A large-aperture telephoto lens comprising seven lens elements in six lens components, the first lens component having a positive lens, the second lens component having a doublet composed of a negative-meniscus lens with its concave side facing an image and a positive-meniscus lens, the third lens component having a negative-meniscus lens with its concave side facing the image, the fourth lens having a meniscus lens with its concave side facing an object, the fifth lens component having a positive-meniscus lens with a surface of a larger curvature facing the object, and the sixth lens component having a positive lens.

BACKGROUND OF THE INVENTION 
The present invention relates to a large-aperture telephotographic lens of 
high performance having F.sub.NO of 1:1.8-2, and an angle of view on the 
order of 18.degree.. 
One generally used telephoto lens having an angle of view on the order of 
18.degree. is known as an Ernostar-type telephoto lens. This type of 
telephoto lens has improved aberrations normally at F.sub.NO of 1:2.8. 
However, with telephoto lenses having larger apertures such as F.sub.NO of 
1:1.8-2, the spherical aberration is increased and is overly compensated 
for with respect to light having short wavelengths (for example, in the 
vicinity of g-line 436 nm). There are known large-diameter telephoto 
lenses which have coped with such problems by thickening front-group 
lenses or using a doublet in the front lens group. However, the prior 
telephoto lenses have proven unsatisfactory in performance. Furthermore, 
since large-aperture lenses are subjected to increased variations in 
various aberrations dependent on the positions of objects, it has been 
quite difficult to keep required high performance over a wide range from 
the infinite distance to close positions. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a large-diameter, 
relatively compact telephoto lens of high performance which will eliminate 
the foregoing difficulties.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described in detail with reference to its 
preferred embodiments. In the following description and in the appended 
claims, a "lens component" may include either a single lens or a plurality 
of lenses grouped together. 
According to the present invention, there is provided a large-aperture 
telephoto lens comprising seven lens elements in six lens components, the 
first lens component having a positive lens L.sub.1, the second lens 
component having a doublet composed of a negative-meniscus lens L.sub.2 
with its surface concave side facing an image and a positive-meniscus lens 
L.sub.3, the third lens component having a negative-meniscus lens L.sub.4 
with its surface concave side facing the image, the fourth lens component 
having a meniscus lens L.sub.5 with its concave side facing an object, the 
fifth lens component having a positive-meniscus lens L.sub.6 with a 
surface of a larger curvature facing the object, and the sixth lens 
component having a positive lens L.sub.7. The large-aperture telephoto 
lens is constructed to meet the following requirements: 
##EQU1## 
EQU .vertline.n.sub.2 -n.sub.3 .vertline.&lt;0.1, .nu..sub.3 -.nu..sub.2 &gt;10, 
0.2f&lt;r.sub.4 &lt;0.35f; (2) 
EQU 0.35f&lt;f.sub.1, 2, 3 &lt;0.55f; (3) 
EQU .vertline.f.sub.5 .vertline.&gt;4f, 0.2f&lt;-r.sub.8 &lt;0.4f; (4) 
EQU 0.35f&lt;r.sub.10 &lt;f; and (5) 
EQU 0.25&lt;f/f.sub.7 &lt;0.45 (6) 
where 
n.sub.i : the refractive index of the ith lens along d-line; 
.nu..sub.i : the Abbe number of the ith lens; 
f : the focal length of the overall lens system; 
r.sub.i : the radius of curvature of the ith surface; 
f.sub.1, 2, 3 : the composite focal length of the first through third 
lenses; and 
f.sub.i : the focal length of the ith lens. 
The above requirements will now be described. 
The requirement (1) relates to a material of the positive lenses in the 
front lens components (the first through third lens components), and is 
concerned with achromatic and Petzval's conditions. If (n.sub.1 
+n.sub.3)/2 in the requirement (1) were smaller than 1.60, then the 
Petzval's sum would be increased, the curvature of an image plane would 
become excessive, the radius of curvature of each of the surfaces of the 
positive lenses would be increased, and various aberrations such as 
spherical aberration would become larger. If (.nu..sub.1 +.nu..sub.3)/2 
were smaller than 50, then chromatic aberration caused by the positive 
lenses would be excessive and could not be compensated for properly. 
