section including the axis AX), the contact points IPA4 to IPF4 and IPA4 to IPF4 are provided when going from the intersection of the optical axes AX to the end of the effective area.

The optical aperture stop ST is disposed on the object side of the first lens L1. Similarly in each embodiment, the optical aperture stop ST may be an aperture stop, a mechanical shutter, or a variable stop.

And, on the image side of the fourth lens L4, the light receiving surface of the image pickup element SR is disposed via a parallel plate FT as a filter. The parallel plate FT is a cover glass or the like of various optical filters or the image sensor SR.

In FIG. 5 to FIG. 10, the numbers ri (i = 1, 2, 3,...) Given to the respective lens surfaces are the i-th lens surfaces when counted from the object side (however, the lens joints). The surface is counted as one surface.), And a surface with an asterisk “*” is an aspherical surface. In addition, both surfaces of the parallel plate FT and the light receiving surface of the imaging element SR are handled as one surface, and the surface of the optical aperture stop ST is also handled as one surface. The meaning of such handling and symbols is the same for each embodiment. However, it does not mean that they are exactly the same. For example, the lens surface arranged closest to the object side is denoted by the same symbol (r1) in each drawing of each embodiment, but the construction described later is used. As shown in the data, this does not mean that these curvatures are the same throughout the embodiments.

Under such a configuration, a light beam incident from the object side sequentially has an optical stop ST, a first lens L1, a second lens L2, a third lens L3, a fourth lens L4, and a parallel plate along the optical axis AX. An optical image of the object is formed on the light receiving surface of the image sensor SR through the FT. In the image sensor SR, the optical image is converted into an electrical signal. This electric signal is subjected to predetermined digital image processing as necessary, and is recorded as a digital video signal in a memory of a digital device such as a digital camera, or other digital signal is transmitted by wired or wireless communication via an interface. Or transmitted to the device.

The construction data of each lens in the imaging optical systems 1A to 1F of each example is as follows.

First, construction data of each lens in the imaging optical system 1A of Example 1 is shown below.

Numerical example 1
Unit mm
Surface data surface number r d nd νd ER
Object ∞ ∞
1 (aperture) ∞ -0.06 0.74
2 * 1.842 0.81 1.54470 56.2 0.77
3 * -5.205 0.05 0.88
4 * 6.438 0.30 1.63470 23.9 0.92
5 * 1.872 0.50 0.93
6 * -8.420 1.03 1.54470 56.2 1.12
7 * -1.044 0.19 1.36
8 * -191.341 0.45 1.54470 56.2 1.52
9 * 1.037 0.49 1.97
10 ∞ 0.11 1.51630 64.1 2.22
11 ∞ 2.25
Image plane ∞
Aspheric data 2nd surface K = 0.61922E + 00, A4 = -0.26416E-01, A6 = -0.36817E-01, A8 = 0.20788E-01, A10 = -0.22500E-01
Third surface K = −0.67572E + 01, A4 = −0.11387E-01, A6 = −0.26456E-01, A8 = 0.69959E-01, A10 = −0.47760E-01
4th surface K = -0.41325E + 01, A4 = -0.34785E-01, A6 = -0.60106E-02, A8 = 0.17883E + 00, A10 = -0.30290E-01, A12 = -0.34564E-01
5th surface K = -0.16739E + 01, A4 = 0.79913E-02, A6 = 0.25187E-01, A8 = -0.19002E-02, A10 = 0.10522E + 00, A12 = -0.80140E-01
6th surface K = 0.48670E + 02, A4 = 0.61034E-01, A6 = −0.12844E + 00, A8 = 0.16286E + 00, A10 = −0.75348E-01, A12 = 0.15723E-01
7th surface K = -0.48941E + 01, A4 = -0.14695E + 00, A6 = 0.12510E + 00, A8 = -0.11833E + 00, A10 = 0.81354E-01, A12 = -0.18372E-01
Eighth surface K = 0.0000E + 03, A4 = −0.21942E + 00, A6 = 0.13554E-01, A8 = 0.66277E-01, A10 = −0.33886E-01, A12 = 0.64851E-02, A14 = − 0.31694E-03
9th surface K = −0.57134E + 01, A4 = −0.16514E + 00, A6 = 0.92086E-01, A8 = −0.39873E-01, A10 = 0.11298E-01, A12 = −0.18667E-02, A14 = 0.13125E-03
Various data focal length (f) 3.55 (mm)
F number 2.4
Diagonal length of imaging surface (2Y) 4.6 (mm)
Back focus (Bf) 0.52 (mm)
Total lens length (TL) 4.45 (mm)
ENTP 0 (mm)
EXTP -2.25 (mm)
H1 -1 (mm)
H2 -3.03 (mm)
Focal length of each lens (mm)
1st lens L1 2.603
Second lens L2 -4.268
Third lens L3 2.086
Fourth lens L4 -1.893

