Compact wide angle lens

A compact wide angle lens comprises, in succession from the object side, a first component which is a positive meniscus lens having its convex surface facing the object side, a second component which is a biconcave lens, a third component which is a positive lens having its surface of sharper curvature facing the object side, a fourth component which is a positive lens, and a fifth component which is a negative meniscus lens having its convex surface facing the image side.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to improvements in a wide angle lens having an angle 
of view of 60.degree.-70.degree. and an F-number of the order of 2.8. 
2. Description of the Prior Art 
In order to shorten the total length of a lens (the distance from the 
foremost surface of the lens to the focal plane thereof), it is known to 
adopt a so-called telephoto type optical system comprising a convergent 
forward group and a divergent rearward group. A wide angle lens using this 
technique is disclosed in Japanese Laid-open Patent Application No. 
76147/1979 (corresponding U.S. Pat. No. 4,204,747), and this is surely 
shorter in total length than former wide angle lenses and succeeds in 
shortening the total length to the same degree as the focal length. On the 
other hand, however, it is a wide angle lens having a large angle of view 
and, therefore, the aperture of the lenses in the rearward group, 
particularly the lens most adjacent to the image side, is very large. Such 
a wide angle lens cannot be said to be compact, and it has other 
disadvantages. That is, if the aperture of the rearward lens is large, a 
large space will be required in a camera body to which such lens is 
mounted, and this is disadvantageous in making the camera compact. 
Further, the marginal light beam may be forced to possibly promote the 
disadvantage of a wide angle lens in which the quantity of marginal light 
is originally small. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a more compact wide 
angle lens which has a total length as short as the focal length thereof 
and yet in which the aperture of the rearward lens is small. 
The invention will become fully apparent from the following detailed 
description thereof taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The wide angle lens according to the present invention employs a negative 
lens as the lens located most adjacent to the image side while basically 
employing the telephoto type system, to thereby enable a decrease in the 
lens aperture and to sufficiently well correct the correspondingly 
occurring aberrations such as distortion, etc. by satisfying certain 
conditions. Specifically, the wide angle lens of the present invention 
has, in succession from the object side, a first component L.sub.1 which 
is a positive meniscus lens having its convex surface facing the object 
side, a second component L.sub.2 which is a biconcave lens, a third 
component L.sub.3 which is a positive lens having its surface of sharper 
curvature facing the object side, a fourth component L.sub.4 which is a 
positive lens, and a fifth component L.sub.5 which is a negative meniscus 
lens having its convex surface facing the image side. Desirably, the wide 
angle lens satisfies the following conditions: 
EQU 0.57f&lt;f.sub.1 &lt;0.74f (1) 
EQU 0.51f&lt;f.sub.4 &lt;1.38f (2) 
EQU d.sub.8 &lt;0.15f (3) 
EQU 0.02f&lt;d.sub.6 &lt;0.12f (4) 
where f represents the total focal length of the entire system, f.sub.1 and 
f.sub.4 represent the focal lengths of the first component L.sub.1 and the 
fourth component L.sub.4, respectively, d.sub.6 represents the air space 
between the third component L.sub.3 and the fourth component L.sub.4, and 
d.sub.8 represents the air space between the fourth component L.sub.4 and 
the fifth component L.sub.5. A diaphragm may desirably be provided between 
the second component L.sub.2 and the third component L.sub.3 or between 
the third component L.sub.3 and the fourth component L.sub.4. 
Generally, to shorten the total length of a lens, as the aforementioned 
telephoto type is typical, it is effective to intensify the refractive 
powers (the inverse number of the focal length) of the positive lens 
located adjacent to the object side and the negative lens located adjacent 
to the image side. In this regard, in the construction of the present 
invention, the first component L.sub.1, which is a positive meniscus lens 
having its convex surface facing the object side, and the fifth component 
L.sub.5, which is a negative meniscus lens having its convex surface 
facing the image side, are closely related to each other with respect to 
the distribution of refractive power. From this point of view, condition 
(1) prescribes the focal length f.sub.1 of the first component L.sub.1. 
Where the refractive power of the first component L.sub.1 is weakened, the 
refractive power of the fifth component L.sub.5 must be intensified to 
keep the total length of the lens short, and, if the refractive power of 
the first component L.sub.1 becomes smaller than the upper limit of 
condition (1), the various aberrations will greatly increase in the fifth 
component L.sub.5 and it will become difficult to correct the various 
aberrations in a well-balanced manner. As the refractive power of the 
first component L.sub.1 is intensified, the total length of the lens can 
be made shorter, but if the refractive power becomes greater than the 
lower limit of condition (1), the negative spherical aberration caused by 
the first component L.sub.1 will increase too much to enable it to be 
sufficiently well corrected even by the other components. 
In the construction of the present invention, the diaphragm is disposed 
between the second and the third component or between the third and the 
fourth component and, in any case, the principal ray of oblique light rays 
is subjected to the converging action of the first component L.sub.1. 
