Large-aperture wide-angle lens system for photographic cameras

A large-aperture wide-angle lens system for photographic cameras with corrected flare due to coma of marginal pencil comprising a flare stop arranged in the vicinity of an intersecting point between the maximum aperture ray of paraxial pencil and the outermost ray of offaxial pencil directing to the marginal portion of image.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
The present invention relates to a retrofocus type large-aperture 
wide-angle lens system for photographic cameras, and more specifically to 
a lens system comprising a flare stop serving to limit flare due to coma 
of offaxial oblique pencil which is likely to be produced in photographing 
at the maximum aperture. 
(b) Description of the Prior Art 
In conventional retrofocus type large-aperture wide-angle lens systems, 
effective aperture of the offaxial oblique pencils was larger than the 
effective diameter of the paraxial maximum aperture on the incidence side. 
Therefore, it was impossible to limit flare due to coma with the effective 
aperture of the lens systems alone when the aperture stop was used in open 
condition. This phenomenon is apt to occur especially in lens systems 
having short total lengths and unavoidably degrades quality of 
photographed images. 
SUMMARY OF THE INVENTION 
A general object of the present invention is to provide a large-aperture 
wide-angle lens system for photographic cameras comprising a flare stop 
which limits flare due to coma by limiting the offaxial oblique pencil. 
An ordinary retrofocus type large-aperture wide-angle lens system consists, 
as shown in FIG. 1, a front diverging lens group L.sub.1, a front 
converging lens group L.sub.2 and a rear converging lens group L.sub.3. An 
aperture stop S is arranged in the airspace formed between the front 
converging lens group L.sub.2 and the rear converging lens group L.sub.3. 
Of the effective pencils passing through the lens system, the paraxial 
pencil directing to the center y.sub.0 of image has a maximum aperture ray 
1.sub.0 (paraxial ray allowed to pass by the maximum aperture stop) whose 
position is determined by the stop S. The marginal pencil (offaxial pencil 
corresponding to the marginal portion y.sub.2 of the image) has a lowest 
ray 1.sub.2 whose position is determined by the aperture of the front lens 
group. In a large-aperture wide-angle lens system, the effective aperture 
of the front lens group is therefore determined by the ray 1.sub.2 of the 
marginal oblique pencil rather than the paraxial pencil. 
In such a large-aperture wide-angle lens system, however, quantity of the 
rays of the pencil which passes within the range of the effective aperture 
to reach an intermediate portion y.sub.1 of image is generally larger than 
required. In an extreme case, aperture efficiency may exceed 100% as 
indicated by curve (1) in FIG. 2 when the aperture stop is opened. This 
phenomenon is likely to occur especially in a lens system having a short 
total length, in which flare due to coma will be aggravated by excessive 
quantity of lower rays of the offaxial pencil as shown in FIG. 3A. In 
order to reduce flare due to coma, it is therefore necessary to limit 
incidence of the lower rays to such a degree as shown in FIG. 3B. However, 
such a measure will unavoidably degrades the aperture efficiency as shown 
in FIG. 3B. Hence, it is desired to reduce flare due to coma while 
controlling degradation in aperture efficiency to the possible minimum 
level, but it will be impossible to satisfy such a desire simply by 
minimizing the effective aperture of a lens system. 
The lens system according to the present invention is characterized in that 
it comprises a flare stop E which is arranged in the vicinity of the 
intersecting point between the ray 1.sub.0 of the paraxial pencil and the 
lowest ray 1.sub.2 of the marginal oblique pencil, and has an effective 
aperture equal to the distance as measured from the optical axis to said 
intersecting point. In other words, the stop E serves for interrupting the 
lowest ray 1.sub.1 of the zonal oblique pencil directing to the 
intermediate image portion y.sub.1, thereby passing lower ray 1 while 
interrupting excessive ray. If the stop E is arranged on the object side 
of the intersecting point between the rays 1.sub.0 and 1.sub.2, ray 
1.sub.2 of the marginal oblique pencil is interrupted, resulting in 
remarkable insufficiency in quantity of the marginal rays. If the stop is 
arranged on the image side of the intersecting point between the rays 
1.sub.0 and 1.sub.2, in contrast, the paraxial pencil will be interrupted, 
thereby reducing the aperture ratio. The vicinity of said intersecting 
point is the position optimum for arranging said stop and most effective 
for limiting flare due to coma without aggravating other aberrations. 