Next, construction data of each lens in the imaging optical system 1B of Example 2 is shown below.

Numerical example 2
Unit mm
Surface data surface number r d nd νd ER
Object ∞ ∞
1 (aperture) ∞ -0.06 0.74
2 * 1.853 0.80 1.54470 56.2 0.77
3 * -5.215 0.05 0.87
4 * 6.392 0.31 1.63470 23.9 0.91
5 * 1.859 0.49 0.92
6 * -8.324 1.03 1.54470 56.2 1.09
7 * -1.033 0.19 1.28
8 * -437.709 0.45 1.54470 56.2 1.43
9 * 1.032 0.50 1.87
10 ∞ 0.11 1.51630 64.1 2.07
11 ∞ 2.10
Image plane ∞
Aspherical data second surface K = 0.64162E + 00, A4 = -0.25436E-01, A6 = -0.36892E-01, A8 = 0.21677E-01, A10 = -0.21474E-01
3rd surface K = -0.92148E + 01, A4 = -0.95609E-02, A6 = -0.24788E-01, A8 = 0.71304E-01, A10 = -0.49069E-01
4th surface K = -0.46304E + 01, A4 = -0.35039E-01, A6 = -0.70042E-02, A8 = 0.11904E + 00, A10 = -0.32540E-01, A12 = -0.35312E-01
5th surface K = -0.17602E + 01, A4 = 0.69022E-02, A6 = 0.26056E-01, A8 = -0.15337E-02, A10 = 0.10274E + 00, A12 = -0.81082E-01
6th surface K = 0.45056E + 02, A4 = 0.61519E-01, A6 = −0.12084E + 00, A8 = 0.16524E + 00, A10 = −0.73974E-01, A12 = 0.13793E-01
7th surface K = -0.47736E + 01, A4 = -0.14524E + 00, A6 = 0.12747E + 00, A8 = -0.11692E + 00, A10 = 0.81714E-01, A12 = -0.18215E-01
Eighth surface K = 0.0000E + 03, A4 = −0.21155E + 00, A6 = 0.14812E-01, A8 = 0.65105E-01, A10 = −0.34010E-01, A12 = 0.66107E-02, A14 = − 0.32092E-03
9th surface K = -0.56695E + 01, A4 = -0.16198E + 00, A6 = 0.91490E-01, A8 = -0.39660E-01, A10 = 0.11280E-01, A12 = -0.18677E-02, A14 = 0.13055E-03
Various data focal length (f) 3.53 (mm)
F number 2.4
Diagonal length of imaging surface (2Y) 4.6 (mm)
Back focus (Bf) 0.52 (mm)
Total lens length (TL) 4.45 (mm)
ENTP 0 (mm)
EXTP -2.26 (mm)
H1 -0.97 (mm)
H2 -3.02 (mm)
Focal length of each lens (mm)
1st lens L1 2.614
Second lens L2 -4.241
Third lens L3 2.063
Fourth lens L4 -1.890

Next, construction data of each lens in the imaging optical system 1C of Example 3 is shown below.

