JPH02222911A - Triplet lens with behind aperture - Google Patents

Abstract

PURPOSE:To compensate astigmatism excellently and to secure the quantity of marginal light by composing the triplet lens of a positive meniscus lens which has its convex surface on the object side, a biconcave lens, a biconvex lens, and a stop S, and satisfying specific conditions. CONSTITUTION:The triplet lens consists of a 1st lens L1 which is the positive meniscus lens having the convex surface on the object side, a 2nd lens L2 which is the biconcave lens, a 3rd lens L3 which is the biconvex lens, and the top S in order from the object side and satisfies the conditions shown by inequalities. In the inequalities, (f) is the focal length of the whole system, f1 the focal length of the lens L1, f2 the focal length of the lens L2, f3 the focal length of the lens L3, SIGMAd the distance from the vertex of the object-side surface of the lens L1 to the vertex of the image-side surface of the lens L3, d1 the on-axis thick ness of the lens L1, d2 the on-axis air gap between the lenses L1 and L2, and d4 the on-axis air gap between the lenses L2 and L3. Consequently, the sufficient quantity of marginal light is secured and the triplet lens which has various aberrations, specially, astigmatism compensated excellently is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention has an angle of view of 60 degrees to 65 degrees, and an F nano<- of 3.
This relates to a tribulet-sotolens with a behind aperture of about 5 to 4.

(Prior Art) Since various aberrations can be corrected with a small number of lenses, Tetsusar type and triplet type lenses are often used as photographic lenses for small cameras.

In particular, it is mechanically advantageous to fix the aperture and extend only the lens during focusing, so a lens configuration with a Pino 1 India aperture is often adopted. As an example of this type of triplet lens, for example, Japanese Patent Application Laid-Open No. 60-177 filed by the same applicant as the present invention
No. 313 is publicly known, and many proposals have been made, but all of them have various problems compared to the Tetsusar type and are not yet satisfactory.

(Problems to be Solved by the Invention) The problem is that it is more difficult to correct the Petzval sum in the triplet type than in the Tetsusar type, and the sagittal image surface tends to remain negative. To improve this, it is desirable to increase the power (refractive power) of each lens,
The occurrence of higher-order spherical aberration and astigmatism is unavoidable. Therefore, even if astigmatism at an intermediate angle of view can be corrected, there remains a large astigmatism in which the meridional image plane is curved negatively and the sagittal image plane is curved positively at the maximum angle of view, so it is not possible to correct the astigmatism over a wide angle of view. Imaging performance could not be maintained.

In recent years, proposals have been made to solve this problem by frequently using high-refractive-index glass; however, this type of glass material is generally expensive, and a cost problem still remains.

In addition, not only triplet type lenses but also lenses with behind apertures tend to suffer from insufficient light at the periphery because the aperture is not located near the center of the optical system. In particular, in order to correct various aberrations, the triplet type lens tends to have a larger central thickness and power than the Tetsusar type lens, making it difficult to secure a sufficient amount of peripheral light.

In other words, this is because as the center thickness of each lens increases, the light flux at the periphery becomes eclipsed, and if the power distribution of each lens is inappropriate, the shape of the entrance pupil at the periphery is greatly reduced and deformed. .

Therefore, the present invention solves all of the above-mentioned problems, and provides a triplet lens that maintains a compact shape, secures sufficient amount of peripheral light, and has a behind aperture that satisfactorily corrects various aberrations, especially astigmatism. The purpose is to provide it at a low cost.

(Means for Solving the Problems) In order to solve these problems, the present invention, as shown in FIG. L1 and the second biconcave lens
This triplet lens has a behind aperture consisting of a lens L, a biconvex 3rd degree lens L, and an aperture S, and further satisfies the following conditions.

(1) 0.9f<fl<1.2f(2) 0
．． 3f<-f2<0.4f(3) 0.3f<f
3<0.4f(4)0.24f<Σd<0.29f
(5) 0.07f<di<01If(8)
1.8<d2/d4<3 However, f Focal length of the entire system fl: Focal length of the first lens L1 f2: Focal length of the second lens L f3; Focal length of the third lens Ls Σd: First The distance along the optical axis from the apex of the object side of lens L to the apex of the image side of third lens L3.

dl: On-axis thickness of the second lens LI d2: On-axis air distance between the first lens and the second lens L.

d4; Axial air distance between the second lens Lt and the third lens L.

(Function) Generally, since a triplet lens has a small number of lenses, there is little freedom in correcting various aberrations. Therefore, it is extremely difficult to correct various aberrations unless the total thickness of the lens and the power (refractive power) arrangement are optimized.

