JP4944436B2 - Zoom lens and imaging apparatus having the same - Google Patents

Description

  The present invention relates to a zoom lens and an image pickup apparatus having the same, and more particularly to a zoom lens used in an image pickup apparatus such as a digital camera, a video camera, and a film camera.

  In recent years, a solid-state image sensor used in an imaging apparatus such as a digital camera has been increasing in the number of pixels and the definition thereof year by year. The size of one pixel (pixel) of the current solid-state imaging device is about several μm.

  Furthermore, the digital camera can easily enlarge the image, and the user can confirm the image at the same pixel size. For this reason, the optical performance required for the taking lens for digital cameras is very high. In particular, when photographing under white light, it is necessary to prevent deterioration in image quality due to color bleeding. Specifically, it is desired that the lateral chromatic aberration is corrected to be very small in the entire visible wavelength range.

  In particular, a zoom lens is more likely to generate chromatic aberration such as lateral chromatic aberration than a photographic lens having a single focal length. For this reason, there is a strong demand for an improvement in the secondary spectrum that tends to be undercorrected in zoom lenses.

  As a telephoto type zoom lens having a relatively high zoom ratio, there is a so-called positive lead type zoom lens in which the most object side lens unit is a lens unit having a positive refractive power.

  Among these types of zoom lenses, there are known zoom lenses in which chromatic aberration is corrected by using anomalous dispersion glass for the first lens group having a high incident height of off-axis chief rays and the positive lens in the final lens group (for example, Patent Documents 1 to 3).

  Further, there is known a positive lead type three-group zoom lens including three groups of positive, negative, and positive refractive power lenses in order from the object side to the image side. In this three-group zoom lens, a zoom lens is known in which a glass material having anomalous dispersion is used for the negative lens in the second lens group having a negative refractive power, and the variation in lateral chromatic aberration during zooming is reduced (Patent Document). 4).

In addition, a zoom lens in which chromatic aberration is favorably corrected using a material having anomalous dispersion with a five-group zoom lens composed of lens groups having positive, negative, positive, negative, and positive refractive power in order from the object side to the image surface side Is known (Patent Documents 5 and 6).
JP-A-6-43363 Japanese Patent Laid-Open No. 2002-62478 JP-A-8-248317 JP 59-198416 JP 2001-350093 A JP 2002-62474 A

  In Patent Documents 1, 2, and 3, correction of chromatic aberration of magnification is not necessarily sufficient for a photographing optical system such as a digital camera in which the solid-state imaging device has been increased in pixels and the pixel pitch has been reduced.

  Further, in Patent Document 4, the variation in chromatic aberration of magnification during zooming is kept small, but the zoom ratio is not always sufficient. The F number is 8, and the brightness is not sufficient.

  In the configuration of the cited document 4, if the zoom ratio is increased, the chromatic aberration of magnification increases, and it becomes difficult to correct this.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a zoom lens that satisfactorily corrects chromatic aberration of magnification that varies with zooming and has good optical performance over the entire zoom range, and an image pickup apparatus using the zoom lens.

The zoom lens according to the present invention includes, in order from the object side to the image side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a negative lens having a negative refractive power. The fourth lens group is composed of a fifth lens group having a positive refractive power. During zooming from the wide-angle end to the telephoto end, the distance between the first lens group and the second lens group becomes large , and the second lens Zoom in which the distance between the third lens group and the third lens group is reduced, the distance between the third lens group and the fourth lens group is increased, and the distance between the fourth lens group and the fifth lens group is reduced. In the lens, the second lens group has an Abbe number of νd2 and a partial dispersion ratio of θ2.
-1.62 × 10 ^-3 · νd2 + 0.642 <θ2
55 ≦ νd2
Having three or more negative lens with a material that satisfies the following condition including a negative lens constructed, the focal length of the second lens group f2, respectively the focal length of the entire system at the wide-angle end and the telephoto end fw, When ft ,
0.3 <| f2 / fw | <1.0
2.5 ≦ ft / fw
It is characterized by satisfying the following conditions.

  According to the present invention, it is possible to obtain a zoom lens and an image pickup apparatus using the same that correct the magnification chromatic aberration that varies with zooming and realize excellent optical performance over the entire zoom range.

  Embodiments of the zoom lens of the present invention and an image pickup apparatus having the same will be described below.