Numerical Example 3
Unit mm
Surface data surface number r d nd νd ER
Object ∞ ∞
1 (aperture) ∞ -0.06 0.74
2 * 1.835 0.81 1.54470 56.2 0.77
3 * -5.270 0.05 0.88
4 * 6.446 0.30 1.63470 23.9 0.92
5 * 1.874 0.51 0.93
6 * -8.456 1.03 1.54470 56.2 1.11
7 * -1.046 0.18 1.36
8 * -244.617 0.45 1.54470 56.2 1.52
9 * 1.035 0.49 1.97
10 ∞ 0.11 1.51630 64.1 2.24
11 ∞ 2.27
Image plane ∞
Aspherical data second surface K = 0.632232E + 00, A4 = -0.26849E-01, A6 = -0.36216E-01, A8 = 0.19442E-01, A10 = -0.21897E-01
Third plane K = -0.60691E + 01, A4 = -0.11741E-01, A6 = -0.24483E-01, A8 = 0.67934E-01, A10 = -0.47494E-01
4th surface K = -0.30629E + 01, A4 = -0.34292E-01, A6 = -0.53860E-02, A8 = 0.11835E + 00, A10 = -0.31083E-01, A12 = -0.35107E-01
5th surface K = -0.16148E + 01, A4 = 0.88283E-02, A6 = 0.24112E-01, A8 = -0.12431E-02, A10 = 0.86186E + 00, A12 = -0.83273E-01
6th surface K = 0.50035E + 02, A4 = 0.61044E-01, A6 = -0.13057E + 00, A8 = 0.16302E + 00, A10 = -0.76267E-01, A12 = 0.166624E-01
7th surface K = -0.49378E + 01, A4 = -0.14761E + 00, A6 = 0.12481E + 00, A8 = -0.11875E + 00, A10 = 0.81339E-01, A12 = -0.18395E-01
8th surface K = 0.0000E + 03, A4 = −0.22206E + 00, A6 = 0.13864E-01, A8 = 0.66675E-01, A10 = −0.33862E-01, A12 = 0.63645E-02, A14 = − 0.29164E-03
9th surface K = −0.57245E + 01, A4 = −0.16591E + 00, A6 = 0.92276E-01, A8 = −0.39887E-01, A10 = 0.11306E-01, A12 = −0.18737E-02, A14 = 0.13246E-03
Various data focal length (f) 3.55 (mm)
F number 2.4
Diagonal length of imaging surface (2Y) 4.6 (mm)
Back focus (Bf) 0.52 (mm)
Total lens length (TL) 4.45 (mm)
ENTP 0 (mm)
EXTP -2.24 (mm)
H1 -1.02 (mm)
H2 -3.04 (mm)
Focal length of each lens (mm)
1st lens L1 2.604
Second lens L2 -4.275
Third lens L3 2.090
Fourth lens L4 -1.891

Next, construction data of each lens in the imaging optical system 1D of Example 4 is shown below.

