JPH0843731A - Wide converter lens - Google Patents

Abstract

(57) [Summary] [Purpose] To obtain a wide converter lens that is mounted in front of the taking lens and displaces the focal length of the entire system to the shorter side. A wide converter lens that is mounted in front of a taking lens and that displaces the focal length of the entire system toward the shorter side, wherein the wide converter lens is a meniscus-shaped positive first lens whose convex surface faces the object side in order from the object side. A lens, a negative meniscus-shaped second lens having a convex surface directed toward the object side, a negative meniscus-shaped third lens having a convex surface directed toward the object side, and a positive fourth lens; The image-side lens surface of the lens and the object-side lens surface of the second lens form an air lens having a positive refractive power.

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 having high optical performance suitable for mounting on a photographing lens such as a zoom lens or a single focal length lens.

[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 is, for example, Japanese Patent Laid-Open No. 59-2.
It can be composed of two lenses having negative and positive refracting powers as proposed in Japanese Patent No. 04817. 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 Patent Laid-Open No. 59-204817, an afocal system is constructed by five lenses as a whole, which is composed of a first group having negative refractive power and a second group having positive refractive power in order from the object side. We have proposed a wide converter lens with a focal magnification of about 0.7.

[0005]

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, when both the front lens group and the rear lens group are composed of a plurality of lenses in order to satisfactorily correct various aberrations and maintain good optical performance when mounted on a taking lens, the lens closest to the object side is Since the lens is located farther from the entrance pupil of the taking lens, as a result, the diameter of the front lens increases, and the size of the entire lens system increases.

On the other hand, when the number of lenses in the entire lens system is reduced and the size is reduced, correction of chromatic aberration of magnification and distortion of various aberrations becomes a problem. Generally, if the front group of negative refracting power is composed of an achromat doublet and a negative lens, it becomes easy to correct chromatic aberration of magnification, but it becomes difficult to correct distortion when the afocal magnification is reduced. Will occur.

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 four lenses having a predetermined shape as a whole, which can be displaced to the shorter side of the focal length of the entire system. Afocal magnification 0.47 ~
It has about 0.56 times, and it can satisfactorily correct various aberrations when mounted, especially distortion, and maintain good optical performance of the entire system. Moreover, the total lens length is short and the front lens It aims to provide wide converter lenses with a small diameter.

[0009]

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 positive meniscus first lens with a convex surface facing the object side,
A negative second meniscus lens having a convex surface directed toward the object side,
It has four lenses, a meniscus-shaped negative third lens having a convex surface facing the object side and a positive fourth lens, and an image-side lens surface of the first lens and an object-side lens of the second lens. It is characterized by forming an air lens having a positive refractive power with the surface.

[0010]

EXAMPLE 1 FIG. 1 is a lens sectional view of Example 1 when the wide converter lens of the present invention is mounted in front of a taking lens. 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 has a positive meniscus first lens C1 having a convex surface facing the object side in order from the object side, and a negative meniscus second lens C2 having a convex surface facing the object side. A negative meniscus third lens C3 having a convex surface directed toward the object side, and a positive fourth lens C4
Is made up of. The first lens C1 to the third lens C3 constitute a front group CA having a negative refractive power, and the fourth lens C4 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. IP is the image plane and wide converter lens C
It is in a fixed position regardless of whether or not it is attached. LS is the 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 this embodiment, the combined focal length of the front group CA is f1 (f1 <0), and the focal length of the rear group CB is f2 (f2).
> 0), and when the principal point distance between the front group CA and the rear group CB is e ', e' = f1 + f2 (a) because the entire system is an afocal system. The afocal magnification α of the wide converter lens C is α = | f1 / f2 | (b), 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 F'of the entire system is displaced to the shorter one as F '= αF.

Here, in order to reduce the afocal magnification α, that is, in order to widen the angle of view, it is necessary to make the absolute value of the focal length f2 very large from the equation (b). Then, the principal point interval e'becomes large according to the equation (a), which leads to an increase in size of the lens system. Therefore, in order to reduce the principal point distance e ′, it is necessary to reduce the absolute values of the focal lengths f1 and f2 while satisfying the expressions (a) and (b). That is, the refracting powers of the front group CA and the rear group CB must be strengthened as a whole.

Generally, when the refracting power of the lens unit is strengthened, large aberrations occur, and it becomes difficult to correct aberrations satisfactorily. In particular, when the afocal magnification α is reduced to achieve a wide angle of view, large Tal-type distortion aberration peculiar to a wide-angle lens occurs.