  FIG. 1 is a cross-sectional view of a principal part of the zoom lens of Example 1, and FIGS. 2 to 4 are aberrations at the wide angle end (short focal length), intermediate focal length, and telephoto end (long focal length) of the zoom lens of Example 1. FIG.

  FIG. 5 is a cross-sectional view of a main part of the zoom lens of Example 2, and FIGS. 6 to 8 are aberration diagrams of the zoom lens of Example 2 at the wide angle end, the intermediate focal length, and the telephoto end.

  FIG. 9 is a cross-sectional view of a main part of the zoom lens of Example 3, and FIGS. 10 to 12 are aberration diagrams of the zoom lens of Example 3 at the wide angle end, the intermediate focal length, and the telephoto end.

  FIG. 13 is a cross-sectional view of a principal part of the zoom lens of Example 4. FIGS. 14 to 16 are aberration diagrams of the zoom lens of Example 4 at the wide angle end, the intermediate focal length, and the telephoto end.

  FIG. 17 is an explanatory diagram showing the relationship between the Abbe number νd and the partial dispersion ratio θgF.

  FIG. 18 is an explanatory diagram of a correction principle when correcting chromatic aberration of magnification in a zoom lens. FIG. 19 is a schematic view of the imaging apparatus of the present invention.

  In the lens sectional views of the zoom lenses of Examples 1 to 4 shown in FIGS. 1, 5, 9, and 13, L1 is a first lens unit having a positive refractive power, and L2 is a second lens having a negative refractive power. The group LR has a plurality of lens groups, and is a subsequent lens group having a positive refractive power as a whole.

  SP is an aperture stop, which is positioned in front of the subsequent lens group LR.

  The subsequent lens unit LR includes, in order from the object side to the image side, a third lens unit L3 having a positive refractive power, a fourth lens unit L4 having a negative refractive power, and a fifth lens unit L5 having a positive refractive power. Yes.

  IP is an image plane on which an imaging plane of a solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor is located. In the lens sectional view, the left side is the object side (front) and the right side is the image side (rear).

  In each embodiment, each lens group is moved as indicated by an arrow during zooming from the wide-angle end to the telephoto end.

  Note that the wide-angle end and the telephoto end are zoom positions when the zooming lens groups are positioned at both ends of a range in which the lens group can be moved in the optical axis direction.

  In each embodiment, during zooming from the wide-angle end to the telephoto end, the first lens unit L1 is moved to the object side so that the distance between the first lens unit L1 and the second lens unit L2 is widened.

  The second lens unit L2 is moved to the image side so that the distance between the second lens unit L2 and the stop SP is narrowed. The third lens unit L3 is moved to the object side so that the distance between the second lens unit L2 and the third lens unit L3 is narrowed.

As shown in the numerical examples, the fourth lens unit L4 moves so that the distance between the third lens unit L3 and the fourth lens unit L4 is increased. Both the fourth lens unit L4 and the fifth lens unit L5 are moved to the object side so that the distance between them is reduced.

  In each embodiment, the third lens unit L3 and the fifth lens unit L5 are integrally moved to simplify the mechanism, but may be moved independently of each other.

  The aperture stop SP may be moved integrally with the third lens unit L3 during zooming or may be moved separately. When integrated, the number of moving units is reduced and the mechanical structure is easily simplified.

  In the aberration diagrams, d and g are d-line and g-line. SC is a sine condition. ΔM and ΔS are meridional image surfaces, sagittal image surfaces, and lateral chromatic aberration is represented by g-line. ω is a half angle of view.

  Next, features of each embodiment will be described.

In each embodiment, the second lens unit L2 has an Abbe number of νd2 and a partial dispersion ratio of θ2,
−1.62 × 10 ⁻³ · νd2 + 0.642 <θ2 (1)
55 ≦ νd2 (2)
It has three or more negative lenses including a negative lens made of a material that satisfies the following conditions.

Furthermore, when the focal length of the second lens unit L2 is f2, and the focal length of the entire system at the wide angle end is fw,
0.3 <| f2 / fw | <1.0 (3)
Is satisfied.

  Here, when the refractive indexes of the glass materials for the wavelength 436 nm (g line), the wavelength 486 nm (F line), the wavelength 588 nm (d line), and the wavelength 656 nm (C line) are ng, nF, nd, and nC, respectively. The Abbe number νd and the partial dispersion ratio θ are as follows.

