JPH04191718A - Front wide converter - Google Patents

Abstract

PURPOSE:To obtain a wide converter which is compact and where aberration is favorably corrected by arranging a first lens with negative refractive power and a second lens with positive refractive power with an air interval in order from the side of an object. CONSTITUTION:When the upper limit of (1) of a formula I is exceeded, compactness of a lens group is spoiled, and when the lower limit is exceeded, positive refractive power of a first lens becomes too strong, and it comes to be difficult to correct negative distortion aberration. When the upper limit of a formula (2) is exceeded, the shape of the first lens becomes a meniscus lens with a strong curvature, the overall length of the lens group becomes longer and the diameter of a front lens becomes bigger. When the upper limit of a formula (3) is exceeded, multiplying chromatic aberration generated at the second lens becomes larger and it comes to be impossible to correct it with the second lens. When slipped out of conditions of a formula (4) and the refractive index of the first lens becomes smaller, negative distortion aberration especially becomes larger, and it is unfavorable.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an afocal front wide converter that is attached in front of a main lens to shorten the focal length.

(Prior art) A format in which a lens component having a negative refractive power and a lens component having a positive refractive power are placed on the object side of a master lens, and are arranged in order from the object side with an appropriate air gap between them. The front wide converter of has been well known for a long time. By attaching such a front wide converter to the master lens, it is possible to easily shorten the focal length of the entire system without changing the F-number, which is very convenient.

Conventional examples of this type of front wide converter include Japanese Patent Application Laid-Open No. 63-253319 and Japanese Patent Application Laid-Open No. 1999-1. -31
Publication No. 9010 etc. can be seen. However,
Looking at the embodiment disclosed in Japanese Patent Publication No. 3-253319, the number of sheets is three or four, and it is somewhat complicated. Further, the device disclosed in Japanese Patent Application Laid-Open No. 1-319010 is simple with only two sheets, but the afocal magnification is about 0.7 times, which leaves some dissatisfaction.

(Problems to be Solved by the Invention) An object of the present invention is to have a single negative and positive lens component so that the afocal magnification is 0 despite having an extremely simple configuration.
．． As much as 5.

Moreover, it is an attempt to obtain a front wide converter in which various aberrations are well corrected.

(Means for Solving the Problem) The front wide converter of the present invention constitutes an afocal system as a whole, and is a cosiverter that is installed in the object side C device of the master system, and has negative refractive power in order from the object side. The first L-lens and the second L-lens with positive refractive power are arranged with an air gap between them. It consists of a positive lens with a strong convexity on the side.

The second front wide converter is characterized by satisfying the following conditions.

(1) 1.6<l fi/R<2.0 (2) 1. o
< (r, +r2) / (r, -r2) <1. ．． 8
(3) 1.1<ν1/ν, (1, 7 (4) X, 55< n + However, fl, focal length of the first lens nl: refractive index of the first lens ν1: Abbe number of the first lens ν, - Abbe number rl of the second lens - Radius of curvature of the object side surface of the first lens r,: Radius of curvature R of the image side surface of the first lens - Effective radius (action) of the first lens Generally, this In order to make the lens system more compact with a wide converter of this kind, it is necessary to set the value of the principal point spacing f, +f of the afocal system as small as possible, where f is the focal length of the second lens. .

Also, if the afocal magnification is m, then m is m--f
, /f, (0<m<1)-(a).

From formula (a), f , = −m f , −(b
), so the principal point spacing of the afocal system is f, +f.

is f, +f, = f, (1-m) - (c)
It is expressed as

From equation (c), it can be seen that in order to make the values of f and +f small and to make the lens system more compact, the refractive power of the second lens must be increased.

Furthermore, from equation (b), it can be seen that when the afocal magnification m is determined, the refractive power of the first lens must also be strong, similar to the second lens.

However, in this type of wide converter, if the refractive powers of the first lens group with negative refractive power and the second lens group with positive refractive power are strengthened, aberration correction, especially correction of negative distortion, becomes difficult. It is well known that it can be difficult.

In order to shorten the overall lens length and to satisfactorily correct various aberrations, it is desirable to satisfy each of the conditional expressions (1) to 4) above.

Conditional expression (1) is a condition for configuring the lens system compactly while sufficiently correcting aberrations, particularly negative distortion aberration. The appropriate value of the focal length of the first lens to be satisfied for this purpose is approximately determined by the maximum effective diameter of the wide converter. .

When the upper limit of the second equation is exceeded, not only the first lens but also the second lens must be used to maintain the predetermined afocal magnification.
The refractive power of the lens also becomes weaker, and the values of the principal point spacing f, +f of the afocal system become larger, which impairs the compactness of the lens system. If the lower limit is exceeded, the negative refractive power of the first lens becomes too strong, making it difficult to correct negative distortion.

Conditional expression (2) is a condition for keeping the lens system compact while suppressing astigmatism generated in the first lens as much as possible by appropriately bending the first lens.