Numerical Example 4
Unit mm
Surface data surface number r d nd νd ER
Object ∞ ∞
1 (aperture) ∞ -0.06 0.74
2 * 1.842 0.82 1.54470 56.2 0.77
3 * -5.214 0.05 0.88
4 * 6.447 0.30 1.63470 23.9 0.92
5 * 1.875 0.50 0.93
6 * -8.425 1.03 1.54470 56.2 1.12
7 * -1.042 0.19 1.36
8 * -176.801 0.45 1.54470 56.2 1.52
9 * 1.035 0.49 1.97
10 ∞ 0.11 1.51630 64.1 2.24
11 ∞ 2.27
Image plane ∞
Aspherical data second surface K = 0.612107E + 00, A4 = −0.26315E-01, A6 = −0.36696E-01, A8 = 0.20896E-01, A10 = −0.22514E-01
Third surface K = −0.68408E + 01, A4 = −0.11301E-01, A6 = −0.26325E-01, A8 = 0.69862E-01, A10 = −0.47577E-01
Fourth surface K = −0.42152E + 01, A4 = −0.34826E-01, A6 = −0.61296E-02, A8 = 0.11791E + 00, A10 = −0.330615E-01, A12 = −0.33979E-01
5th surface K = -0.16858E + 01, A4 = 0.78428E-02, A6 = 0.50554E-01, A8 = -0.17820E-02, A10 = 0.10411E + 00, A12 = -0.78809E-01
6th surface K = 0.48646E + 02, A4 = 0.60894E-01, A6 = -0.12851E + 00, A8 = 0.16273E + 00, A10 = -0.75630E-01, A12 = 0.15865E-01
7th surface K = -0.48956E + 01, A4 = -0.14730E + 00, A6 = 0.12537E + 00, A8 = -0.11826E + 00, A10 = 0.81325E-01, A12 = -0.18400E-01
8th surface K = 0.0000E + 03, A4 = −0.21941E + 00, A6 = 0.13863E-01, A8 = 0.62625E-01, A10 = −0.33859E-01, A12 = 0.64559E-02, A14 = − 0.31228E-03
9th surface K = −0.57312E + 01, A4 = −0.16465E + 00, A6 = 0.91848E-01, A8 = −0.39804E-01, A10 = 0.11298E-01, A12 = −0.18707E-02, A14 = 0.13175E-03
Various data focal length (f) 3.55 (mm)
F number 2.4
Diagonal length of imaging surface (2Y) 4.6 (mm)
Back focus (Bf) 0.52 (mm)
Total lens length (TL) 4.45 (mm)
ENTP 0 (mm)
EXTP -2.25 (mm)
H1 -1 (mm)
H2 -3.03 (mm)
Focal length of each lens (mm)
1st lens L1 2.605
Second lens L2 -4.274
Third lens L3 2.080
Fourth lens L4 -1.888

Next, construction data of each lens in the imaging optical system 1E of Example 5 is shown below.

Numerical Example 5
Unit mm
Surface data surface number r d nd νd ER
Object ∞ ∞
1 (aperture) ∞ -0.06 0.73
2 * 1.860 0.80 1.54470 56.2 0.77
3 * -5.201 0.05 0.88
4 * 7.177 0.33 1.63470 23.9 0.92
5 * 1.922 0.48 0.94
6 * -8.415 1.03 1.54470 56.2 1.13
7 * -1.014 0.18 1.35
8 * -411.733 0.45 1.54470 56.2 1.53
9 * 1.017 0.45 1.98
10 ∞ 0.11 1.51630 64.1 2.21
11 ∞ 2.24
Image plane ∞
Aspheric data 2nd surface K = 0.64790E + 00, A4 = -0.25198E-01, A6 = -0.36700E-01, A8 = 0.19492E-01, A10 = -0.19750E-01
3rd surface K = -0.93736E + 01, A4 = -0.97166E-02, A6 = -0.25706E-01, A8 = 0.73936E-01, A10 = -0.51589E-01
4th surface K = 0.26553E + 01, A4 = -0.31698E-01, A6 = -0.73939E-02, A8 = 0.11582E + 00, A10 = -0.34375E-01, A12 = -0.31283E-01
Fifth surface K = -0.16396E + 01, A4 = 0.89770E-02, A6 = 0.26413E-01, A8 = -0.66480E-02, A10 = 0.99281E-01, A12 = -0.74104E-01
6th surface K = 0.47683E + 02, A4 = 0.60002E-01, A6 = -0.11217E + 00, A8 = 0.15822E + 00, A10 = -0.74908E-01, A12 = 0.14858E-01
7th surface K = -0.48009E + 01, A4 = -0.14410E + 00, A6 = 0.13159E + 00, A8 = -0.11660E + 00, A10 = 0.80392E-01, A12 = -0.18493E-01
8th surface K = 0.0000E + 03, A4 = -0.20152E + 00, A6 = 0.15242E-01, A8 = 0.63266E-01, A10 = -0.34233E-01, A12 = 0.66441E-02, A14 =- 0.28591E-03
9th surface K = -0.57647E + 01, A4 = -0.15568E + 00, A6 = 0.88198E-01, A8 = -0.38649E-01, A10 = 0.11175E-01, A12 = -0.19011E-02, A14 = 0.13705E-03
Various data focal length (f) 3.52 (mm)
F number 2.4
Diagonal length of imaging surface (2Y) 4.6 (mm)
Back focus (Bf) 0.57 (mm)
Total lens length (TL) 4.45 (mm)
ENTP 0 (mm)
EXTP -2.22 (mm)
H1 -0.93 (mm)
H2 -2.95 (mm)
Focal length of each lens (mm)
First lens L1 2.620
Second lens L2 -4.238
Third lens L3 2.017
Fourth lens L4 -1.861

