JP2000066091A - Image pickup lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image pickup lens which accomplishes an F-number of 2.8 and an viewing angle of 40 deg. despite of few number of lens configuration, and has a long back focal length required for an electronic pickup element and excellent telecentricity, and moreover a very sharp optical focus performance. SOLUTION: Sequentially from an object side, this image pickup lens is configured of a 1st lens L1 with a negative refractive power, a 2nd lens L2 with a positive refractive power, a diaphragm S, a 3rd lens L3 with a negative refractive power, a 4th lens L4 with a negative refractive power, and a 5th lens L5 with a positive refractive power. In this case, expressing that Bf is a back focal length, f is a focal length of the entire system, and α is an angle formed by a principal ray going to the maximum height of the image with the optical axis, the conditional expressions 0.6<Bf/f<0.8, and |α|<10 deg. are satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an imaging lens,
More specifically, the present invention relates to a telecentric imaging lens used for a small electronic imaging device (CCD).

[0002]

As optical systems for small CCDs, Japanese Patent Application Laid-Open Nos. Hei 7-218825, Hei 7-218826, and Hei 8-122636 have been proposed.
JP-A-8-220428 and JP-A-9-220428
The thing disclosed by 230232 gazette etc. is known. However, these imaging lenses have a problem that the number of lens components is large in order to make the F-number small and bright, and that the distortion is large and the optical performance is not always sufficient. In view of these prior arts, the present invention achieves an F-number of 2.8 and an angle of view of 40 ° with a small number of lens components, and a long back focus and good telecentricity required for an electronic image sensor. An object of the present invention is to obtain an imaging lens having city and extremely high optical imaging performance.

[0003]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem. That is, a first lens having a negative refractive power and a second lens having a positive refractive power are arranged in order from the object side. It is composed of a lens, an aperture, a third lens having a negative refractive power, a fourth lens having a positive refractive power, and a fifth lens having a positive refractive power. Bf: Back focus length f: Focal length α of the whole system : When the angle formed with the optical axis of the principal ray that reaches the maximum image height, the following conditional expression is satisfied: 0.6 <Bf / f <0.8 ＜ (1) | α | <10 ° ‥‥ (2) An imaging lens characterized in that: The principal ray is a ray passing through the center of the stop.

In an image pickup lens using a small CCD as an image pickup device, a low-pass filter made of a quartz plate or the like is provided between the image pickup lens and the image pickup device, that is, in a back focus of the image pickup lens, in order to cut a high frequency component of light. Inserted. Conditional expression (1) is an expression that defines an appropriate range of the back focus Bf of the imaging lens. If the lower limit of the conditional expression (1) is exceeded and the back focus Bf becomes excessively short, it becomes impossible to secure a sufficient space for inserting an optical low-pass filter or the like. On the other hand, if the back focus Bf becomes excessively longer than the upper limit of the conditional expression (1), a retrofocus lens composed of a first lens having a negative refractive power and second to fifth lenses having a positive combined refractive power. Each refractive power of the configuration becomes too strong, and it becomes difficult to correct aberration.

[0005] In addition, since the CCD cannot exhibit sufficient performance unless the light incident on the CCD is incident at an angle close to the vertical, the imaging lens system needs to have a sufficient telecentricity. Conditional expression (2) defines an appropriate range of the telecentricity of the imaging lens system. When the upper limit of conditional expression (2) is exceeded, the amount of light incident on a small-sized image sensor such as a CCD is sufficiently effective. Can no longer be used. Note that the position where the principal ray reaching the image plane intersects the optical axis, that is, the position of the exit pupil may be on the object side (imaging lens side) or on the opposite side of the image plane, but generally, It is preferable that the position of the exit pupil is formed on the object side of the image plane. The value of α in each embodiment described later is positive when the position of the exit pupil is on the object side (imaging lens side) of the image plane.