When the upper limit of this equation is exceeded, the shape of the first lens becomes a meniscus lens with a strong curvature, so the position of the principal point of the first lens moves relatively toward the image side.

Therefore, in order to satisfy the conditions for keeping the converter afocal, it is necessary to increase the air distance between the first lens and the second lens, which increases the overall length of the lens system and increases the diameter of the front lens.

Moreover, when the lower limit of the formula is exceeded, the occurrence of astigmatism in the first lens becomes large, and the meridional image plane becomes undersized.

Conditional expression (3) maintains good axial and lateral chromatic aberrations. When the upper limit of this equation is exceeded, the chromatic aberration generated in the second lens becomes large.

Moreover, when the lower limit is exceeded, the chromatic aberration of magnification generated in the first lens becomes large, and it becomes impossible to correct this with the second lens.

Conditional expression (4) corrects distortion aberration, and if this condition is violated and the refractive index of the first lens becomes small, negative distortion becomes particularly large, which is undesirable.

(Example) Examples of the wide converter of the present invention will be shown below. In the table, r is the radius of curvature of each lens surface, d is the lens thickness or lens spacing, n is the refractive index, ν is the Abbe number, and m is the afocal magnification.

First embodiment Magnification m=0.51 Surface NCLr d na 1)a
l 40.350 +, 00 1. ．． 713
00 53.92 9.138 14.00 3 -52.230 2.60 +, 58144
40.74 -14.298 1.65 1f, l/R=1.84 (r, +r2)/(r, -
r2)=1.59ν1/ν,=1.324 Second example magnification m=0.51 Surface Nci r d nd
v, 11 46.469 1.00 1.658
44 50.98.911 14.00 3 -46.312 2,60 1,58144
40.74 -13.864 1.65 If, I/R=1.84 (r, +r2)/(r, -
r2) = l, 47 sl/s, = 1.251 Third embodiment Magnification m = 0.51 Surface Na r d nd
Vd1 51.943 1,00 1.6229
9 58,22 8.714 14.00 3 -46.779 2.60 1,54814
45.84 -13.336 1.65 if, l/R=1,84 (r, +r2)/(r, -
r2)=1.40ν1/ν, = 1,271 4th embodiment Magnification m=0.51 Surface NCLr d n, vdl
82.226 0.70 1.772
50 49.62 9.754 12.0
03 -173.082 2,60 +,
60342 38.04 -15.532
1.651f+i/R==1,73 (r,+r2
)/(r, -r2)=1. ．． 27shi1/ν, = 1.
305 Fifth Example Magnification m=o, 51 plane ha, r d
n+1 ν d
l 45.895 0,70 1.69680
55.52 9.204 14.00 3 -47.090 2,30 1.63930
44.94 -14.701 1.65 1 f+ I/R=1.86 (r, "r
#)/(r+-r2)=1.50ν1/ν,=1
．． An example of a photographic lens equipped with each of the above converters is shown below.

Focal length f=11. ,728 Angle of view 2ω=39°
14' Back Focus f! l=2.255 plane Nαr d n, IV.

l oo 1,00 1.51
633 64.12 Beauty 2.7
0 3 5.900 2.15 1,7995
2 42,24 8.885 1.05 5 -17.088 0.70 1.603
42 38,06 4.226 0.
50 7 6.149 0.85 1.7552
0 27.58 3.741 2.80
1,77250 49.69 ~9.7
11 2.55 10 oo6,0(11,5163364,
111■ (Effects of the Invention) As seen in the above-mentioned embodiments and their aberration diagrams, the front wide converter of the present invention has a simple configuration of only two elements, yet has a magnification of 0.5x. We were able to obtain a wide converter that is compact, has all aberrations well corrected, and is highly practical.

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing the front wide converter of the present invention attached to a photographic lens, and Figs. 2 to 6 show aberrations of the first to fifth embodiments respectively attached to the photographic lens. Curve diagram, FIG. 7 is an aberration curve diagram of the photographing lens.

Claims (1)

[Claims] 1) In a wide converter that constitutes an afocal system as a whole and is placed on the object side of the master system, in order from the object side, the first lens has a negative refractive power and a positive refractive power. A front lens characterized in that the first lens is a negative meniscus lens with a convexity facing the object side, and the second lens is a positive lens with a strong convexity facing the image side. Wide converter 2) Claim 1, characterized in that it satisfies each of the following conditions:
Front wide converter 1.6<｜f_1 described in section
|/R<2.0 1.0<(r_1+r_2)/(r_1-r_2)<1
．． 81.1<ν_1/ν_2<1.7 1.55<n_1 However, f_1: Focal length of the first lens n_1: Refractive index of the first lens ν_1: Abbe number of the first lens ν_2: Abbe number of the second lens r_1: Radius of curvature of the object-side surface of the first lens r_2: Radius of curvature of the image-side surface of the first lens R: Indicates the effective radius of the first lens.