JPH01144007A - Rear focus type triplet lens - Google Patents

Abstract

PURPOSE:To decrease the load of focusing by constituting a lens system in such a manner that the lens system satisfy specific conditions and focusing can be executed by moving a 3rd group lens. CONSTITUTION:This lens consists, successively from an object side, of a 1st group lens of a positive lens, the convex face of which is directed to the object side, a 2nd group lens of a biconcave lens, and the 3rd group lens of a biconvex lens and is so constituted as to satisfy the conditions expressed by equation I-equation V and to execute focusing by extending only the 3rd group lens. In equation I-equation V, (f) is the focal length of the entire system; f23 is the combined focal length of the 2nd group lens and the 3rd group lens; f3 is the focal length of the 3rd group lens; D23 is the spacing between the 2nd group lens and the 3rd group lens at the time of infinite distance; D3 is the thickness of the 3rd group lens; R is the radius of curvature of the face on the object side of the 3rd group lens. Rear focusing is thereby enabled while the constitution is simple. The stop position is fixed arbitrarily and the lens is as bright as F/1.6-F/2.5 aperture ratio. The small-size and excellent- performance lens which is well corrected of various aberrations is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a rear focus type triplet lens used in consumer video cameras, still video cameras, and the like.

[Conventional technology]

Currently, lenses for consumer video cameras or still video cameras have a zoom ratio of 3 to 6 and an aperture ratio of 1.
Zoom lenses with a size of 1.2 and 6 are the most popular.

This conventional zoom lens occupies a relatively small size compared to the size of the camera. This is because the demand for miniaturization of lenses was not as strong as that for electrical systems.

However, in the future, as camera bodies become much smaller, lighter, and lower in cost, it will become increasingly necessary to reduce the size, weight, and cost of lens systems.

In order to reduce the size and weight of this lens system, small, lightweight, and low-cost single-focal-length lenses are attracting attention in place of large, high-cost zoom lenses.

The triplet type has been known for a long time as a lens system that is small, lightweight, low cost, and has a small number of elements.A normal triplet is located between the first and second group lenses or between the second and third group lenses. In many cases, an aperture is provided between the lenses, which has the disadvantage that the aperture must be moved together with the lens during focusing. In particular, the aperture of a video camera is large and heavy, making it difficult to make it movable.

To provide a rear focus type triplet lens in which the diaphragm is fixed during focusing in a zoom type lens system, and focusing is performed only by a third group lens in order to lighten the load of focusing as much as possible.

[Failure to solve the problem]

The rear focusing triplet lens consists of, in order from the object side, a positive first group lens with a convex surface facing the object side, a biconcave second group lens, and a biconvex third group lens. and the following conditions (1) to (5)
In addition, focusing is performed by extending only the third lens group.

(1) lf/f□31 < 0.4 (2) 0.
4 < f3/f < 1.6 (3) 0.02 <
D23/f< 0.4(4) 0.02<D3/
f < 0.4 (5) 0.6 < dark 5 where f is the focal length of the entire system, f23 is the combined focal length of the second group lens and third group lens, f3 is the focal length of the third group lens, D23 is the distance between the second group lens and the third group lens at infinity, D3 is the thickness of the third group lens, and R is the radius of curvature of the object side surface of the third group lens.

The aperture of this lens system may be located in front of the lens system, between the first group lens and the second group lens, between the second group lens and the third group lens, or at the rear of the third group lens. Also, by satisfying the above conditions, spherical aberration and astigmatism become a problem, especially in the case of serial focusing.

Problems such as eliminating fluctuations caused by focusing such as coma aberration have been solved.

Next, the above conditions (1) to (5) will be explained.

Condition (1) defines the combined focal length of the first group lens and the second group lens, and is provided to reduce fluctuations in spherical aberration due to focusing. This condition (
If the upper limit of 1) is exceeded, spherical aberration will be overcorrected when focusing on a close object point. On the other hand, if the lower limit is exceeded, spherical aberration will be insufficiently corrected when focusing on a close object point, which is undesirable.

Condition (2) defines the focal length of the third group lens, and is similarly provided to reduce fluctuations in spherical aberration due to focusing. If the upper limit of condition (2) is exceeded, spherical aberration will be overcorrected when focusing on a close object point. If the lower limit is exceeded, conversely, spherical aberration will be insufficiently corrected when focusing on a close object point, and at the same time, chromatic aberration will occur significantly at high image heights.

In the lens system of the present invention, only the third group lens is moved during focusing, so the conditions regarding the third group lens are strict. Condition (3) was established for this purpose.
, (4), (5).

Condition (3) defines the distance between the second group lens and the third group lens with respect to an object point at infinity. If the upper limit of condition (3) is exceeded, comatic aberration will significantly occur when focusing on a close object point. If the lower limit of condition (3) is exceeded, there is a risk that the third group lens will come into contact with the second group lens during focusing on a close object point.