Therefore, as a result of repeated studies while changing the positive power distribution between the first lens L1 and the third lens L, the following was discovered.

(a) The larger the power distribution to the third lens L and the smaller the power distribution to the second lens L, the smaller the curvature of the image plane due to the angle of view and the smaller the astigmatism difference.

(b) The larger the absolute value of the power of each lens, the more the curvature of the image plane due to the angle of view increases.

(C) The smaller the power of the first lens L1, the less deformation of the entrance pupil shape.

As a result, in the present invention, as shown above, conditional expressions (1) to (3) define appropriate power distribution for each lens, and achieve both good correction of astigmatism and securing of peripheral light amount. We have achieved this.

First, according to conditional expression (1), the power of the second lens Lt is weakened as much as possible to suppress deformation of the entrance pupil shape, thereby ensuring a sufficient amount of peripheral light. However, if this lower limit is exceeded, the refractive power of the first lens L1 becomes excessive, and the shape of the entrance pupil is reduced and deformed, resulting in insufficient peripheral light quantity, which is not preferable.

On the other hand, when the upper limit is exceeded, the refractive power of the first lens L decreases, and the height of incidence of the land ray (ray from infinity parallel to the optical axis) increases, resulting in a higher-order spherical surface. Significant aberrations occur.

Furthermore, conditional expressions (2) and (3) are for correcting the Petzval sum and the curvature of the image plane in a well-balanced manner. If the lower limit of conditional expressions (2) and (3) is exceeded, the Petzval sum can be made smaller, and - Although it seems like a correction for field curvature, higher-order field curvature On the contrary, the curvature of field increases. Conversely, conditional expressions (2) and (3
) If the upper limit is exceeded, the Petzval sum becomes excessive and the sagittal image plane remains negative. In addition, it is more desirable that the upper limit in condition (2) is 0.37 f and the lower limit is 0.32 f, and furthermore, the lower limit in condition (3) is 0.33 f.
F is desirable.

By the way, it is desirable for the total thickness of the lens to be large to some extent in terms of aberration correction, but if you try to ensure the amount of peripheral light at the same time, the effective diameter of the first lens L1 becomes too large, which goes against the compactness that is the objective of the present invention. become. That is, ensuring good aberration correction and sufficient amount of peripheral light are both contradictory to making the lens system more compact.

Therefore, in the present invention, in order to ensure the amount of peripheral light while achieving good aberration correction and to achieve compactness, the optimum overall length of the lens system is determined by conditional expression (4), and conditional expression (5) is determined. The optimum center thickness of the first lens L is defined as follows. If the upper limits of conditional expressions (4) and (5) are exceeded, it will be difficult to make it compact; if the lower limits are exceeded, it will not only be difficult to correct comatic aberration and spherical aberration, but also It becomes difficult to ensure brightness.

Furthermore, conditional expression (6) is intended to satisfactorily correct distortion by defining an appropriate ratio of each air gap formed between each lens. Exceeding the upper and lower limits of this condition is undesirable because significant distortion occurs.

Furthermore, in order to simultaneously and reliably achieve good image plane correction and cost reduction, it is desirable to satisfy the following conditions.

(7) 1.80<nl<l, 78(8)
1.59<n2<1.89 (9) 1.75<
n3<1.85 where n L n 2 and n3 are respectively the second lens Ll and the second lens L! , the refractive index of the third lens L.

In correcting the Petzval sum, it is desirable to make the refractive index of the positive lens as high as possible; however, glass materials with high refractive indexes are expensive, and their use poses the problem of increased costs.

Therefore, in the present invention, the refractive index of the third lens L, which has a large share of positive refractive power, is increased, and the second lens L, which has a negative refractive power, has a high refractive index.
By lowering the refractive index of the lens L, the refractive index of the first lens L1 is suppressed to a relatively small value, thereby reducing the overall cost. Note that it is desirable that the refractive index of the first lens L1 be smaller than that of the third lens L3.

However, if the lower limits of conditions (7) and (9) are exceeded, it is difficult to correct the Petzval sum, and conversely, if the upper limits of conditions (7) and (9) are exceeded, expensive glass materials must be used, which increases the cost. Invite up.

Furthermore, when the lower limit of conditional expression (8) is exceeded, higher-order spherical aberration becomes significant, and conversely, when the upper limit is exceeded, it becomes difficult to correct the Petzval sum.

Furthermore, in order to achieve sufficient aberration correction in the present invention,
It is desirable to satisfy the following conditions.

