Wide angle zoom lens system

A miniaturized, super-wide angle zoom lens system of the two lens group type wherein barrel shaped distortion aberration in the wide angle position is compensated for by providing one of the lenses with an aspherical surface.

BACKGROUND OF THE INVENTION 
This invention relates to a miniaturized zoom lens system having a super 
wide viewing angle of 84.degree. or 94.degree. and good compensation for 
various aberrations. These criteria are achieved by, inter alia, providing 
one of the lenses with an aspherical surface. 
The lens system of the present invention is a so-called two lens group type 
of zoom lens system comprising a first divergent lens group and a second 
convergent lens group. The movement characteristics of the two lens groups 
are illustrated in FIG. 1, where f.sub.1 (f.sub.1 &lt;0) is the focal length 
of the first lens group, f.sub.2 (f.sub.2 &gt;0) is the focal length of the 
second lens group, and l is the distance between the first and second lens 
groups. The relation of these parameters to the overall focal length f is 
as follows: 
##EQU1## 
The back focal length f.sub.B is given by: 
##EQU2## 
From equation (1): 
##EQU3## 
From equation (2): 
##EQU4## 
The overall length L is: 
EQU L=l+f.sub.B ( 5) 
From equations (3), (4) and (5): 
##EQU5## 
In equation (6), if dL/df =0, then 
EQU f=.vertline.f.sub.1 .vertline. (7) 
From equation (3), the distance l between the first and second lens groups 
is longest in the wide angle position, and the longer the overall focal 
length f the shorter the distance l becomes. From equation (4), the back 
focal length f.sub.B has a minimum value in the wide angle position, and 
the longer the overall focal length the longer the back focal length 
proportionately becomes. From equation (7), the overall length L has a 
minimum value when the overall focal length f is equal to the absolute 
value .vertline.f.sub.1 .vertline. of the focal length of the first lens 
group. Accordingly, whenever the overall focal length f is longer or 
shorter than .vertline.f.sub.1 .vertline., the overall length L becomes 
longer. 
From the above analysis, since the distance l between the first and second 
lens groups becomes longest at the wide angle position, the aperture of 
the first lens group tends to increase as the viewing angle widens. This 
tendency is effective to extremely widen the viewing angle. If the 
miniaturization of the lens system is unreasonably compatible with 
widening the viewing angle to a super-wide angle, the barrel shaped 
distortion aberration in the wide angle position is abrupty increased, and 
it is impossible to compensate for this aberration by using a spherical 
lens system. 
SUMMARY OF THE INVENTION 
In order to overcome the above defect, the present invention provides a 
miniaturized zoom lens system in which the barrel shaped distortion 
aberration in the wide angle position is compensated for by using a single 
aspherical lens, and various other aberrations are also compensated for 
throughout the overall zoom range.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The zoom lens system of the present invention is composed of a first 
divergent lens group and a second convergent lens group. The overall focal 
length is varied by the relative mechanical movement of the first and 
second lens groups, and at the same time the image focus is maintained 
constant. The first divergent lens group comprises at least two positive 
lenses and at first two negative lenses, and includes, in order from the 
object side, a positive lens having convex surface facing the object, a 
negative lens group, and a positive meniscus lens having a convex surface 
facing the object. The second convergent lens group comprises at least two 
positive lenses in its end units facing both the object and image sides, 
and includes positive, negative and positive lens units or positive, 
negative, positive, negative and positive lens units. Further, a single 
aspherical surface is provided either on an arbitrary surface (the i-th 
surface) of the first lens group of on an arbitrary surface (the j-th 
surface) of the second lens group. The overall zoom lens system must 