Condition (4) defines the thickness of the third group lens. If the thickness of the third group lens is increased, the height of the chief ray on the image side surface of the third group lens becomes high, and astigmatism and coma aberration occur. In particular, in the lens system of the present invention, since focusing is performed by moving the third group lens toward the image side, the height of the principal ray on the image side surface of the third group lens becomes even higher, which increases astigmatism and coma aberration. Occurs noticeably. For the above reasons, the third
An upper limit was placed on the thickness of the lens group to prevent deterioration in the performance of the lens system. Further, it is preferable to reduce the thickness of the third lens group from the viewpoint of correcting aberrations, but due to processing problems, a lens with a small effective diameter results in a dark optical system as a whole.

Based on the above, the lower limit of condition (4) was set in order to maintain appropriate brightness of the lens system. In other words, when the upper limit of condition (4) is exceeded, astigmatism and coma aberration worsen. Moreover, if the lower limit is exceeded, a bright lens system cannot be obtained.

Condition (5) also defines the curvature of the object-side lens surface of the third group lens. The curvature of this lens surface affects the spherical aberration of the entire lens system, especially the 3rd spherical aberration during focusing.
Spherical aberration changes as the group lens moves. Condition (5)
If the curvature of this surface exceeds the upper limit and becomes large, spherical aberration will be over-corrected when focusing on a close object point, and if the lower limit is exceeded and the curvature becomes small, spherical aberration will be under-corrected overall.

For the above reasons, the upper and lower limits of condition (5) were set.

〔Example〕

Next, examples of the rear focus type triplet lens of the present invention will be shown.

Example 1 f=9, F/1.7 r1=4.7643 ds ”2.4713 nt =1.75500
! /, =52.33r2=76.8889 dz = 0.2000 rs=ω (aperture) d, =0.6000 r4=-5,8348 d4=0.200On2=1.59270 9t =3
5.29rs = 4.0747 ds = 1.1001 ra = 13.8843 do = 1.0000 ns = 1.72000
νs = 50.25r, = -5,3267 1'/l = 0.088, 'S/f = 0
．． 607il”/=r6/=1.54,”Vl=d5
/f=0.12f DVl=dI/f=0. i 1 , Δ=0.0
48 Example 2 f=9, F/2.5 r+=4.1559 d1=1.2244 nt=1.75500
j/1=52.33r2=66.6048 a2=0.3000 r,=oO (diaphragm) d3=0.6519 r,=6.0914 di=0.2000 12=1.59270
Vt = 35.29r5 = 3.6875 ds = 1.5o03 ra = 18.2103 da = 0.8000 n, = 1.72000
j/3=50.25r7=5.4765 +f/l=0.083, /f=0.625
f12 f・R/ = r6/f= 2.02, n+3/ =
d54=0.167DV=d6/f=0. os9.
Δ=0.048 Example 3 f=9, F/2. O r, = 4.1681 di = 1.6158 nt = 1.75500
shi, = 52.33 rz = 32.0151 d2 = 0.4000 r, = ω (aperture) d3 = 0.6000 r4 = 6.4386 di = 0.2000 nz = 1.6,36
36! /2 = 35.37r5 = 3.717
6 d5=0.9255 ra=11.1342 da=1.3000 ns=1.72000
ν3=50.25rf=-5,5499 1f/I=0.161, /f=0.590
fu f・”/””6/f= 1.237, D23/f=
d5/f=0.103Dη=dη=0.144, Δ
= 0.050 Example 4 f = 9, F/2.0 rt = ω (aperture) d, = 0.5000 r2 = 4.3689 d2 = 1.300 On + = 1.81600 Si + =
46.62r3: 37.1633 ds = 0.3923 r, = 8.9528 d+ = O-6538 R2 = 1.66680
1/2 = 33.04rs = 3.7553 d5 = 1.4000 ra = 25.4501 da = 2.3910 n3 = 1.81600
v, = 46.62r7 = 7.5098 1/, 21 = 0.163, f3/f = 0.8
16R/=r6/f=2.33, D2s/=ds/
,=0.155Dv=d6/f=0.266, Δ
= 0.047 Example 5 f=9, F/2. O r+=4.3026 d+=1.5042 n+=1.75500 sit=52.33r2=44.6993 dz=0.3000 r3=ω (aperture υ) ds=0.5980 r+=6.5017 d* =0.2000 nt=1.59270
! /2=35.29r5 ” 3.8970 d,, =1.2562 ra=15.5014 do =0.8000 13 =1.7,2000
νs = 50.25r7 = -5,7303 1f/I = 0.076, f3,7/f = 0
．． 656R7=r6/=1.72, Dzs7
．． = d5/f= 0.140f D3/=d6/f=0.089. Δ=0.048 Example 6 f=9, F/2.0 r, =4.3951 d+ =1.0OOOn+ =1.75500 1/+
=52.33r2"59.3070 d2=0.9575 r3=-8,2505 d3=0.6727 n2=1.59270 1/
2 = 35.29r, = 3.6123 da = 0.6000 r, = oo (diaphragm) d, = 0.6000 ra = 11.7471 da = 0.8000 ns = 1.72000
9s = 50.25rv:: 6.5915 If/l = 0.080, /f= 0.6
63f12 f・”/1=”//f=1.30, D2Vf=
”/1 = 0.067DVf=d6/f=0.089
．． Δ=0.048 Example 7 f=9, -F/2. O r+ = 3.3022 d+ = 1. oooo n,=1.84100 1'
+=43.23r2=10.2295 d2==0.5000 r3”” 21.4498 d3=0.2000 n2=1.66680 1/
2=33.04r+=2.9775 d+=1.2000 r5=8.7033 ds=0.7000 n3=1.75500
J/3 = 52.33ra = 10.4388 da = 0.1000 r-t = CI: l (aperture) ,, fx If/ l = 0.074, / = 0.710"/
=rf/f=0.967, D23/f=dVf=
0.133DV=dt/f=0.076, Δ=0
．． 047 Example 8 f=12, F/2. O r+=5.7336 d+ ”2.0934 n+ =1.75500
I)+ =52.33r2=89.0698 d2=0.4000 r3"ω (aperture) d3=0.8000 r<"" 8.4472 d4=0.2667 n2=1.59270 ν
2 = 35.29rs = 5.0207 d5 = i, 8467 ra = 21.5002 da ””1.4000 n3 = 1.72000
! '3 = 50.25r7 = 7.4942! f/l = 0.054, '34 = 0
．． 656f+2 ”/1 = r1/f= 1.79, ””/1 =
dVl = 0.154D3/f= d6/f= 0
．． t 17 , Δse 0.047 but rl+r2+...
. is the radius of curvature of each lens surface, dl, d2. ... is the wall thickness and air gap of each lens, nl+n2. n3 is the refractive index of each lens, and si1. C2. ν is the Atsube number of each lens,
Δ is the object distance 2001! This is the amount of movement of the third group lens when focusing on +11 (--L times).