Next, in the present invention, the first lens is formed in a meniscus shape with the convex surface facing the object side, the object-side lens surface of the second lens is formed in a convex shape toward the object side, and The third lens is formed to have a greater curvature than the side lens surface, the third lens is formed to have a biconcave shape, the image side lens surface of the fourth lens is formed to have a shape convex to the image side, and the object side lens is formed. The fifth lens is formed so as to have a curvature stronger than the surface, and the object-side lens surface of the fifth lens is formed to have a shape convex toward the object side and to have a curvature greater than the image-side lens surface. preferable. With these lens shapes, good optical performance can be achieved.

Next, in the present invention, the distance between the first lens having a negative refractive power and the second lens having a positive refractive power is an important distance for realizing a high-performance optical system. is not. Therefore, the first lens and the second lens are
It can be a separate lens or a cemented lens. When the first lens and the second lens are cemented, the manufacturing tolerance can be further increased, so that the optical system can be manufactured easily.

Next, in the present invention, when D is the distance between the second lens and the third lens on the optical axis, the following conditional expression is satisfied: 0.15 <D / f <0.25 (3) Preferably, it is satisfied. Second lens and third
Since the interval D on the optical axis with respect to the lens is a space including the stop, the behavior of the peripheral ray on the axis and the principal ray off the axis can be controlled by the interval D. If the lower limit of conditional expression (3) is exceeded, it will be difficult to ensure telecentricity. On the other hand, when the value exceeds the upper limit of the conditional expression (3), the on-axis peripheral light entering the third lens approaches the optical axis, so that the back focus Bf becomes short, and it becomes difficult to correct aberration of off-axis light. In addition, it is preferable that the stop of the present invention be disposed closer to the second lens than the midpoint of the distance D on the optical axis between the second lens and the third lens. With this configuration, good telecentricity can be ensured, and the off-axis aberration can be satisfactorily corrected in combination with the operation of the rear group after the third lens.

[0009]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an imaging lens according to a first embodiment of the present invention, and FIG. 3 shows an imaging lens according to a second embodiment. The imaging lens of any of the embodiments uses a CCD as an imaging device. The imaging lens of the first embodiment includes, in order from the object side, a first lens L ₁ having a negative refractive power, a second lens L ₂ having a positive refractive power, a stop S, and a third lens having a negative refractive power. L ₃ , a fourth lens L ₅ having a positive refractive power, and a fifth lens L ₅ having a positive refractive power. The first lens L ₁ is formed in a meniscus shape with a convex surface facing the object side. Object-side lens surface of the second lens L ₂ is formed into a shape convex to the object side, it is formed and to have a stronger curvature than the image side lens surface.
The third lens L ₃ are formed in a biconcave shape. Image-side lens surface of the fourth lens L ₄ is formed in the shape of a convex to the image side, it is formed and to have a stronger curvature than an object-side lens surface. Object-side lens surface of the fifth lens L ₅ is formed in a shape convex to the object side, it is formed and to have a stronger curvature than the image side lens surface. Further, the imaging lens of the second embodiment shown in FIG. 3 differs from the first embodiment in that the first lens L ₁ and the second lens L ₂ are joined. In both embodiments, the stop S is more than the middle point of the distance D on the optical axis between the second lens L ₂ and the third lens L ₂ than the middle point.
It is disposed on the lens L ₂ side.

Tables 1 and 2 below show the specifications of the first embodiment and the second embodiment, respectively. In [Main Specifications] of both tables, 2
A represents the angle of view. In [Lens Specifications], No. 1 column
The number of each lens surface from the object side, the second column r is the radius of curvature of each lens surface, the third column d is the distance on the optical axis from each lens surface to the next lens surface, the fourth column n _d The refractive index for the d-line of a lens satisfying from each lens surface to the next lens surface (the air is blank) for the d-line, the fifth column v _d is the Abbe number based on the d-line of each lens, and the sixth column is the number of each lens Represents The values of the parameters in the conditional expressions (1) to (3) are shown in [Values for Conditional Expressions] in both tables.