νd = (nd−1) / (nF−nC)
θ = (ng−nF) / (nF−nC)
In the zoom lens of each embodiment, for example, if the second lens unit L2 is composed of two or less negative lenses, the following problem occurs. Since anomalous dispersion glass generally has a low refractive index, the curvature is tight (the radius of curvature is small) in order to obtain a desired refractive power. As a result, negative distortion increases mainly at the wide-angle end, and it is difficult to correct this.

  FIG. 17 is a graph showing the relationship between the Abbe number νd of the optical material and the partial dispersion ratio θ. In FIG. 17, a point A indicates a value of a product name PBM2 (νd = 36.26, θ = 0.5828) manufactured by OHARA INC.

  Point B shows the value of the product name NSL7 (νd = 60.49, θ = 0.5436) manufactured by OHARA INC.

  A line connecting points A and B is defined as a reference line. Many high-dispersion glasses having an Abbe number νd smaller than about 35 are located above the reference line. Moreover, many low dispersion glasses having an Abbe number νd of about 35 to 60 are located below the reference line.

  On the other hand, there is an anomalous dispersion glass having an Abbe number νd of 60 or more and positioned above the reference line. For the low dispersion glass, it is effective for correcting the secondary spectrum to use a glass positioned above the reference line, and the correction effect increases as the distance from the reference line increases.

  Conditional expression (1) defines the anomalous dispersion of the lens material used for the g-line and F-line with reference to the partial dispersion reference line. Specifically, the partial dispersion ratio θ2 in the conditional expression (1) represents the partial dispersion ratio of the lens material used for the g-line and the F-line, and the partial dispersion ratio in the case where the right side of the conditional expression (1) is the reference glass represents θ2.

  That is, the lens material satisfying the conditional expression (1) is generally called anomalous dispersion glass, and the case where the partial dispersion ratio θ2 is larger than the reference line is relatively g line compared to the reference glass. This indicates that the refractive index of is large.

  In order to reduce lateral chromatic aberration over the entire zoom range, it is necessary to control the lateral chromatic aberration coefficient of the entire system to a value close to zero throughout the entire zoom range.

Here, the lateral chromatic aberration coefficient T is expressed as follows: when the refractive power of the lens is φ, the axial ray incident height incident on the lens surface is h, the off-axis principal ray incident height incident on the lens surface is hb, and the Abbe number is νd.
T = Σ (h · hb · φ / νd)
It is represented by

  Accordingly, the variation in lateral chromatic aberration is dominated by the influence of the lens group having a large variation in the off-axis principal ray incident height hb. Next, the influence of the lens group having a large absolute value of the refractive power φ, that is, the second lens group L2 in each embodiment, becomes large.

  FIGS. 18A and 18B are schematic diagrams of paraxial refractive power arrangements at the wide-angle end and the telephoto end of the zoom lens according to each embodiment.

  In the paraxial refractive power arrangement of FIG. 18, the first lens unit L1 having a positive refractive power, the second lens unit L2 having a negative refractive power, and the subsequent lens unit LR having a positive refractive power are sequentially arranged from the object side to the image side. Have. As the focal length of the entire system increases, the distance between the first lens unit L1 and the second lens unit L2 increases and the distance between the second lens unit L2 and the subsequent lens unit LR decreases.

  In such a zoom lens, an off-axis principal ray will be considered. At the wide-angle end, as shown in FIG. 18A, and at the telephoto end, as shown in FIG. 18B, the principal ray passes through each lens group. Become. In FIGS. 18A and 18B, refraction at the subsequent lens unit LR on the image side from the stop SP is omitted.

  In the conventional zoom lens, it is assumed that the lateral chromatic aberration of the g-line and the C-line is corrected so as to be at the same position on the image plane. In this case, the position on the image plane is shifted to the outside of the optical axis La at the wide-angle end and shifted to the inside of the optical axis La at the telephoto end with respect to the d-line.

  At this time, the lateral chromatic aberration of g-line is corrected according to the following principle. Assume that anomalous dispersion glass is used as the material of the negative lens of the second lens unit L2 having a negative refractive power. In this case, the force that bends the g-line inside the optical axis La increases.

  This is because the refractive power of the g-line of the anomalous dispersion glass is relatively higher than that of a normal glass material.

  Here, when attention is paid to the height hb of the off-axis principal ray at the wide-angle end and the telephoto end, the height hb is small at the telephoto end, so that the influence of the second lens unit L2 is smaller than that at the wide-angle end.