The requirement (2) is directed to the doublet of the second lens 
component. According to the present invention, the doublet employed as the 
second lens component compensates to a high degree for spherical 
aberration with respect to wavelengths in the vicinity of the g-line. By 
determining the refractive index, the Abbe number, and the radius of 
curvature r.sub.4 of a material of the doublet according to the 
requirement (2), a hyperchromatic surface can be provided in the doublet 
for independent control of chromatic aberrations caused by the first and 
second lens components. If the value .vertline.n.sub.2 -n.sub.3 .vertline. 
in the requirement (2) were greater than 0.1, then the hyperchromatic 
effect would be reduced resulting in difficulty to correct the chromatic 
aberrations independently. If .nu..sub.3 -.nu..sub.2 were smaller than 10, 
then chromatic correction at the hyperchromatic surface, especially 
spherical aberration correction with respect to light having short 
wavelengths could not be effective. If r.sub.4 were below the lower limit, 
chromatic aberration would become excessive, and there would be difficulty 
in fabricating the lens. If r.sub.4 exceeded the upper limit, then 
chromatic aberration could not be corrected sufficiently, and spherical 
aberration with respect to short-wavelength light would be corrected quite 
excessively. 
The requirement (3) is concerned with the combined focal length of the 
first and second lens components, and is required to render the lens 
smaller in size. If the combined focal length f.sub.1, 2, 3 were smaller 
than the lower limit of the range of the requirement (3), the lens could 
be smaller in size, but the entrance height of incident light falling on 
the third lens component would be reduced and the third lens component 
would have excessive negative power, so that spherical aberration and coma 
of higher order would be caused, distortion would become excessive, and 
correction for the fourth lens component and the succeeding lens 
components would be rendered difficult to carry out. If f.sub.1, 2, 3 were 
larger than the upper limit, then the power of each lens would be reduced 
making it effective to correct aberrations, but the lens would be larger 
in size. 
The requirement (4) is directed to the fourth lens component. The fourth 
lens has no large power, but serves to correct spherical aberration and 
coma of higher order which have been generated and left by the lens 
components up to the third lens component as the fourth lens component has 
a surface having a large radius of curvature which is concentric with 
respect to aperture. More specifically, by maintaining .vertline.f.sub.5 
.vertline. above 4f, the eighth surface r.sub.8 is given a large radius of 
curvature as defined by the requirement (4) to cause aberration of higher 
order at the eighth and ninth surfaces without affecting aberration of 
lower order, thus cancelling out remaining aberration generated up to the 
third lens component. If -r.sub. were smaller than the lower limit of the 
requirement (4), then aberration of higher order would overly be 
generated, and coma would particularly be caused with respect to rays of 
light travelling outside the optical axis. If -r.sub.8 were greater than 
the upper limit, then the remaining aberration of higher order up to the 
third lens component could not be compensated for, and hence the object of 
the fourth lens component could not be achieved. 
The requirement (5) is needed for correcting distortion. With the lens 
system according to the present invention, spherical aberration, coma and 
the like which have produced by the front lens components or the first 
through third lens components are substantially corrected up to the fourth 
lens component. However, distortion still remains uncorrected. By 
determining the radius of curvature r.sub.10 of the surface of the fifth 
lens component which faces the object as defined by the requirement (5), 
the lens system has a sufficiently low distortion. If r.sub.10 were larger 
than the upper limit of the requirement (5), then no distortion could be 
corrected, and if r.sub.10 were smaller than the lower limit, then 
distortion would be corrected overly. 
The present invention provides a large-aperture telephoto lens having 
improved aberrations by meeting the above requirements. Variations in 
aberrations when an object is moved become manifest themselves especially 
where the lens has a general overall lens movement system. As an example, 
FIG. 2 illustrative of aberrations caused by a lens according to the first 
Example when the object is at infinite distance, and FIG. 4 is 
illustrative of aberrations caused at a magnification of about -1/10 with 
the lens of first Example controlled by an overall lens movement system. 
Study of these figures clearly indicates worse aberrations. 
The present invention eliminates the problem of worse aberrations by 
employing a lens movement system in which the six lens component is fixed 
and the first through fifth lens components are movable upon focusing. 
More specifically, aberrations can be improved over a wide range from the 
infinite distance to close locations by defining the focal length f.sub.7 
of the fixed sixth lens component according to the requirement (6). If 
f/f.sub.7 were smaller than the lower limit of the range of the 
requirement (6), then no aberration could be corrected with respect to an 
object at a close distance, and f/f.sub.7 were larger than the upper 
limit, then the first through fifth lens components would have to be moved 
an increased interval, resulting in an elongated lens barrel, and 
aberrations with respect to an object at a short distance would be 
corrected excessively. 