satisfy the following conditions: 
EQU f.sub.W &lt;.vertline.f.sub.1 .vertline.&lt;1.5 f.sub.T, (1) 
EQU 0.4&lt;l.sub.W /f.sub.W &lt;1.0, (2) 
EQU 3.0&lt;L/f.sub.W &lt;6.0, (3) 
if K=i or j (the K-th surface is aspherical), 
EQU 0&lt;.vertline..phi..sub.K .vertline.&lt;0.35, (4) 
where: .phi..sub.i &gt;0 and .phi..sub.j &lt;0, 
EQU 0&lt;.vertline.h.sub.WK.sup.2 h.sub.WK.sup.2 .phi..sub.K &lt;0.2, (5) 
and 
EQU -0.2&lt;h.sub.WK h.sub.WK.sup.3 .phi..sub.K &lt;0, (6) 
wherein: 
f.sub.1 is the focal length of the first lens group, 
f.sub.W is the focal length in the wide angle position, 
f.sub.T is the focal length in the telescopic or narrow angle position, 
l.sub.W is the distance between the first and second lens groups in the 
wide angle position, 
L is the maximum overall length, 
.phi..sub.K is the solid or cubic aspherical coefficient, 
h.sub.WK is the height of the parallel light beam near the optical axis 
passing through the K-th surface in the wdie angle position and limited by 
the stop diaphragm, and 
h.sub.WK is the height of the angled light beam passing through the center 
of the stop diaphragm in the wide angle position in the K-th lens surface. 
To prevent the aperture of the first lens group from increasing as the 
overall length is shortened, a positive lens having a convex surface 
facing the object is disposed at the front or object side of the first 
divergent lens group. To prevent various aberrations when focusing on a 
small object, a positive meniscus lens having a convex surface facing the 
object is disposed at the final or end position on the image side in the 
first lens group. 
To prevent variations of and increases in the spherical aberration when the 
focal length is varied, at least two positive lenses are positioned on the 
object side in the second convergent lens group, and to prevent variations 
of and increases in the astigmatism aberration and the image distortion, 
at least two positive lenses are positioned on the image side therein. 
Conditions (1) and (2) above relate the power of the lens system. As 
previously mentioned, the overall length L is maximum when the overall 
focal length f is equal to the absolute value .vertline.f.sub.1 .vertline. 
of the focal length of the first lens group. Accordingly, the power of the 
first lens group is defined throughout the zoom range from f.sub.W to 
f.sub.T and at its peripheral ends to miniaturize the maximum length of 
the zoom lens system, as specified in Condition (1). If the value of 
.vertline.f.sub.1 .vertline. decreases below the minimum limiting value, 
the power of the first lens group becomes too strong to enable adequate 
compensation for the various aberrations and distortions. Conversely, if 
.vertline.f.sub.1 .vertline. exceeds the maximum limiting value, although 
the various aberrations can be easily compensated, the amount of zoom 
movement increases to thereby enlarge the overall lens system and work 
against miniaturization. 
The maximum value of Condition (2) also defines a miniaturization limit. 
Below the minimum value of Condition (2), the back focal length is 
shortened and the zoom ratio becomes too small. To overcome this defect 
the power of the first lens group must be increased, but this leads to 
difficulties in compensating for various aberrations and distortions as 
mentioned above. 
Condition (3) is derived from Conditions (1) and (2), and relates to the 
maximum length of the lens system. Above the maximum aberration 
compensation is easily achieved, but the length is imcompatible with 
miniaturization. Below the minimum value the powers of the first and 
second lens groups become too strong, aberration compensation is 
difficult, and the zoom ratio must be reduced. 
Conditions (4), (5) and (6) relate to the shape or configuration of the 
aspherical surface. This will be illustrated with reference to FIGS. 2 and 
3. In FIG. 2 the vertex of the aspherical surface is the origin O, and the 
X-axis is coincident with the direction of light propagation. The 
coordinates (x, y) of a point P on the aspherical surface is defined as 
follows: 
##EQU6## 
where C is the curvature near the optical axis (the reverse number of the 
radius of curvature). 