Next, construction data of each lens in the imaging optical system 1F of Example 6 is shown below.

Numerical Example 6
Unit mm
Surface data surface number r d nd νd ER
Object ∞ ∞
1 (aperture) ∞ -0.06 0.74
2 * 1.838 0.81 1.54470 56.2 0.77
3 * -5.349 0.05 0.88
4 * 6.470 0.30 1.63470 23.9 0.92
5 * 1.882 0.50 0.93
6 * -8.422 1.02 1.54470 56.2 1.12
7 * -1.039 0.19 1.35
8 * -359.933 0.45 1.54470 56.2 1.53
9 * 1.030 0.51 1.98
10 ∞ 0.15 1.51630 64.1 2.24
11 ∞ 2.28
Image plane ∞
Aspherical data second surface K = 0.64587E + 00, A4 = −0.26148E-01, A6 = −0.35239E-01, A8 = 0.19715E-01, A10 = −0.21206E-01
Third surface K = −0.80138E + 01, A4 = −0.10339E-01, A6 = −0.22766E-01, A8 = 0.68437E-01, A10 = −0.48178E-01
4th surface K = -0.46053E + 01, A4 = -0.34957E-01, A6 = -0.56803E-02, A8 = 0.11818E + 00, A10 = -0.31044E-01, A12 = -0.35791E-01
5th surface K = -0.17006E + 01, A4 = 0.74300E-02, A6 = 0.22742E-01, A8 = -0.15821E-02, A10 = 0.010891E + 00, A12 = -0.83496E-01
6th surface K = 0.48552E + 02, A4 = 0.61719E-01, A6 = -0.12921E + 00, A8 = 0.16470E + 00, A10 = -0.76050E-01, A12 = 0.16457E-01
7th surface K = −0.48849E + 01, A4 = −0.14736E + 00, A6 = 0.12585E + 00, A8 = −0.11823E + 00, A10 = 0.81714E-01, A12 = −0.18179E-01
8th surface K = 0.0000E + 03, A4 = −0.21942E + 00, A6 = 0.13989E-01, A8 = 0.67012E-01, A10 = −0.33825E-01, A12 = 0.346346E-02, A14 = − 0.29396E-03
9th surface K = −0.57105E + 01, A4 = −0.16502E + 00, A6 = 0.92459E-01, A8 = −0.39898E-01, A10 = 0.11322E-01, A12 = −0.18707E-02, A14 = 0.13084E-03
Various data focal length (f) 3.55 (mm)
F number 2.4
Diagonal length of imaging surface (2Y) 4.6 (mm)
Back focus (Bf) 0.48 (mm)
Total lens length (TL) 4.45 (mm)
ENTP 0 (mm)
EXTP -2.29 (mm)
H1 -1 (mm)
H2 -3.07 (mm)
Focal length of each lens (mm)
1st lens L1 2.616
Second lens L2 -4.289
Third lens L3 2.074
Fourth lens L4 -1.884

Here, the total lens length (TL) of the above-mentioned various data is the total lens length (distance from the first lens object side surface to the imaging surface) when the object distance is infinite. ENTP is the distance from the entrance pupil to the first surface (aperture). Here, the entrance pupil is equal to the aperture, and is 0. EXTP is the distance from the final surface (cover glass image surface side) to the exit pupil, H1 is the distance from the first surface (aperture) to the object side principal point, and H2 is the final surface (cover glass image). This is the distance from the image side principal point to the image side principal point.

