JPH0843732A - Wide converter lens - Google Patents

Abstract

PURPOSE:To obtain a wide converter lens attached in front of a photographic lens and displacing the focal distance of an entire system to shorter focal distance. CONSTITUTION:This wide converter lens C attached in front of the photographic lens M and displacing the focal destance of the entire system to the shorter focal distance has three lenses, namely a meniscus negative first lens C1 whose convex face faces an object side, a negative second lens C2 whose strongly negative refractive surface faces an image surface side, and a positive third lens C3 which has stronger positive refractive surface on the image surface side in comparison with the object side and whose both lens surfaces are convex face in this order from the object side. The refractive index of the matelial of the lens C3 is made smaller than the refractive indexes of the materials of the lenses C1 and C2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide converter lens which is mounted in front of a photographing lens and displaces the focal length of the entire photographing system to the shorter side, and is particularly used for a photographic camera, a video camera, an electronic camera and the like. The present invention relates to a wide converter lens which is suitable for mounting on a photographing lens such as a zoom lens or a single focal length lens and which is suitable for downsizing and weight reduction of the entire lens system and has high optical performance.

[0002]

2. Description of the Related Art Conventionally, various front wide converter lenses have been proposed which are mounted in front of a taking lens and change the focal length of the entire taking system to a shorter one while keeping the focal plane of the entire system at a fixed position. Has been done.

In many cases, this wide converter lens has two lens groups, a front lens group having a negative refractive power and a rear lens group having a positive refractive power, which are arranged at a principal point distance by a sum of focal lengths of both lens groups. , As a whole, constitutes an afocal system.

Therefore, the simplest lens system can be composed of two lenses having negative and positive refracting powers, as proposed in Japanese Patent Laid-Open No. 4-70616. However, in order to obtain a predetermined optical performance when the lens unit is mounted on a taking lens, it is necessary to configure each lens group with a plurality of lenses in terms of aberration correction.

For example, in Japanese Unexamined Patent Publication No. 59-204817, an afocal system is composed of five lenses as a whole, which is composed of a first group having a negative refractive power and a second group having a positive refractive power in order from the object side. We have proposed a wide converter lens with a focal magnification of about 0.7.

[0006]

Generally, the front type wide converter lens mounted in front of the taking lens has an advantage that the F number and the focal plane of the entire system can be kept unchanged even if the wide converter lens is mounted. Have

However, since the front wide converter lens is mounted in front of the taking lens, the entire lens system tends to be large and heavy. In particular, if each lens group is composed of multiple lenses when it is desired to obtain a predetermined optical performance when it is attached to a photographing lens system, or when the refractive power of each lens group is increased for downsizing, the lens closest to the object side Since the position of is far from the entrance pupil of the taking lens, it is inevitable that the lens system becomes large.

On the other hand, in order to downsize the wide converter lens by reducing the number of constituent lens systems, among the various aberrations, reduction of distortion and field curvature remains a problem until the end. Distortion aberration is reduced by dispersing the refracting power by configuring the front group of negative refracting power with two concave lenses, as seen in JP-A-4-116511 and JP-A-4-260609. I am trying to However, in the above publication, the afocal magnification of the converter is changed from 0.65 times to 0.8 times, and it is disclosed about reduction of distortion and field curvature when the afocal magnification is further reduced while keeping downsizing. Not not.

According to the present invention, by appropriately setting the lens configuration, the lens is mounted on the object side of the taking lens and is composed of three lenses having a predetermined shape as a whole which can be displaced to the focal length of the entire system and to the shorter one. The purpose of the invention is to provide a wide converter lens that has a small afocal magnification and that effectively corrects various aberrations of the entire system when mounted on the object side of the taking lens, especially distortion and field curvature. To do.