The first term of equation (8) depends only on the curvature C near the 
axis. The second and following terms define the aspherical gradient. The 
coefficient A.sub.1 of the second term is related to the coefficient .phi. 
of the solid or cubic aspherical surface as follows: 
EQU .phi.=8(N'-N)A.sub.1, 
where 
N is the refractive index of the medium before the aspherical surface, and 
N' is the refractive index of the medium after the aspherical surface. 
The aspherical surface coefficient relates to the solid or cubic aberration 
coefficient in the theory of aberrations, and requires that the following 
amounts of aberration variations be considered in using the aspherical 
surface: 
EQU .DELTA.I=h.sup.4 .phi., 
EQU .DELTA.II=h.sup.3 h.phi., 
EQU .DELTA.III=h.sup.2 h.sup.2 .phi., 
EQU .DELTA.IV=h.sup.2 h.sup.2 .phi., and 
EQU .DELTA.V=hh.sup.3 .phi., 
where: 
I is the spherical coefficient, 
II is the coma aberration coefficient, 
III is the astigmatism aberration coefficient, 
IV is the spherical deficiency surface curve aberration coefficient, 
V is the distortion aberration coefficient, and 
h and h are the amounts of tracking near the axis. 
The term h represents the height of the focused image on the optical axis 
formed by the light beam passing through each lens surface in parallel 
with the optical axis as shown in FIG. 3. 
The term h is the vertical distance from the optical axis to the 
intersection of each lens surface with the slanting light beam which 
passes through the center of the stop diaphragm. 
Conditions (4), (5) and (6) result in the barrel shaped distortion 
aberration being well compensated for by the single aspherical lens 
surface. 
In Condition (4), when the aspherical surface is disposed in the first lens 
group the stipulation that .phi..sub.i &gt;0 applies; when the aspherical 
surface is disposed in the second lens group .phi..sub.j &lt;0 applies. If 
the value .vertline..phi..sub.K .vertline. exceeds the maximum limit, as 
the viewing angle becomes large the compensation of the distortion 
aberration becomes excessive. This leads to an increase in the amount of 
distortion turning, and hence to difficulty in compensating for 
astigmatism and image distortion. 
Condition (5) relates to the astigmatism aberration and the image 
distortion. Above the maximum value, when the aspherical surface is 
positioned in the first lens group the image surface is too 
under-compensated, while when the aspherical surface is positioned in the 
second lens group the image surface is too over-compensated. 
Condition (6) relates to the distortion aberration. Below the minimum 
value, as the viewing angle increases the distortion compensation becomes 
excessive and the amount of distortion turning increases. Also, it is 
difficult to compensate for the image curvature distortion. 
The detailed parameters for four Examples of the present invention are 
listed below, wherein: 
.omega. is the half-viewing angle, 
r is the radius of curvature, 
d is the thickness of the lens or the distance between adjacent lenses, 
N is the refractive index at the d-line, 
.nu. is the Abbe number, 
A.sub.1, A.sub.2, A.sub.3 and A.sub.4 are the aspherical coefficients, 
h is the height of the parallel light beam near the optical axis in the 
wide angle position, 
.alpha. is the angle of the near optical axis light beam in the wide angle 
position, 
h is the height of the angled light beam that passes through the center of 
the stop diaphragm in the wide angle position, and 
.alpha. is the angle of the latter light beam in the wide angle position. 
EXAMPLE 1 
______________________________________ 
1 : 3.5 f = 1.0-1.46 .omega. = 42.0.degree.-31.3.degree. 
Surface No. r d N .upsilon. 