[0011]

[Table 1] [Main _{specifications] f = 8.3304 F NO = 2.8} 2A = 40.0 ° Bf = 5.9336 α = 2.79 ° [ Lens _{Data] No r d n d ν d} 1 8.4673 1.0000 1.69680 55.61 L 1 2 3.4626 0.9653 3 _{4.0897 1.8149 1.79668 45.42 L 2 4 -16.6458} 0.1000 5 - 1.6702 S 6 -4.9007 1.0000 1.86074 23.00 L 3 7 7.0001 0.7817 8 -25.0416 2.3049 1.69680 55.61 L 4 9 -4.8643 0.1000 10 8.1126 2.3311 1.71300 53.97 L 5 11 -67.1215 [ condition Corresponding value] (1) Bf / f = 0.712 (2) | α | = 2.79 ° (3) D / f = 0.212

[0012]

[Table 2] [Main _{specifications] f = 8.3304 F NO = 2.8} 2A = 40.4 ° Bf = 5.7394 α = 2.78 ° [ Lens _{Data] No r d n d ν d} 1 16.5407 1.0000 1.51680 64.12 L 1 2 3.0480 1.8749 1.79668 _{45.42 L 2 3 -29.1219 0.1000 4 -} 1.2547 S 5 -3.6402 1.0000 1.86074 23.00 L 3 6 9.6443 0.6956 7 -31.0660 2.3414 1.69680 55.61 L 4 8 -4.7219 0.1000 9 9.2693 2.3581 1.71300 53.97 L 5 10 -24.6662 [ values for conditional expressions] (1) Bf / f = 0.689 (2) | α | = 2.78 ° (3) D / f = 0.163

FIG. 2 shows the spherical aberration and astigmatism of the first embodiment.
4 shows distortion and lateral aberration. In each aberration diagram, Y represents the image height. In the astigmatism diagram, a dotted line M represents a meridional image plane, and a solid line S represents a sagittal image plane. Similarly,
FIG. 4 shows various aberrations of the second embodiment. As is clear from the aberration diagrams, the required lens configuration and the conditional expressions (1) to
By satisfying (3), it can be seen that both examples have excellent imaging performance.

[0014]

As described above, according to the present invention, although the number of lenses is small, the F-number reaches 2.8, the angle of view reaches 40 °, and it has a long back focus and good telecentricity. Thus, an imaging lens for a small electronic imaging device having high optical imaging performance could be obtained.

[Brief description of the drawings]

FIG. 1 is an optical path diagram of a first embodiment.

FIG. 2 is an aberration diagram of the first example.

FIG. 3 is an optical path diagram of a second embodiment.

FIG. 4 is an aberration diagram of the second example.

[Explanation of symbols]

L _{1 to} L ₅ … Lens S… Aperture

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H087 KA03 LA01 NA02 PA04 PA05 PA17 PA18 PB05 QA02 QA07 QA17 QA21 QA26 QA34 QA41 QA46 RA32

Claims (5)

[Claims] 1. A first lens having a negative refractive power in order from the object side.
It is composed of a lens, a second lens having a positive refractive power, a stop, a third lens having a negative refractive power, a fourth lens having a positive refractive power, and a fifth lens having a positive refractive power. An imaging lens characterized by satisfying the expression. 0.6 <Bf / f <0.8 (1) | α | <10 ° (2) where Bf: back focus length f: focal length of the entire system α: principal ray reaching the maximum image height Is the angle formed by the optical axis of 2. The first lens is formed in a meniscus shape with a convex surface facing the object side. The object-side lens surface of the second lens is formed in a convex shape on the object side, and the image-side lens surface. The third lens is formed in a biconcave shape, the image-side lens surface of the fourth lens is formed in a convex shape on the image side, and the object-side lens surface That the object-side lens surface of the fifth lens is formed to have a shape convex toward the object side and to have a stronger curvature than the image-side lens surface. The imaging lens according to claim 1, wherein: 3. The imaging lens according to claim 1, wherein the first lens and the second lens are cemented. 4. The imaging lens according to claim 1, wherein the following conditional expression is satisfied. 0.15 <D / f <0.25 (3) where D is the distance between the second lens and the third lens on the optical axis. 5. The apparatus according to claim 1, wherein the stop is disposed closer to the second lens than a midpoint of an interval between the second lens and the third lens on the optical axis. Or the imaging lens according to item 4.