[0010]

The wide converter lens of the present invention is mounted on the front side of a photographing lens and displaces the focal length of the entire system to the shorter side. The wide converter lens is arranged in order from the object side. A negative meniscus first lens whose convex surface faces the object side,
A negative second lens having a strong negative refracting surface facing the image side, and a third lens having a positive positive refracting surface on the image side as compared to the object side. It is characterized in that the refractive index of the material of the third lens is smaller than the refractive index of the material of the first and second lenses.

[0011]

EXAMPLE 1 FIG. 1 is a lens sectional view of Example 1 in which the wide converter lens of the present invention is mounted in front of a taking lens (master lens). 2 to 4 are lens cross-sectional views of Numerical Examples 1 to 3 of the wide converter lens of the present invention, and FIG. 5 is a lens cross-sectional view of the taking lens of FIG. In the figure, C is a wide converter lens, which is composed of an afocal system as a whole.

The wide converter lens C of this embodiment is a negative meniscus first lens C having a convex surface facing the object side.
1. Negative second lens C with a strong negative refracting surface facing the image side
2. The third lens C3 has a positive lens having a positive refractive surface facing the image side more strongly than the object side, and having a convex positive surface. The first lens C1 and the second lens C2 constitute a front group CA having a negative refractive power, and the third lens C3 constitutes a rear group CB having a positive refractive power.

Reference numeral M denotes a photographing lens (master lens), which is composed of a lens having a single focal length or a zoom lens. An image surface 4 is at a fixed position regardless of whether the wide converter lens C is attached or not. 5 is an optical axis.

In general, since the photographing lens itself performs photographing, the aberration of the photographing lens is favorably corrected by itself. Therefore, in order to obtain a good optical performance as a whole when the wide converter lens is attached, it is necessary to achieve good aberration correction by the wide converter lens alone.

In the present embodiment, the combined focal length of the front group CA is f1, the focal length of the rear group CB is f2, the front group CA and the rear group C.
When the principal point distance from B is e ′, e ′ = f1 + f2 (1) because the entire system is an afocal system. The afocal magnification α of the wide converter lens C is α = | f1 / f2 | (2), and generally | f1 | <f2 and α <1.

As a result, when the wide converter lens C is attached to the taking lens M having the focal length F, the focal length Fa of the entire system is displaced to the shorter one as Fa = α · F.

In order to widen the angle of view of the entire system when the afocal magnification α is reduced and the lens is attached to the taking lens,
From the equation (2), it is necessary to increase the absolute value of the focal length f2 of the rear lens group CB. Then, from the formula (1), the principal point distance e ′ between the front group and the rear group becomes long, and the entire lens system becomes large.

Therefore, in order to reduce the principal point interval e ', the absolute values of the focal lengths f1 and f2 of the front lens group and the rear lens group are (1),
It is necessary to reduce the size while satisfying the expression (2). That is, it is necessary to strengthen the refractive powers of the front group and the rear group as a whole. Generally, when the refracting power of the lens group is strengthened, a large amount of aberration occurs, and it becomes difficult to correct aberrations satisfactorily. In particular, when the afocal magnification α is reduced to achieve a wide angle of view, a lot of Tal-type distortion aberration peculiar to a wide-angle lens occurs.

Therefore, in this embodiment, even if the refractive powers of the front lens group CA and the rear lens group CB are increased and the principal point spacing e'is set small, the lens shapes of the three lenses forming the wide converter lens C are as described above. By configuring, the taking lens M
When the lens is mounted on, the focal length of the entire system is displaced to a shorter side by a predetermined amount, and on-axis aberrations such as spherical aberration and off-axis aberrations are well corrected.

As shown in FIG. 1, the distortion aberration mainly occurs in the negative front group CA when considering the path of the light rays.