______________________________________ 
1 2.6088 0.1464 
1.58913 
61.1 
2 8.3383 0.0041 
3 2.0830 0.0488 
1.79952 
42.2 
first lens 4 0.6508 0.3091 
group 5 13.9462 0.0529 
1.67790 
50.7 
6 1.3168 0.1952 
7 1.0677 0.1114 
1.80518 
25.4 
8 1.9415 0.6480 
9 1.7490 0.2916 
1.80610 
40.9 
10 -6.6348 0.0907 
11 0.8200 0.1041 
1.80610 
40.9 
12 2.6537 0.0936 
second 13 -2.3505 0.1594 
1.84666 
23.9 
lens 14 0.8293 0.0732 
group 15 -8.5416 0.0915 
1.51633 
64.1 
16 -0.9506 0.0041 
17 14.4470 0.0944 
1.51633 
64.1 
18 -1.2857 
______________________________________ 
______________________________________ 
focal length of the overall 
lens system d.sub.8 
______________________________________ 
1.00 0.6480 
1.20 0.3715 
1.46 0.1268 
______________________________________ 
The third surface is aspherical. 
A.sub.1 =0.975539.times.10.sup.-2 
A.sub.2 =0.159788.times.10.sup.-1 
A.sub.3 =-0.839908.times.10.sup.-2 
A.sub.4 =-0.703569.times.10.sup.-3 
The stop diaphragm surface is positioned at 0.0407 before the ninth 
surface. 
______________________________________ 
Surface No. 
h .alpha. --h --.alpha. 
______________________________________ 
0.000 -1.000 
1 1.000 0.226 -0.799 -1.180 
2 0.979 0.157 -0.690 -1.132 
3 0.979 0.532 -0.686 -1.395 
4 0.964 -0.652 -0.648 -0.599 
5 1.166 -0.596 -0.463 -0.622 
6 1.185 -1.205 -0.443 -0.393 
7 1.420 -0.135 -0.366 -0.670 
8 1.428 -0.727 -0.325 -0.535 
Diaphram surface 
1.870 -0.727 0.000 -0.535 
9 1.899 0.148 0.022 -0.525 
10 1.875 0.376 0.107 -0.512 
11 1.841 2.186 0.153 -0.362 
12 1.715 1.665 0.174 -0.414 
13 1.559 1.103 0.213 -0.491 
14 1.464 -0.391 0.255 -0.751 
15 1.493 -0.481 0.310 -0.770 
16 1.522 0.345 0.356 -0.576 
17 1.520 0.399 0.359 -0.564 
18 1.495 1.000 0.394 -0.405 
______________________________________ 
f.sub.1 = 1.376 
l.sub.W = 0.6480 
L = 4.014 
.phi..sub.3 = 0.062397 
.DELTA.IV = 0.02814 
.DELTA.V = -0.01972 
EXAMPLE 2 
______________________________________ 
1 : 3.5 f = 1.0-1.46 .omega. = 42.0.degree.-31.3.degree. 
Surface No. r d N .upsilon. 
______________________________________ 
1 2.9614 0.1422 
1.58913 
61.1 
2 9.1414 0.0041 
3 1.8778 0.0488 
1.80610 
40.9 
first 4 0.6423 0.3088 
lens 5 7.1790 0.0528 
1.67790 
50.7 
group 6 1.2023 0.1881 
7 1.1365 0.1219 
1.80518 
25.4 
8 2.2346 0.6412 
9 1.7117 0.2832 
1.80610 
40.9 
10 -5.6534 0.0926 
11 0.8377 0.1016 
1.80610 
40.9 
12 2.6212 0.0913 
second 13 -2.1878 0.1593 
1.84666 
23.9 
lens 14 0.8525 0.0731 
group 15 -8.3985 0.0914 
1.51633 
64.1 
16 -0.9428 0.0041 
17 -34.2275 0.0943 
1.51633 
64.1 
18 -1.1567 
______________________________________ 
______________________________________ 
focal length of the overall 
lens system d.sub.8 
______________________________________ 
1.00 0.6412 
1.20 0.3652 
1.46 0.1209 
______________________________________ 
The seventh surface is aspherical. 
A.sub.1 =0.252119.times.10.sup.-1 
A.sub.2 =0.104986 
A.sub.3 =0.200580 
A.sub.4 =-0.338823 
The stop diaphragm is arranged at 0.0488 before the ninth surface. 