(10) 17<shil-shi2ku25(11) 4
9<ν1 (12) 2<C1-C3ku14(13)0.2
8f<rl<0.33f(14)O15f<
-r3<0.8f(15)0. 9<-r5/r6<1
．． 1 However, νl: Atsube number of the first lens L.

C2: 2nd lens L! Atsbe number.

ν3: Atsube number of the third lens L3.

rl: radius of curvature of the object side of the first lens L.

r3: radius of curvature of the object side of the second lens L.

r5 The radius of curvature of the object side of the second third lens L3.

r6: radius of curvature of the image side surface of the third lens L.

f: Focal length of the entire system.

It is.

Conditional expression (10) relates to correction of axial chromatic aberration. When the upper limit of this conditional expression is exceeded, the dispersion of the second lens L1 becomes relatively small and the axial chromatic aberration becomes overcorrected. On the other hand, when the lower limit is exceeded, the dispersion of the first lens L becomes relatively large, resulting in insufficient correction of axial chromatic aberration.

Furthermore, conditional expressions (11) and (12) relate to correction of off-axis chromatic aberration.

Generally, it is difficult to correct off-axis aberrations in a lens having a behind aperture, and chromatic aberration is no exception. Therefore, the bending of chromatic aberration of magnification depending on the angle of view becomes more pronounced as the angle of view becomes larger, making it difficult to widen the angle of view.

This is because the contribution of the second lens L to oblique light rays increases rapidly compared to the third lens L when the angle of view is increased. Therefore, the second lens L and the third lens L are determined by conditional expression (6) as described above.

In the case of the present invention, which reduces the distance between the two lenses and suppresses the difference in contribution between the second lens L and the third lens L3 to oblique rays, it is possible to suppress the curve of chromatic aberration of magnification to a small level. However, since the first lens L exceeds the range of conditional expression (11),
It is not preferable to configure the lens with high-dispersion glass because it will fundamentally increase chromatic aberration of magnification.

In addition, the aberration structure of triplet lenses (including chromatic aberration)
is over-corrected by the first lens L and the second lens L, and the third lens I5. It has an aberration structure that cancels out the excess. Therefore, in the case of a lens with a behind aperture, if the third lens L is made of glass with a lower dispersion than the first lens, the chromatic aberration for oblique rays will be overcorrected compared to on-axis, and the so-called Chromatic coma aberration occurs and off-axis performance deteriorates. Therefore, the lower limit of conditional expression (12) is set as the limit value that can correct the coma aberration of this color in a well-balanced manner. On the other hand, if the third lens L is made of an excessively high dispersion glass compared to the second lens Ll, the chromatic aberration for oblique rays will be insufficiently corrected compared to the axial one, which will also occur due to the third lens L. Since the chromatic coma aberration increases, it becomes difficult to correct the chromatic coma aberration. Therefore, the upper limit of conditional expression (12) is set as the limit value that can correct the coma aberration of this color in a well-balanced manner.

Conditional expression (13) relates to correction of spherical aberration. If this upper limit is exceeded, spherical aberration will be over-corrected, making it impossible to maintain balance with the sagittal image plane; conversely, if the lower limit is exceeded,
Spherical aberration is undercorrected.

Conditional expression (14) relates to correction of astigmatism. If this upper limit is exceeded, the meridional image surface is strongly curved in the negative direction, and conversely, if the lower limit is exceeded, the meridional image surface is strongly curved in the positive direction, making it difficult to correct astigmatism.

Conditional expression (15) relates to correction of coma aberration. If this upper limit is exceeded, an extremely large amount of extroverted coma occurs, and if the lower limit is exceeded, an introverted coma occurs, resulting in deterioration of the off-axis performance.

In addition, in the present invention, it is preferable that the value of conditional expression (15) is approximately 1. Specifically, when the radii of curvature of the object side surface and the image side surface of the third lens L are r5 and r6, respectively. , (1θ) r5w−r6 is preferably substantially satisfied.

If the radius of curvature of both surfaces of the third lens L3 is configured to be - as shown in conditional expression (16), both surfaces of the third lens L can be processed in the same process, and furthermore, the front and back sides of the lens can be checked during assembly. This has great advantages in the manufacturing process, such as eliminating the need for additional steps, and has a significant effect on cost reduction.

Furthermore, when the present invention is configured to satisfy the above conditional expression (1B), the third lens L
It is desirable that the axial thickness d5 of , satisfy the following conditions.

(17) 0.07f<d5<O, if conditional expression (17
) defines the optimal axial thickness of the third lens L. If the upper and lower limits of conditional expression (17) are exceeded, it becomes difficult to correct coma aberration.