  Therefore, satisfying conditional expression (1) can greatly improve the secondary spectrum of lateral chromatic aberration at the telephoto end without greatly deteriorating lateral chromatic aberration at the telephoto end.

  Therefore, when the anomalous dispersion of the lens material used for the second lens unit L2 is reduced beyond the lower limit of conditional expression (1), it is difficult to sufficiently reduce the lateral chromatic aberration.

  Further, if the lower limit of conditional expression (2) is exceeded, the achromaticity of each lens group becomes insufficient, and aberration fluctuation due to zooming of lateral chromatic aberration and axial chromatic aberration becomes large, which is not good.

  In each embodiment, aberration variation during zooming is reduced so as to satisfy the conditional expression (3).

  Conditional expression (3) defines an appropriate range of the focal length of the second lens unit L2 responsible for most of the zooming action. It is assumed that the refractive power of the second lens unit L2 increases beyond the lower limit of conditional expression (3). In this case, the amount of movement of the second lens unit L2 due to zooming is reduced, but various aberrations that occur in the second lens unit L2 increase further, making it difficult to correct aberration variations due to zooming.

  On the other hand, it is assumed that the refractive power of the second lens unit L2 becomes smaller than the upper limit of the conditional expression (3). In this case, the amount of movement of the second lens unit L2 due to zooming increases, and the entire optical system increases in size.

  In addition, the lens diameter made of the anomalous dispersion material used for the second lens unit L2 is increased accordingly, which increases the weight and complicates the lens barrel structure.

  More preferably, the numerical ranges of conditional expressions (2) and (3) should be set as follows.

60 ≦ νd2 (2a)
0.4 <| f2 / fw | <0.9 (3a)
As described above, in the zoom lens of each embodiment, the second lens unit L2 that greatly affects the chromatic aberration of magnification of the entire zoom lens includes the negative lens that satisfies the predetermined relationship described above. As a result, various aberrations are satisfactorily corrected over the entire zoom range while maintaining a high zoom ratio, and optical performance with high resolution and high contrast is obtained.

In each embodiment, when the focal lengths of the entire system at the wide-angle end and the telephoto end are respectively fw and ft, 2.5 ≦ ft / fw ( 4 )
Is satisfied.

Conditional expression ( 4 ) relates to the zoom ratio. In each embodiment, a zoom lens having a zoom ratio of 2.5 or more is obtained by appropriately setting the refractive power and movement condition of each lens group.

  When the zoom ratio is smaller than 2.5, the zoom ratio becomes insufficient as a photographing lens for a single-lens reflex camera, which is not preferable.

More preferably, the numerical value of conditional expression ( 4 ) should be set as follows.

3.0 ≦ ft / fw ( 4a )
In each embodiment, the second lens unit L2 includes a negative lens, a negative lens, and a plurality of lenses in order from the object side to the image side.

  This reduces aberration fluctuations during zooming.

  In each embodiment, some lens surfaces are aspherical in order to satisfactorily correct various aberrations without significantly increasing the number of lenses.

  In the lens configurations of the embodiments, it is preferable that at least one surface of the second lens unit L2 or the fifth lens unit L5 has an aspherical shape.

  According to this, it becomes easy to satisfactorily correct mainly astigmatism and distortion at the wide-angle end.

  An aperture stop SP is disposed on the image side of the second lens unit L2 and in the subsequent lens unit LR or on the object side of the subsequent lens unit.

  As a result, the entire lens system is reduced in size while preventing an increase in the effective diameter of the front lens.

  The numerical examples 1 to 4 corresponding to the first to fourth examples are shown below. In each numerical example, i indicates the order of the surfaces from the object side, ri is the radius of curvature of each surface, di is the distance between the i-th surface and the (i + 1) -th surface, and ni and νi are d-lines, respectively. Refractive index and Abbe number based on. When the aspherical shape is X with the displacement in the optical axis direction at the position of the height h from the optical axis as X with respect to the surface vertex,

It is represented by However, R is a paraxial radius of curvature, K is a conic constant, and C4, C6, C8, C10, and C12 are aspherical coefficients. “E ^−X ” means [× 10 ^−X ]. f denotes a focal length, Fno denotes an F number, and L1 to L5 denote first to fifth lens groups.

  The final value of the center thickness and the distance di is the distance between the lens surface and the image surface.

  Further, << Table 1 >> shows the relationship between the above-described conditional expressions and numerical values in the numerical examples.