______________________________________ 
Surface No. 
h .alpha. -h --.alpha. 
______________________________________ 
0.000 -1.000 
1 1.000 0.199 -0.788 -1.157 
2 0.982 0.136 -0.684 -1.113 
3 0.982 0.557 -0.680 -1.405 
4 0.967 -0.656 -0.642 -0.599 
5 1.169 -0.546 -0.457 -0.642 
6 1.186 -1.215 -0.437 -0.396 
7 1.415 -0.212 -0.363 -0.652 
8 1.429 -0.727 -0.318 -0.539 
Diaphram Sur- 
face 1.860 -0.727 0.000 -0.538 
9 1.895 0.165 0.026 -0.525 
10 1.869 0.432 0.109 -0.510 
11 1.829 2.192 0.156 -0.360 
12 1.706 1.668 0.176 -0.414 
13 1.554 1.066 0.214 -0.497 
14 1.462 -0.385 0.257 -0.752 
15 1.490 -0.477 0.312 -0.771 
16 1.519 0.355 0.358 -0.575 
17 1.517 0.332 0.360 -0.580 
18 1.497 1.000 0.397 -0.403 
______________________________________ 
f.sub.1 = -1.375 
l.sub.W = 0.6412 
L = 3.995 
.phi..sub.7 = 0.16240 
.DELTA.IV = 0.04284 
.DELTA.V = -0.01099 
EXAMPLE 3 
______________________________________ 
1 : 3.5 f = 1.0-01.46 w = 42.0.degree.-31.3.degree. 
Surface No. r d N .upsilon. 
______________________________________ 
1 2.9645 0.1546 
1.58913 
61.1 
2 12.7739 0.0041 
3 2.0214 0.0488 
1.80610 
40.9 
first 4 0.6757 0.3091 
lens 5 14.6011 0.0529 
1.67790 
50.7 
group 6 1.1185 0.1737 
7 1.0523 0.1114 
1.80518 
25.4 
8 2.1592 0.6516 
9 1.7350 0.3063 
1.80610 
40.9 
10 -8.4074 0.0549 
11 0.8439 0.1041 
1.79952 
42.2 
12 3.3875 0.0927 
second 13 -2.3504 0.1594 
1.84666 
23.9 
lens 14 0.8386 0.0732 
group 15 -9.5985 0.0915 
1.51633 
64.1 
16 -1.1917 0.0041 
17 30.7196 0.1301 
1.51633 
64.1 
18 -0.9926 
______________________________________ 
______________________________________ 
focal length of the overall 
lens system d.sub.8 
______________________________________ 
1.00 0.6516 
1.20 0.3739 
1.46 0.1280 
______________________________________ 
The eighteenth surface is aspherical. 
A.sub.1 =0.634163.times.10.sup.-1 
A.sub.2 =0.109390 
A.sub.3 =0.470527 
A.sub.4 =-5.122549 
The stop diaphragm is arranged at 0.0651 before the ninth surface. 
______________________________________ 
Surface No. 
h .alpha. --h --.alpha. 
______________________________________ 
0.000 -1.000 
1 1.000 0.199 -0.801 -1.159 
2 0.981 0.153 -0.688 -1.127 
3 0.980 0.544 -0.683 -1:400 
4 0.965 -0.607 -0.645 -0.630 
5 1.153 -0.554 -0.451 -0.651 
6 1.170 -1.263 -0.430 -0.390 
7 1.390 -0.200 -0.362 -0.667 
8 1.402 -0.723 -0.321 -0.548 
Diaphragm 
surface 1.826 -0.723 0.000 -0.548 
9 1.873 0.148 0.036 -0.531 
10 1.848 0.325 0.126 -0.519 
11 1.830 2.059 0.154 -0.373 
12 1.711 1.655 0.176 -0.414 
13 1.558 1.094 0.214 -0.492 
14 1.463 -0.384 0.257 -0.751 
15 1.491 -0.464 0.312 -0.768 
16 1.519 0.194 0.358 -0.612 
17 1.518 0.220 0.360 -0.606 
18 1.500 1.000 0.412 -0.392 
______________________________________ 
f.sub.1 = -1.384 
l.sub.W = 0.6516 
L = 4.022 
.PHI..sub.18 = -0.26195 
.DELTA.IV = -0.1000 
.DELTA.V = -0.02747 
EXAMPLE 4 
______________________________________ 
1 : 4.0 f = 1.0-1.40 .omega.= 47.3.degree.-37.7.degree. 