Examples of the present invention will be described. 
EXAMPLE 1 
______________________________________ 
f = 135, FNO 1:2, 2.omega. = 18.degree. 
f.sub.1,2,3 = 0.436 f, f.sub.5 = 10.04 f, f/f.sub.7 = 0.330 
______________________________________ 
r.sub.1 
64.573 d.sub.1 
11.45 n.sub.1 1.61800 
.nu..sub.1 63.4 
L.sub.1 
r.sub.2 
3710.762 d.sub.2 
0.10 
r.sub.3 
50.679 d.sub.3 
4.05 n.sub.2 1.74950 
.nu..sub.2 35.3 
L.sub.2 
r.sub.4 
38.886 d.sub.4 
10.07 n.sub.3 1.72916 
.nu..sub.3 54.7 
L.sub.3 
r.sub.5 
94.734 d.sub.5 
3.68 
r.sub.6 
383.622 d.sub.6 
10.16 n.sub.4 1.68893 
.nu..sub.4 31.1 
L.sub.4 
r.sub.7 
29.564 d.sub.7 
26.73 
r.sub.8 
-41.519 d.sub.8 
3.55 n.sub.5 1.80518 
.nu..sub.5 25.4 
L.sub.5 
r.sub.9 
-41.500 d.sub.9 
16.70 
r.sub.10 
70.634 d.sub.10 
4.05 n.sub.6 1.70000 
.nu..sub.6 48.1 
L.sub.6 
r.sub.11 
119.936 d.sub.11 
4.05 
r.sub.12 
250.369 d.sub.12 
3.54 n.sub.7 1.70000 
.nu..sub.7 48.1 
L.sub.7 
r.sub.13 
1996.195 
______________________________________ 
EXAMPLE 2 
______________________________________ 
f = 135, FNO 1:1.8, 2.omega. = 18.degree. 
f.sub.1,2,3 = 0.476 f, f.sub.5 = 5.57 f, f/f.sub.7 = 0.318 
______________________________________ 
r.sub.1 
71.398 d.sub.1 
12.90 n.sub.1 1.62041 
.nu..sub.1 60.3 
L.sub.1 
r.sub.2 
4020.868 d.sub.2 
0.10 
r.sub.3 
52.330 d.sub.3 
4.82 n.sub.2 1.72342 
.nu..sub.2 37.9 
L.sub.2 
r.sub.4 
31.309 d.sub.4 
15.0 n.sub.3 1.69680 
.nu..sub.3 55.5 
L.sub.3 
r.sub.5 
94.161 d.sub.5 
4.05 
r.sub.6 
461.054 d.sub.6 
8.17 n.sub.4 1.67270 
.nu..sub.4 32.1 
L.sub.4 
r.sub.7 
30.267 d.sub.7 
34.38 
r.sub.8 
-41.362 d.sub.8 
4.37 n.sub.5 1.70154 
.nu..sub.5 41.2 
L.sub.5 
r.sub.9 
-40.027 d.sub.9 
6.87 
r.sub.10 
71.985 d.sub.10 
4.05 n.sub.6 1.72000 
.nu..sub.6 46.0 
L.sub.6 
r.sub.11 
143.722 d.sub.11 
4.0 
r.sub.12 
248.646 d.sub.12 
4.0 n.sub.7 1.70000 
.nu..sub.7 48.1 
L.sub.7 
r.sub.13 
1506.638 
______________________________________ 
FIGS. 2 and 6 are illustrative of aberrations of the lenses according to 
Examples 1 and 2 with an object at infinite distance. The large-aperture 
telephoto lenses having F.sub.NO of 1:2-F.sub.NO of 1:1.8 have 
sufficiently small aberrations. FIGS. 3 and 7 illustrate aberrations of 
the lenses according to Examples 1 and 2 at a magnification of -1/10 with 
d.sub.11 being 20.64 and 20.42. FIGS. 4 and 8 show aberrations of the 
lenses in which the first through fifth lens components are moved to 
provide the same magnification. With the lens movement system according to 
the present invention, aberrations will become worse to a small degree, 
and the lens has high performance over a wide range from the infinite 
distance to short distances.