Furthermore, in conditional expressions (4) and (5) as described above, the total thickness of the lens system and the second lens L! Since the axial thickness d1 of the third lens L is defined as the axial thickness d1 of the third lens L, if the axial thickness d5 of the third lens L is made slightly thicker so as to satisfy conditional expression (17), the first lens L and the second lens L! The air distance d2 between the second lens L and the third lens L can be simultaneously narrowed.

Therefore, the spacing ring disposed between the second lens and the third lens, which was conventionally necessary, can be eliminated, and it becomes possible to adopt a configuration in which all the lenses are brought into contact with each other. This makes it possible to reliably improve manufacturing accuracy and significantly reduce costs.

(Example) In Tables 1 to 4 below, the values of specifications of the first to fourth examples of the present invention are summarized. In the table below, [ is the focal length of the entire system, B
f is the back focus, FN is the F number, 2ω is the angle of view, r is the radius of curvature, d is the surface spacing, ν is the Abbe number, and n is the refractive index at the d-line (λw 587.6 nm).

2ω-63,04°-56,536 2,9236,3 1,82004 -55,099(B f ) f =lOO,O,B f =87.60. F N
=4002ω=62.90' f-100,0, B f =84.31, FN=3.8
52ω-65,40°31.747 48.752 -72.558 35.534 80.205 -80.205 9.69 4.41 2.94 1.75 9.10 (Bf) ν 49.7 32. 2 46.5 1.77250 1.67270 1.80420 -54.968 (B f) Table 5 below lists numerical values corresponding to conditions of the embodiments of the present invention.

1st Example fl/f 1.075 f2/f -0,335 f3/f O,353 Σd/f O,253 dl/f Q,086 d2/d4 2.750 nl 1670 n2 1620 n 3 1. 804 S1-S2 21.3 ν157.6 SIL-ν3 11.2 r 1 / f O, 288 Second Example 1, 004 1, 623 64B 1.797 23.2 57.0 11, B O , 278 d5/f 0, 075 0, 072 Figures 2, 4, 6, and 8 are in order from 1 to 8.
This is an aberration diagram of the fourth embodiment, and in the astigmatism diagram, a broken line indicates a meridional image plane, and a solid line indicates a sagittal image plane.

Comparison of each aberration diagram clearly shows that various aberrations have been corrected extremely well and that the lens has excellent imaging performance. In particular, it can be seen that astigmatism at the periphery is kept extremely small and corrected with good balance, and chromatic coma aberration is also corrected with good balance.

(Effects of the Invention) As described above, according to the present invention, the behind diaphragm is compact, yet secures a sufficient amount of peripheral light, and is capable of good correction of various aberrations, especially astigmatism at the maximum angle of view and chromatic aberration for oblique rays. It is possible to achieve a triplet lens with a low cost.

In addition, since the triplet lens according to the present invention has a relatively thin total lens thickness, when a rear converter lens is attached to the image side, the distance between principal points can be made small, so compared to the power of the rear converter lens, Higher magnification can be obtained. Therefore, the lens according to the present invention is also suitable as a so-called master lens for a multifocal camera.

[Brief explanation of the drawing]

Figures 1, 3, 5, and 7 are shown in order from 1 to 7, respectively.
It is a lens cross-sectional configuration diagram of the fourth embodiment, and FIGS. 2, 4, 6, and 8 are aberration diagrams of the first to fourth embodiments, respectively. (Description of main parts) Ll...--First lens Lr--Second lens Lr-m-Third lens S-----Aperture

Claims (1)

[Claims] In order from the object side, the first lens L_1 is a positive meniscus lens with a convex surface facing the object side, and the second lens L_1 is a biconcave lens.
A triplet lens having a behind aperture, which is composed of a lens L_2, a third lens L_3 which is a biconvex lens, and an aperture S, and satisfies the following conditions. (1) 0.9f<f1<1.2f (2) 0.3f<-f2<0.4f (3) 0.3f<f3<0.4f (4) 0.24f<Σd<0.29f ( 5) 0.07f<d1<0.1f (6) 1.8<d2/d4<3 where, f: focal length of the entire system. f1: Focal length of the first lens L_1. f2: Focal length of the second lens L_2. f3: Focal length of the third lens L_3. Σd: Distance along the optical axis from the apex of the object side of the first lens L_1 to the apex of the image side of the third lens L_3. d1: On-axis thickness of the first lens L_1. d2: On-axis air distance between the first lens L_1 and the second lens L_2. d4: On-axis air distance between the second lens L_2 and the third lens L_3.