  Next, an embodiment in which the zoom lens shown in Embodiments 1 to 4 is applied to an imaging apparatus will be described with reference to FIG.

  FIG. 19 is a schematic view of the main part of a single-lens reflex camera. In FIG. 19, reference numeral 10 denotes a photographing lens having the zoom lens 1 according to the first to fourth embodiments.

  The zoom lens 1 is held by a lens barrel 2 that is a holding member.

Reference numeral 20 denotes a camera body. The camera body 20 includes a quick return mirror 3, a focusing screen 4, a penta roof prism 5, an eyepiece lens 6, and the like. The quick return mirror 3 reflects the light beam from the photographing lens 10 upward. The focusing screen 4 is disposed at the image forming position of the taking lens 10. The penta roof prism 5 converts the reverse image formed on the focusing screen 4 into an erect image. An observer observes the erect image through the eyepiece 6, 7 is a photosensitive surface, and a solid-state imaging device (photoelectric conversion device) or a silver salt film for receiving an image of a CCD sensor or a CMOS sensor is arranged. The At the time of photographing, the quick return mirror 3 is retracted from the optical path, and an image is formed on the photosensitive surface 7 by the photographing lens 10.

  The benefits described in the first to fourth embodiments are effectively enjoyed in the optical apparatus disclosed in the present embodiment.

Lens cross-sectional view at the wide-angle end of the zoom lens of Example 1 Aberration diagram at the wide-angle end of the zoom lens of Example 1 Aberration diagram at the intermediate zoom position of the zoom lens of Example 1 Aberration diagram at the telephoto end of the zoom lens of Example 1 Lens sectional view at the wide-angle end of the zoom lens according to Embodiment 2 Aberration diagram at the wide-angle end of the zoom lens of Example 2 Aberration diagram at the middle zoom position of the zoom lens of Example 2 Aberration diagram at the telephoto end of the zoom lens of Example 2 Lens sectional view at the wide-angle end of the zoom lens according to Embodiment 3 Aberration diagram at the wide-angle end of the zoom lens in Example 3 Aberration diagram at the middle zoom position of the zoom lens of Example 3 Aberration diagram at the telephoto end of the zoom lens in Example 3 Lens sectional view at the wide-angle end of the zoom lens according to Embodiment 4 Aberration diagram at wide-angle end of zoom lens in Example 4 Aberration diagram at intermediate zoom position of zoom lens of Example 4 Aberration diagram at the telephoto end of the zoom lens in Example 4 Explanatory drawing showing the relationship between Abbe number νd and partial dispersion ratio θ Schematic diagram for explaining the correction principle of lateral chromatic aberration in the zoom lens of the present invention Schematic diagram of main parts of an imaging apparatus of the present invention

Explanation of symbols

L1 1st lens group L2 2nd lens group L3 3rd lens group L4 4th lens group L5 5th lens group LR Subsequent lens group SP Aperture stop IP Image plane d d line g g line S.L. C Sine condition ΔS Sagittal image plane ΔM Meridional image plane

Claims (5)

In order from the object side to the image side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a negative refractive power, and a positive lens The zoom lens is composed of a fifth lens unit having a refractive power. During zooming from the wide-angle end to the telephoto end, the distance between the first lens unit and the second lens unit increases , and the second lens unit and the third lens unit In the zoom lens in which the distance between the third lens group and the fourth lens group is increased, and the distance between the fourth lens group and the fifth lens group is decreased. When the group has an Abbe number of νd2 and a partial dispersion ratio of θ2,
-1.62 × 10 ^-3 · νd2 + 0.642 <θ2
55 ≦ νd2
Having three or more negative lens with a material that satisfies the following condition including a negative lens constructed, the focal length of the second lens group f2, respectively the focal length of the entire system at the wide-angle end and the telephoto end fw, When ft ,
0.3 <| f2 / fw | <1.0
2.5 ≦ ft / fw
A zoom lens characterized by satisfying the following conditions: The zoom lens according to claim 1 , wherein the second lens group includes a negative lens, a negative lens , and a plurality of lenses in order from the object side to the image side. The zoom lens according to claim 1 or 2, characterized in that the aperture stop is disposed on the image side of the second lens group. The zoom lens according to any one of claims 1 to 3, characterized in that to form an image on the solid-state imaging device. 5. An imaging apparatus comprising: the zoom lens according to claim 1 ; and a solid-state imaging device that receives an image formed by the zoom lens.