Therefore, in this embodiment, the front group CA is composed of two negative lenses C1 and C2 to disperse the negative refracting power, thereby reducing the occurrence of distortion. Further, the Petzval sum is reduced by making the refractive index of the material of the positive third lens C3 lower than that of the material of the negative first and second lenses, and this also causes a problem when the angle of view is widened. The effective field curvature is effectively reduced. In addition, the positive third lens C3 is configured to have a convex lens shape with both positive and negative refracting surfaces facing the image side as compared to the object side, so that various aberrations can be satisfactorily corrected and as described above. In order to reduce the Petzval sum, a material whose refractive index is smaller than that of the first and second lenses is used as the third lens C3.

In particular, in this embodiment, the refractive power of the image-side lens surface of the third lens C3 is strengthened, and the convex surface is directed toward the diaphragm of the master lens M, so that pincushion distortion aberration is caused. Front group CA with strong negative refractive power generated
The Tal-type distortion that occurs in 1 is corrected well.

In the present invention, when the radiuses of curvature of the object-side and image-side lens surfaces of the third lens C3 are R5 and R6, respectively, -0.6 <(R6 + R5) / (R6-R5) <0. It is good to satisfy the condition (1).

Conditional expression (1) is for properly setting the lens shape of the third lens C3 and mainly for favorably correcting distortion. When the lens shape of the third lens C3 approaches the plano-convex lens beyond the lower limit of conditional expression (1), the third lens C3 is provided in order to obtain the positive refractive power required for the rear group CB with a material having a relatively low refractive index. The curvature of the image side lens surface of C3 becomes too tight, and off-axis aberrations, particularly pincushion type distortion and astigmatism, occur in large numbers, which is not preferable.

On the contrary, if the curvature of the lens surface on the image side becomes too loose beyond the upper limit of the conditional expression (1), pincushion distortion aberration generated in the rear lens group CB decreases and the tal lens generated in the front lens group CA. Type distortion aberration cannot be canceled, and a large amount of tal type distortion aberration is generated as a whole, which is not good.

In the present invention, it is preferable to set the upper limit of conditional expression (1) to about "-0.17" because it is possible to satisfactorily correct various aberrations.

Next, numerical examples of the wide converter lens of the present invention and the photographing lens (master lens) to which the wide converter lens is attached will be shown. In the numerical example, Ri is the i-th radius of curvature in order from the object side, Di is the i-th lens thickness and air gap in order from the object side, and Ni and νi are the glass of the i-th lens in order from the object side, respectively. The refractive index and the Abbe number. In the master lens, R14 and R15 are glass materials such as a face plate.