Therefore, positive distortion occurs here, and after the principal ray 
has passed through the diaphragm, the distortion will increase in the 
positive direction due to the diverging action of the fifth component 
L.sub.5, thus resulting in excessively large positive distortion. 
Conditions (2) and (3) are provided to correct this. Condition (2) is 
concerned with the focal length f.sub.4 of the fourth component L.sub.4, 
which is a positive lens, and it is for correcting the positive distortion 
by causing negative distortion to occur due to the converging action of 
this component. But if the upper limit of this condition is exceeded, the 
converging action will become small and sufficient correction will become 
impossible. Also, the function of the fourth component L.sub.4 is to make 
the incidence position of the light beam to the fifth component L.sub.5 
near to the optical axis due to the converging action thereof and to 
reduce the diverging action imparted by the fifth component, as well as to 
keep the aperture of the fifth component small. From this point of view, 
it is disadvantageous if the upper limit of condition (2) is exceeded. 
Conversely, if the lower limit of condition (2) is departed from, the 
refractive power of the fourth component L.sub.4 will become strong and, 
in order to provide a desired converging action as to the entire lens 
system under condition (1), the converging action of the third component 
L.sub.3 must be reduced. By this, the principal point of the entire system 
will be moved toward the image side and necessarily the total length of 
the lens will become large. Also, in this case, large negative spherical 
aberration will occur, which may be corrected by weakening the refractive 
power of the third component L.sub.3. But doing so will likewise result in 
an increased total length of the lens, which is contrary to the object of 
the present invention. 
Condition (3) is concerned with the air space d.sub.8 between the fourth 
component L.sub.4 and the fifth component L.sub.5 and it is for correcting 
distortion in combination with said condition (2). The light beam exitting 
from the fourth component L.sub.4 is subjected to the converging action of 
this component, but oblique light rays are in a direction away from the 
optical axis. Therefore, if the air space d.sub.8 becomes large, the 
incidence positive to the fifth component L.sub.5 will become high and the 
diverging action imparted by this component tends to become strong. If the 
air space d.sub.8 exceeds condition (3), the diverging action in the fifth 
component L.sub.5 will become unnecessarily large and it will become 
difficult to correct distortion. Further, in this case, the effective 
diameter of the fifth component L.sub.5 will become excessively large, 
contrary to the object of the present invention. Accordingly, a smaller 
value for this air space d.sub.8 is more advantageous. However, since the 
fifth component L.sub.5 is a negative meniscus lens and the surface 
thereof adjacent to the fourth component L.sub.4 is a concave surface of 
considerably sharp curvature, there is a danger that the fifth component 
may contact the fourth component L.sub.4 in the marginal portion thereof. 
Therefore, it is desirable that the air space d.sub.8 be greater than 
0.05f. 
Condition (4) is for prescribing the air space d.sub.6 between the third 
component L.sub.3 and the fourth component L.sub.4 to enable good 
correction of the curvature of the image field. If the upper limit of this 
condition is exceeded, the meridional image plane will be sharply curved 
positively, and if the lower limit of this condition is departed from, the 
meridional image plane will be sharply curved negatively. In any case, 
even if the meridional image plane can be corrected by the other 
components, the astigmatic difference will become large and it will be 
difficult to obtain a good image plane. In the present invention, high 
dispersion flint glass is used for the second component L.sub.2, and the 
forward three components L.sub.1, L.sub.2 and L.sub.3 have a construction 
similar to that of the conventional triplet or tessar type lens, but the 
combined focal length f.sub.12 of the first component L.sub.1 and the 
second component L.sub.2 should desirably be designed to .infin. or 
negative, namely, to a touch of divergence. According to this, the 
diverging action of the second component L.sub.2 can be intensified and 
the correction of the chromatic aberration of the entire lens system 
according to the present invention can be accomplished more effectively. 
Embodiments of the present invention will hereinafter be described. A first 
embodiment of the present invention, as shown in the lens construction 
view of FIG. 1, has a diaphragm between the second component L.sub.2 and 
the third component L.sub.3. A second embodiment shown in FIG. 3 has a 
diaphragm between the third component L.sub.3 and the fourth component 
L.sub.4 and in other respects, it is substantially similar to the first 
embodiment. A third embodiment, as shown in FIG. 5, has a diaphragm 
between the second and the third component similarly to the first 
embodiment, but a cemented surface is provided on the third component 
L.sub.3 to thereby further improve the chromatic aberration. 
Numerical data of each embodiment will be shown below, and the various 
aberrations in the first, second and third embodiments are illustrated in 
FIGS. 2A-2C, 4A-4C and 6A-6C, respectively. In the tables below, r 
represents the radius of curvature of each lens surface, d represents the 
center thicknesses of and the air spaces between the respective lenses, n 
and .nu. represent the refractive index and the Abbe number, respectively, 
of each lens, and the subscripts represent the order from the object side. 
______________________________________ 
Focal length f = 100 F 2.8 Angle of view 62.degree. 