In the above surface data, the surface number corresponds to the number i of the symbol ri (i = 1, 2, 3,...) Given to each lens surface shown in FIGS. The surface marked with * in the number i indicates an aspherical surface (aspherical refractive optical surface or a surface having a refractive action equivalent to an aspherical surface).

“R” is the radius of curvature of each surface (unit: mm), and “d” is the distance (axis) between the lens surfaces on the optical axis in the infinitely focused state (focused state at infinity). “Top”), “nd” indicates the refractive index of each lens with respect to the d-line (wavelength 587.56 nm), “νd” indicates the Abbe number, and “ER” indicates the effective radius (mm). Since each surface of the optical aperture stop ST, both surfaces of the parallel flat plate FT, and the light receiving surface of the image sensor SR is a flat surface, the radius of curvature thereof is ∞ (infinite).

The above-mentioned aspheric surface data includes the quadric surface parameter (cone coefficient K) and the aspheric surface coefficient Ai (i = 4, 6, 6) of the surface that is an aspheric surface (the surface with the number i added to * in the surface data). 8, 10, 12, 14, 16).

In each embodiment, the shape of the aspheric surface is defined by the following equation when the surface vertex is the origin, the X axis is taken in the optical axis direction, and the height in the direction perpendicular to the optical axis is h.

X = (h ² / R) / [1+ (1− (1 + K) h ² / R ² ) ^1/2 ] + ΣA _i · h ⁱ
Where Ai is an i-th order aspheric coefficient, R is a reference radius of curvature, and K is a conic constant.

Note that the paraxial radius of curvature (r) described in the claims, embodiments, and examples is in the vicinity of the center of the lens (more specifically, within 10% of the lens outer diameter) in the actual lens measurement scene. The approximate curvature radius when the shape measurement value in the center region of the curve is fitted by the least square method can be regarded as the paraxial curvature radius. For example, when a secondary aspherical coefficient is used, a curvature radius that takes into account the secondary aspherical coefficient in the reference curvature radius of the aspherical definition formula can be regarded as a paraxial curvature radius (for example, reference literature). (See pages 41-42 of “Lens Design Method” by K. Matsui, Kyoritsu Publishing Co., Ltd.).

In the aspheric data, “En” means “10 to the power of n”. For example, “E + 001” means “10 to the power of +1”, and “E-003” means “10 to the power of −3”.

FIG. 11 to FIG. 16 show aberrations in the imaging lenses 1A to 1F of the respective examples under the lens arrangement and configuration as described above.

FIGS. 11 to 16 show aberration diagrams at a distance of infinity, and (A), (B), and (C) in each figure are spherical aberrations (sinusoidal conditions) (LONGITUDINAL) in this order, respectively. SPHERICAL ABERRATION), astigmatism (ASTIGMATISM)
FIELD CURVE) and distortion (DISTORTION) are shown respectively. The abscissa of the spherical aberration represents the focal position shift in mm, and the ordinate represents the value normalized by the maximum incident height. The horizontal axis of astigmatism represents the focal position shift in mm, and the vertical axis represents the image height in mm. The horizontal axis of the distortion aberration represents the actual image height as a percentage (%) with respect to the ideal image height, and the vertical axis represents the image height in mm. Moreover, in the figure of astigmatism, the broken line represents the result on the tangential (meridional) surface, and the solid line represents the result on the sagittal (radial) surface.

In the diagram of spherical aberration, the aberrations of two light beams, ie, the d-line (wavelength 587.56 nm) as a solid line and the g-line (wavelength 435.84 nm) as a broken line are shown. The diagrams of astigmatism and distortion are the results when the d-line (wavelength 587.56 nm) is used.

Table 1 shows numerical values obtained when the above-described conditional expressions (1) to (6) are applied to the imaging optical systems 1A to 1F of Examples 1 to 6 listed above.