* Wide Converter Lens <Numerical Example 1> f = 0.56 FNo = 1: 2.5 2ω = 78.5 ° R 1 = 2.179 D 1 = 0.171 N 1 = 1.83400 ν 1 = 37.2 R 2 = 1.253 D 2 = 0.764 R 3 = -9.846 D 3 = 0.128 N 2 = 1.77250 ν 2 = 49.6 R 4 = 2.234 D 4 = 0.547 R 5 = 3.803 D 5 = 0.663 N 3 = 1.70154 ν 3 = 41.2 R 6 = -2.694 α = 0.56 f1 = −9.00 f2 = 16.07 e ′ = 7.07 <Numerical Example 2> f = 0.56 FNo = 1: 2.5 2ω = 78.5 ° R 1 = 2.440 D 1 = 0.171 N 1 = 1.69680 ν 1 = 55.5 R 2 = 1.203 D 2 = 0.719 R 3 = 125.562 D 3 = 0.114 N 2 = 1.77250 ν 2 = 49.6 R 4 = 1.804 D 4 = 0.455 R 5 = 2.398 D 5 = 0.857 N 3 = 1.53172 ν 3 = 48.9 R 6 = -2.178 α = 0.56 f1 = -9.00 f2 = 16.07 e ′ = 7.07 <Numerical Example 3> f = 0.49 FNo = 1: 2.5 2ω = 86.0 ° R 1 = 2.172 D 1 = 0.171 N 1 = 1.69680 ν 1 = 55.5 R 2 = 1.241 D 2 = 0.885 R 3 = 20.256 D 3 = 0.114 N 2 = 1.77 250 ν 2 = 49.6 R 4 = 1.739 D 4 = 0.792 R 5 = 6.312 D 5 = 0.857 N 3 = 1.53172 ν 3 = 48.9 R 6 = -1.958 α = 0.49 f1 = -10.00 f2 = 20.41 e '= 10.41 * Shooting lens <Numerical Example 1> f = 1 FNo = 1: 2.5 2ω = 49.1 ° R 1 = 2.253 D 1 = 0.285 N 1 = 1.58144 ν 1 = 40.8 R 2 = -33.536 D 2 = 0.028 R 3 = 3.295 D 3 = 0.100 N 2 = 1.77250 ν 2 = 49.6 R 4 = 0.626 D 4 = 1.142 R 5 = -3.369 D 5 = 0.714 N 3 = 1.60342 ν 3 = 38.0 R 6 = -1.120 D 6 = 0.214 R 7 = (Aperture) D 7 = 0.357 R 8 = 0.936 D 8 = 0.285 N 4 = 1.72000 ν 4 = 50.3 R 9 = -39.450 D 9 = 0.021 R10 =- 2.917 D10 = 0.285 N 5 = 1.80518 ν 5 = 25.4 R11 = 0.842 D11 = 0.185 R12 = 2.154 D12 = 0.314 N 6 = 1.69680 ν 6 = 55.5 R13 = -1.513 D13 = 0.342 R14 = ∞ D14 = 0.657 N 7 = 1.51633 ν 7 = 64.2 R15 = ∞ Table 1 shows the numerical values corresponding to the numerical conditional expressions of each numerical example.

[0029]

[Table 1]

[0030]

As described above, according to the present invention, when the lens is mounted in front of the photographing lens and the focal length of the entire photographing system is displaced to the shorter side, it is composed of three lenses satisfying the above-mentioned conditions. As a result, the afocal magnification was widened to about 0.5 times, while the lens system was downsized and the optical performance of the entire system when mounted on the taking lens system, particularly distortion and field curvature, were favorably maintained. Wide converter lens can be achieved.

[Brief description of drawings]

FIG. 1 is a lens sectional view when a wide converter lens of the present invention is mounted in front of a taking lens.

FIG. 2 is a lens cross-sectional view of Numerical Example 1 of a wide converter lens according to the present invention.

FIG. 3 is a lens cross-sectional view of a numerical example 2 of the wide converter lens of the present invention.

FIG. 4 is a lens sectional view of a numerical example 3 of the wide converter lens of the present invention.

5 is a lens cross-sectional view of the taking lens of FIG.

FIG. 6 is a diagram of various aberrations when the wide converter lens according to Numerical Example 1 of the present invention is mounted in front of the taking lens.

FIG. 7 is a diagram showing various aberrations when the wide converter lens of Numerical Example 2 of the present invention is mounted in front of the taking lens.

FIG. 8 is a diagram showing various aberrations when the wide converter lens of Numerical Example 3 of the present invention is mounted in front of the taking lens.

9 is a diagram of various aberrations of the taking lens of FIG.

[Explanation of symbols]

 C Wide converter lens CA Front group CB Rear group M Photographic lens d d line g g line ΔM meridional image surface ΔS sagittal image surface

Claims (2)

[Claims] 1. A wide converter lens which is mounted in front of a taking lens and which displaces the focal length of the entire system to the shorter side, wherein the wide converter lens is a meniscus negative lens whose convex surface faces in order from the object side to the object side. First lens,
A negative second lens having a strong negative refracting surface facing the image side, and a third lens having a positive positive refracting surface on the image side as compared to the object side. A wide converter lens having the refractive index of the material of the third lens is smaller than that of the material of the first and second lenses. 2. When the radius of curvature of the i-th lens surface from the object side is Ri, the condition of −0.6 <(R6 + R5) / (R6-R5) <0 is satisfied. 1 wide converter lens.