Surface No. r d N .upsilon. 
______________________________________ 
1 2.5388 0.2441 
1.48749 
70.1 
2 6.5644 0.0049 
3 1.6844 0.0879 
1.80400 
46.6 
first 4 0.6835 0.4150 
lens 5 -9.7648 0.0732 
1.80400 
46.6 
group 6 1.2853 0.1992 
7 1.3068 0.1367 
1.80518 
25.4 
8 3.8921 0.5273 
9 6.6455 0.2529 
1.83400 
37.2 
10 -2.7428 0.1270 
11 1.6557 0.1499 
1.83400 
37.2 
12 27.9064 0.0859 
13 -0.9111 0.1445 
1.84666 
23.9 
14 -1.6620 0.0327 
second 15 -2.4266 0.1108 
1.50048 
65.9 
lens 16 -1.1002 0.0273 
group 17 -16.1119 0.1382 
1.84666 
23.9 
18 1.4403 0.0923 
19 -5.2884 0.1465 
1.50048 
65.9 
20 -0.9789 0.0049 
21 -4.5065 0.1465 
1.51821 
65.0 
22 -2.2867 
______________________________________ 
______________________________________ 
focal length of the overall 
lens system d.sub.8 
______________________________________ 
1.00 0.5273 
1.20 0.2227 
1.40 0.0226 
______________________________________ 
The third surface is aspherical. 
A.sub.1 =0.409896.times.10.sup.-1 
A.sub.2 =0.593034.times.10.sup.-2 
A.sub.3 =-0.167327.times.10.sup.-1 
A.sub.4 =0.157683.times.10.sup.-1 
The stop diaphragm surface is arranged at 0.0684 before the eleventh 
surface. 
______________________________________ 
Surface No. 
h .alpha. -h --.alpha. 
______________________________________ 
0.000 -1.000 
1 1.000 0.193 -1.020 -1.197 
2 0.968 0.121 -0.824 -1.135 
3 0.968 0.585 -0.819 -1.528 
4 0.940 -0.526 -0.745 -0.647 
5 1.157 -0.622 -0.478 -0.608 
6 1.182 -1.365 -0.453 -0.323 
7 1.452 -0.465 -0.389 -0.564 
8 1.487 -0.775 -0.347 -0.492 
9 1.872 -0.539 -0.103 -0.505 
10 1.960 0.061 -0.020 -0.511 
Diaphragm 
surface 1.957 0.061 0.000 -0.511 
11 1.954 1.051 0.030 -0.496 
12 1.869 0.994 0.070 -0.498 
13 1.784 -0.673 0.113 -0.603 
14 1.836 0.268 0.160 -0.521 
15 1.827 -0.111 0.176 -0.558 
16 1.835 0.729 0.217 -0.459 
17 1.816 0.633 0.230 -0.471 
18 1.768 -0.413 0.265 -0.627 
19 1.806 -0.585 0.322 -0.658 
20 1.863 0.373 0.386 -0.459 
21 1.861 0.589 0.389 -0.414 
22 1.805 1.000 0.428 -0.317 
______________________________________ 
f.sub.1 = -1.298 
l.sub.W = 0.5273 
L = 4.952 
.PHI..sub.3 = 0.26365 
.DELTA.IV = 0.1657 
.DELTA.V = -0.1402