As described above, the imaging optical systems 1A to 1F in Embodiments 1 to 6 have a four-lens configuration and satisfy the above-described conditions. As a result, the imaging optical systems 1A to 1F have a brightness of about F2.4. Thus, various aberrations can be corrected more favorably while reducing the size of the conventional optical system. The imaging optical systems 1A to 1F in the first to sixth embodiments are sufficiently reduced in size when mounted on the imaging device 21 and the digital device 3, particularly when mounted on the portable terminal 5. A pixel imaging device 17 can be employed.

For example, a high-pixel image sensor 17 having a class (grade) of about 8M to 16M pixels such as 8M pixel, 10M pixel, and 16M pixel has a short pixel pitch when the size of the image sensor 17 is constant (pixel The imaging optical systems 1A to 1F need a resolution corresponding to the pixel pitch, and are defined by, for example, specifications when the imaging optical system 1 is evaluated with the required resolution, for example, with MTF. Although it is necessary to suppress various aberrations within a predetermined range, in the imaging optical systems 1A to 1F in Examples 1 to 6, the various aberrations are suppressed within the predetermined range as shown in each aberration diagram. Accordingly, since the imaging optical systems 1A to 1F in Examples 1 to 6 correct various aberrations satisfactorily, the imaging optical systems 1A to 1F are preferably used for the imaging element 17 of the class of 5M to 8M pixels, for example.

In Examples 1 to 6, the principal ray incident angle of the light beam incident on the imaging surface of the solid-state imaging device is not necessarily sufficiently small at the periphery of the imaging surface. However, as described above, shading can be corrected by hardware or software. Such shading countermeasures alleviate the demand for shading, so that the imaging optical systems 1A to 1F of the present embodiment are further downsized.

This specification discloses various modes of technology as described above, and the main technologies are summarized below.

An imaging optical system according to an aspect includes, in order from the object side to the image side, a stop, a first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a positive refractive power. It consists of a lens and a fourth lens having negative refractive power whose both surfaces are concave, and satisfies the conditional expression (1) above. The imaging optical system having such a configuration has a bright four-lens configuration of about F2.4, is smaller, and can correct various aberrations better.

In another aspect, in the imaging optical system described above, preferably, the first lens has a convex shape on both sides.

In another aspect, in the above-described imaging optical system, preferably, the second lens has a meniscus shape with a convex surface facing the object side.

In another aspect, in the above-described imaging optical system, it is preferable that the first lens and the second lens satisfy the conditional expression (2).

In another aspect, in the above-described imaging optical system, preferably, the image side surface of the fourth lens has an aspherical shape, has a negative refractive power at the center thereof, and approaches the effective area end. When the negative refractive power becomes weak and the lens cross section along the optical axis moves from the intersection of the optical axes toward the effective region end, the vertical contact is provided, and the conditional expression (3) is satisfied.

In another aspect, in the above-described imaging optical system, it is preferable that the fourth lens satisfies the conditional expression (4).

In another aspect, in the above-described imaging optical system, preferably, the second lens satisfies the conditional expression (5). In another aspect, in the above-described imaging optical system, Preferably, the third lens satisfies the conditional expression (6).

In another aspect, in the above-described imaging optical system, it is preferable that all of the first to fourth lenses are resin material lenses formed of a resin material.

An imaging apparatus according to another aspect includes any one of the above-described imaging optical systems and an imaging element that converts an optical image into an electrical signal, and the imaging optical system receives a light receiving surface of the imaging element. An optical image of the object can be formed thereon.

According to this configuration, it is possible to provide an image pickup apparatus using the image pickup optical system having a bright four-lens configuration of about F2.4 while being able to correct various aberrations satisfactorily while being small. Therefore, such an imaging apparatus can be reduced in size and performance.

According to another aspect of the present invention, a digital apparatus includes the above-described imaging device, and a control unit that causes the imaging device to perform at least one of photographing a still image and a moving image of the subject, and imaging optics of the imaging device. The system is assembled so that an optical image of the subject can be formed on the imaging surface of the imaging device. Preferably, the digital device comprises a mobile terminal.