Therefore, in this embodiment, even if the refractive powers of the front group CA and the rear group CB are both strengthened and the principal point spacing e'is set small, the lens shapes of the four lenses forming the wide converter lens C are described above. With such a configuration, when mounted on the taking lens M, the focal length of the entire system can be displaced to a shorter side by a predetermined amount, and on-axis aberrations such as spherical aberration and off-axis aberrations, especially distortion aberration, can be corrected well. I am able to do it. Distortion aberration is considered from the way of the off-axis chief ray,
It occurs mainly in the front group CA having a negative refractive power.

Therefore, in the present invention, the front group CA is composed of three lenses having the above-mentioned shapes. As a result, the Tal type distortion produced by the negative second and third lenses and the positive first
The pincushion distortion aberrations that occur in the lenses cancel each other out in a well-balanced manner, reducing the distortion aberrations as a whole. In particular, by disposing the positive first lens on the most object side where the amount of off-axis rays from the optical axis is large, it is possible to effectively cancel the Tal-type distortion aberration of the front group CA, which has a negative refracting power as a whole. There is. The front lens group CA is a combination of the first lens element C1 and the second lens element C2, which is a so-called distortion canceling doublet, and the third lens element C3, which is negative, is provided so that the front lens group CA has a predetermined negative refracting power as a whole. It consists of two lenses.

Further, an air lens is formed by the image side surface of the first lens C1 and the object side surface of the second lens C2, and the refractive power thereof is made positive. Then, since the positive lens is arranged in a place where the amount of off-axis rays separated from the optical axis is large, the effect of reducing the distortion aberration together with the first lens C1 is obtained.

The wide converter lens which is the object of the present invention has been achieved by adopting the above-mentioned lens structure. To further correct various aberrations and obtain high optical performance, at least one of the following conditions is required. It is good to satisfy one.

(1-1) The radius of curvature of the ith lens surface from the object side is Ri, the refractive index of the material of the ith lens is Ni,
When the combined focal length from the first lens to the third lens is f1

[0023]

[Equation 2] To satisfy the condition.

Conditional expression (1) relates to the lens configuration of the front group CA, and particularly relates to the refractive power of the air lens formed by the image side lens surface of the first lens C1 and the object side lens surface of the second lens C2. This is mainly for favorably correcting distortion. When the lower limit of conditional expression (1) is exceeded and the refracting power of the air lens approaches 0 or becomes negative, a large amount of Tal-type distortion occurs in the air lens, and the positive first lens arranged closest to the object side. The effect of one lens is canceled. Conversely, if the positive refracting power of the air lens becomes too strong beyond the upper limit of conditional expression (1), it is good in terms of correction of distortion aberration, but refraction of off-axis light by the air lens becomes large and coma aberration becomes large. Is often generated, which is not preferable.

(1-2) When the refractive index and the Abbe number of the material of the i-th lens from the object side are Ni and νi, respectively, 1.7 <N2 (2) ν1 <35 (3) 45 <ν 2 (4) is to be satisfied.

The conditional expression (2) relates to the refractive index of the material of the second lens C2, mainly for properly setting the Petzval sum and correcting the field curvature well. If the condition (2) is not satisfied, the field curvature will be undercorrected, which is not preferable.

Conditional expressions (3) and (4) relate to dispersion of the materials of the first lens C1 and the second lens C2, and are mainly for favorably correcting chromatic aberration. Outside the range of conditional expressions (3) and (4), lateral chromatic aberration increases, which is not good.

Next, numerical examples of the wide converter lens of the present invention and the photographic 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. The last two lens surfaces of the taking lens are glass blocks such as face plates and filters. Table 1 shows the relationship between the above-mentioned conditional expressions and various numerical values in the numerical examples.