Among the above examples, Example 1 has a lens configuration shown in FIG.
The aperture is located between the first group lens and the second group lens. In this embodiment, focusing is performed only by extending the third lens group. The aberration situation for an object point at infinity in this embodiment is shown in FIG. 5, and the aberration situation when the magnification is -1 is shown in FIG.

Examples 2.3 and 5.8 also have the lens configuration shown in FIG. The aberration conditions of the object point at infinity and when the magnification is 1 in these Examples are shown in FIGS. 7 and 8 for Example 2, and FIG. 9 for Example 3.
10, Example 5 is shown in FIGS. 13 and 14, and Example 8 is shown in FIGS. 19 and 20.

Embodiment 4 is as shown in FIG. 2, and the diaphragm is located in front of the lens system. The aberration situation of the object point at infinity in this example is the first
FIG. 12 shows the aberration situation when focusing at -5 times as shown in FIG. 1.

Embodiment 6 has a through structure shown in FIG. 3, with the aperture diaphragm located between the second group lens and the third group lens.

The aberration situation for an object point at infinity in this embodiment is shown in FIG. 15, and the aberration situation at -5 times is shown in FIG. 16.

Embodiment 7 has a lens configuration shown in FIG. 4, in which the diaphragm is located after the lens system. The aberration situation of this embodiment is shown in FIG. 17 for the infinite relic point, and in FIG. 18 for the 1× magnification.

〔Effect of the invention〕

Although the lens system of the present invention has a very simple configuration with three lenses, it is capable of rear focusing, the aperture position can be fixed at will, and it is bright with a front diameter ratio of ~1.6 to 5.5. It is suitable for compact electronic still cameras that have excellent performance and are compact with various aberrations well corrected.

[Brief explanation of the drawing]

FIGS. 1 to 4 are diagrams showing the lens structure of an embodiment of the present invention, and FIGS. 5 to 20 are aberration curve diagrams of the embodiment of the present invention. Applicant Olympus Optical Industry Co., Ltd.

Claims (1)

[Claims] Consisting of, in order from the object side, a first group lens which is a positive lens with a convex surface facing the object side, a second group lens which is a biconcave lens, and a third group lens which is a biconvex lens, and satisfies the following conditions. A rear focusing triplet lens that performs focusing by moving the third lens group. (1) |f/f_2_3|<0.4 (2) 0.4<f_3/f<1.6 (3) 0.02<D_2_3/f<0.4 (4) 0.02<D_3/f <0.4 (5) 0.6<R/f<5 where f is the focal length of the entire system, f_2_3 is the combined focal length of the second and third group lenses, and f_3 is the focal length of the third group lens. , D_2_3 is the air distance between the second group lens and the third group lens at infinity, D_3 is the thickness of the third group lens, and R is the radius of curvature of the object side surface of the third group lens.