According to this configuration, it is possible to provide a digital device or a portable terminal using an imaging optical system having a bright four-lens configuration of about F2.4, which can correct various aberrations in a small size, but is excellent. it can. Accordingly, such digital devices and portable terminals can be reduced in size and performance.

This application is based on Japanese Patent Application No. 2011-113743 filed on May 20, 2011, the contents of which are included in the present application.

In order to express the present invention, the present invention has been properly and fully described through the embodiments with reference to the drawings. However, those skilled in the art can easily change and / or improve the above-described embodiments. It should be recognized that this is possible. Therefore, unless the modifications or improvements implemented by those skilled in the art are at a level that departs from the scope of the claims recited in the claims, the modifications or improvements are not covered by the claims. It is interpreted that it is included in

According to the present invention, an imaging optical system, an imaging device, and a digital device can be provided.

Claims (12)

1. From the object side to the image side,
   Aperture,
   A first lens having a positive refractive power;
   A second lens having negative refractive power;
   A third lens having positive refractive power;
   A fourth lens having negative refractive power, both surfaces of which are concave,
   An imaging optical system characterized by satisfying conditional expression (1) below.
   −1000 <(r1 + r4) / (r1−r4) <− 55 (1)
   However,
   r1: radius of curvature of object side surface in first lens r4: radius of curvature of image side surface of second lens
2. The imaging optical system according to claim 1, wherein both surfaces of the first lens are convex.
3. The imaging optical system according to claim 1, wherein the second lens has a meniscus shape with a convex surface facing the object side.
4. The imaging optical system according to any one of claims 1 to 3, wherein the first lens and the second lens satisfy a conditional expression (2) below.
   1 <f12 / f <1.7 (2)
   However,
   f12: Composite focal length of the first lens and the second lens f: Focal length of the entire imaging optical system
5. The image side surface of the fourth lens has an aspherical shape, has a negative refractive power at the center thereof, and the negative refractive power becomes weaker toward the end of the effective region, and in the contour line of the lens cross section along the optical axis. 5. The device according to claim 1, wherein the contact point has a perpendicular contact when it goes from the intersection of the optical axes toward the end of the effective region, and satisfies the following conditional expression (3): Imaging optical system.
   0.05 <T4 / f <0.17 (3)
   However,
   T4: thickness on the optical axis of the fourth lens f: focal length of the entire imaging optical system
6. The imaging optical system according to any one of claims 1 to 5, wherein the fourth lens satisfies the following conditional expression (4).
   0.1 <(r7 + r8) / (r7−r8) <1 (4)
   However,
   r7: radius of curvature of object side surface in the fourth lens r8: radius of curvature of image side surface of the fourth lens
7. The imaging optical system according to any one of claims 1 to 6, wherein the second lens satisfies the following conditional expression (5).
   1.6 <r3 / f <2.2 (5)
   However,
   r3: radius of curvature of image side surface of second lens f: focal length of entire imaging optical system
8. The imaging optical system according to any one of claims 1 to 7, wherein the third lens satisfies the following conditional expression (6).
   0.1 <T3 / f <0.6 (6)
   However,
   T3: thickness on the optical axis of the third lens f: focal length of the entire imaging optical system
9. 9. The imaging optical system according to claim 1, wherein all of the first to fourth lenses are resin material lenses made of a resin material.
10. The imaging optical system according to any one of claims 1 to 9,
    An image sensor that converts an optical image into an electrical signal,
    An image pickup apparatus, wherein the image pickup optical system is capable of forming an optical image of an object on a light receiving surface of the image pickup element.
11. An imaging device according to claim 10;
    A controller that causes the imaging device to perform at least one of still image shooting and moving image shooting of a subject;
    A digital apparatus, wherein an imaging optical system of the imaging apparatus is assembled on an imaging surface of the imaging element so that an optical image of the subject can be formed.
12. The digital device according to claim 11, comprising a mobile terminal.

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