______________________________________ 
First Embodiment 
r.sub.1 = 33.422 
d.sub.1 = 8.570 
n.sub.1 = 1.77279 
.nu..sub.1 = 49.4 
L.sub.1 
r.sub.2 = 86.715 
d.sub.2 = 5.713 
r.sub.3 = -193.623 
d.sub.3 = 2.857 
n.sub.2 = 1.78470 
.nu..sub.2 = 26.1 
L.sub.2 
r.sub.4 = 44.440 
d.sub.4 = 4.571 
r.sub.5 = 38.296 
d.sub.5 = 9.998 
n.sub.3 = 1.62374 
.nu..sub.3 = 47.0 
L.sub.3 
r.sub.6 = 108.753 
d.sub.6 = 4.285 
r.sub.7 = 92.359 
d.sub.7 = 11.141 
n.sub.4 = 1.80740 
.nu..sub.4 = 35.4 
L.sub.4 
r.sub.8 = -131.181 
d.sub.8 = 12.283 
r.sub.9 = -22.687 
d.sub.9 = 2.857 
n.sub.5 = 1.78797 
.nu..sub.5 = 47.5 
L.sub.5 
r.sub.10 = -42.682 
.SIGMA.d = 62.274 
Back focal length: Bf = 40.415 
Total length: T.L = 102.689 = 10.27f 
f.sub.1 = 65.8 = 0.658f 
f.sub.4 = 68.7 = 0.687f 
f.sub.12 = -539.6 
Second Embodiment 
r.sub.1 = 32.280 
d.sub.1 = 8.570 
n.sub.1 = 1.77279 
.nu..sub.1 = 49.4 
L.sub.1 
r.sub.2 = 76.797 
d.sub.2 = 5.713 
r.sub.3 = -203.021 
d.sub.3 = 2.857 
n.sub.2 = 1.78470 
.nu..sub.2 = 26.1 
L.sub.2 
r.sub.4 = 42.069 
d.sub.4 = 4.571 
r.sub.5 = 36.999 
d.sub.5 = 9.998 
n.sub.3 = 1.62374 
.nu..sub.3 = 47.0 
L.sub.3 
r.sub.6 = -8133.457 
d.sub.6 = 9.141 
r.sub.7 = 100.449 
d.sub.7 = 5.428 
n.sub.4 = 1.80740 
.nu..sub.4 = 35.4 
L.sub.4 
r.sub.8 = -592.361 
d.sub.8 = 10.284 
r.sub.9 = 22.130 
d.sub.9 = 2.857 
n.sub.5 = 1.78797 
.nu..sub.5 = 47.5 
L.sub.5 
r.sub.10 = -40.439 
.SIGMA.d = 59.417 
Back focal length: Bf = 40.387 
Total length: T.L = 99.804 = 0.998f 
f.sub.1 = 66.5 = 0.665f 
f.sub.4 = 106.7 = 1.067f 
f.sub.12 = -388.0 
Third Embodiment 
r.sub.1 = 32.565 
d.sub.1 = 8.570 
n.sub.1 = 1.78100 
.nu..sub.1 = 44.5 
L.sub.1 
r.sub.2 = 74.220 
d.sub.2 = 5.713 
r.sub.3 = -204.935 
d.sub.3 = 2.857 
n.sub.2 = 1.78472 
.nu..sub.2 = 25.7 
L.sub.2 
r.sub.4 = 42.997 
d.sub.4 = 4.571 
r.sub.5 = 38.167 
d.sub.5 = 2.857 
n.sub.3 = 1.62374 
.nu..sub.3 = 47.0 
L.sub.3 
r'.sub.5 = 30.280 
d'.sub.5 = 7.142 
n.sub.4 = 1.62230 
.nu.'.sub.3 = 53.1 
r.sub.6 = -2093.039 
d.sub.6 = 7.999 
r.sub.7 = 105.514 
d.sub.7 = 8.570 
n.sub.5 = 1.79504 
.nu..sub.4 = 28.4 
L.sub.4 
r.sub.8 = -458.112 
d.sub. 8 = 10.284 
r.sub.9 = -22.159 
d.sub.9 = 2.857 
n.sub.6 = 1.78797 
.nu..sub.5 = 47.5 
L.sub.5 
r.sub.10 = -39.381 
.SIGMA.d = 61.417 
Back focal length: Bf = 40.223 
Total length: T.L = 101.640 = 1.016f 
f.sub.1 = 68.1 = 0.681f 
f.sub.4 = 108.6 = 1.086f 
f.sub.12 = -374.9 
______________________________________ 
According to the present invention as described above, a more compact wide 
angle lens can be provided which has the total length thereof kept as 
short as the focal length thereof and yet in which the effective diameter 
of the lens most adjacent to the image side is 0.46f to 0.48f, which is 
considerably smaller than 0.56f to 0.58f which is the effective diameter 
of the lens most adjacent to the image side in the conventional wide angle 
lens disclosed in Japanese Laid-open Patent Application No. 76147/1979 
(corresponding U.S. Pat. No. 4,204,747), and which has a sufficiently good 
imaging performance in practical use.