* Wide Converter Lens <Numerical Example 1> f = 0.50 FNo = 1: 2.5 2ω = 84.8 ° R 1 = 10.884 D 1 = 0.428 N 1 = 1.76182 ν 1 = 26.5 R 2 = 25.036 D 2 = 0.021 R 3 = 4.674 D 3 = 0.214 N 2 = 1.77250 ν 2 = 49.6 R 4 = 1.416 D 4 = 0.714 R 5 = 7.274 D 5 = 0.128 N 3 = 1.77250 ν 3 = 49.6 R 6 = 1.832 D 6 = 0.919 R 7 = 3.486 D 7 = 0.571 N 4 = 1.53172 ν 4 = 48.9 R 8 = -2.539 D 8 = 0.428 α = 0.50 f1 = -1.43 f2 = 2.86 e '= 1.43 <Numerical example 2> f = 0.55 FNo = 1: 2.5 2 ω = 79.4 ° R 1 = 6.480 D 1 = 0.571 N 1 = 1.69895 ν 1 = 30.1 R 2 = 15.937 D 2 = 0.021 R 3 = 4.826 D 3 = 0.214 N 2 = 1.77250 ν 2 = 49.6 R 4 = 1.800 D 4 = 0.714 R 5 = 9.084 D 5 = 0.142 N 3 = 1.77250 ν 3 = 49.6 R 6 = 2.282 D 6 = 0.613 R 7 = 2.297 D 7 = 0.500 N 4 = 1.53172 ν 4 = 48.9 R 8 = -26.557 D 8 = 0.428 α = 0.55 f1 = −2.20 f2 = 4.00 e ′ = 1.80 <Numerical Example 3> f = 0.47 FNo = 1: 2.5 2ω = 88.4 ° R 1 = 8.183 D 1 = 0.357 N 1 = 1.84666 ν 1 = 23.8 R 2 = 19.496 D 2 = 0.0 21 R 3 = 5.041 D 3 = 0.214 N 2 = 1.77250 ν 2 = 49.6 R 4 = 1.306 D 4 = 0.571 R 5 = 9.390 D 5 = 0.128 N 3 = 1.77250 ν 3 = 49.6 R 6 = 1.346 D 6 = 0.740 R 7 = 2.626 D 7 = 0.714 N 4 = 1.51823 ν 4 = 59.0 R 8 = -1.957 D 8 = 0.428 α = 0.47 f1 = -1.07 f2 = 2.29 e '= 1.21 <Numerical Example 4〉 f = 0.56 FNo = 1: 2.5 2ω = 78.4 ° R 1 = 5.901 D 1 = 0.571 N 1 = 1.69895 ν 1 = 30.1 R 2 = 18.882 D 2 = 0.021 R 3 = 5.738 D 3 = 0.214 N 2 = 1.80400 ν 2 = 46.6 R 4 = 2.049 D 4 = 0.571 R 5 = 7.116 D 5 = 0.142 N 3 = 1.77250 ν 3 = 49.6 R 6 = 1.840 D 6 = 1.654 R 7 = 1.954 D 7 = 0.500 N 4 = 1.53172 ν 4 = 48.9 R 8 = 44.983 D 8 = 0.428 α = 0.56 f1 = −2.14 f2 = 3.83 e ′ = 1.68 * 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 = ∞

[0030]

[Table 1]

[0031]

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 four lenses satisfying the above-mentioned conditions. As a result, a wide converter lens that maintains a wide angle of view with an afocal magnification of about 0.5x, while maintaining good compactness of the lens system and optical performance of the entire system when attached to the shooting lens system, especially distortion Can be achieved.

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view when a wide converter lens according to Numerical Example 1 of the present invention is mounted in front of a taking lens.

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

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

FIG. 4 is a lens cross-sectional view of Numerical Example 4 of a wide converter lens according to 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.

FIG. 9 is a diagram of various types of aberration when a wide converter lens according to Numerical Example 4 of the present invention is mounted in front of a taking lens.

FIG. 10 is a diagram of various types of aberration of the taking lens in 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 (3)

[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 in the shorter direction, wherein the wide converter lens is a positive meniscus lens whose convex surface faces in order from the object side to the object side. First lens,
A negative second meniscus lens having a convex surface directed toward the object side,
It has four lenses, a meniscus-shaped negative third lens having a convex surface facing the object side and a positive fourth lens, and an image-side lens surface of the first lens and an object-side lens of the second lens. Wide converter lens characterized by forming an air lens of positive refractive power with the surface. 2. The radius of curvature of the ith lens surface from the object side is Ri, the refractive index of the material of the ith lens is Ni, and the first
Let f1 be the combined focal length from the lens to the third lens. The wide converter lens according to claim 1, which satisfies the following condition. 3. The condition satisfying 1.7 <N2 ν1 <35 45 <ν2 when the refractive index and the Abbe number of the material of the ith lens from the object side are Ni and νi, respectively. 1 or 2 wide converter lens.