However, a large-aperture wide-angle lens system must comprise a large 
number of lens components and have a short total length, within which 
airspace is naturally limited. It is therefore impossible in most cases to 
arrange the flare stop E in an airspace. From the viewpoints of machining 
precision and effective aperture, it is also impossible to manufacture 
very thin lens components. Therefore, the intersecting point between the 
rays 1.sub.0 and 1.sub.2 is located, in relatively numerous cases, within 
a lens component belonging to the front converging lens group L.sub.2 as 
shown in FIG. 4, thereby making it impossible to arrange the stop E at its 
optimum position. This problem can be solved by dividing said lens 
component into two elements in the vicinity of the intersecting point 
between the rays 1.sub.0 and 1.sub.2, and arranging said stop E in the 
airspace formed by dividing said lens component. In such a case, said lens 
component should preferably be so divided as not to aggravate aberrations 
and so as to facilitate to arrange said stop E. The lens elements formed 
by dividing said lens component and arranged on both sides of said stop E 
may have flat or curved surfaces having large radii of curvature r and r' 
which may be equal or different and refractive indices which may be equal 
or different. 
The stop E may not be arranged in airspace formed between two separate lens 
elements. Speaking concretely, the stop E can be arranged as a thin 
ring-shaped stop E fitted between the surfaces r and r' which are cemented 
after splitting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the large-aperture wide-angle lens system for 
photographic cameras has such a composition as shown in FIG. 7. That is to 
say, the lens system consists of eight components of ten lens elements and 
has the following numerical data: 
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f = 100 
Aperture ratio: 
1:2 
Field angle: 
92.degree. 
r.sub.1 = 205.67 
d.sub.1 = 18.60 
n.sub.1 = 1.639 
.nu..sub.1 = 44.9 
r.sub.2 =529.30 
d.sub.2 = 0.60 
r.sub.3 = 101.53 
d.sub.3 = 6.28 
n.sub.2 = 1.734 
.nu..sub.2 = 51.5 
r.sub.4 = 52.977 
d.sub.4 = 23.26 
r.sub.5 = 227.95 
d.sub.5 = 5.63 
n.sub.3 = 1.7725 
.nu..sub.3 = 49.6 
r.sub.6 = 62.233 
d.sub.6 = 10.70 
r.sub.7 = 640.47 
d.sub.7 = 15.86 
n.sub.4 = 1.61659 
.nu..sub.4 = 36.6 
r.sub.8 = .infin. 
d.sub.8 = 3.26 = d 
r.sub.9 = .infin. 
d.sub.9 = 23.26 
n.sub.5 = 1.61659 
.nu..sub.5 = 36.6 
r.sub.10 = -2015.3 
d.sub.10 = 0.47 
r.sub.11 = 110.65 
d.sub.11 = 6.98 
n.sub.6 = 1.6968 
.nu..sub.6 = 55.5 
r.sub.12 = 68.419 
d.sub.12 = 36.28 
n.sub.7 = 1.5934 
.nu..sub.7 = 34.8 
r.sub.13 = -170.19 
d.sub.13 = 17.67 
r.sub.14 = -1046.5 
d.sub.14 = 17.58 
n.sub.8 = 1.72 
.nu..sub.8 = 43.7 
r.sub.15 = -88.372 
d.sub.15 = 6.98 
n.sub.g = 1.84666 
.nu..sub.9 = 23.9 
r.sub.16 = 212.47 
d.sub.16 = 10.00 
r.sub.17 = 232.47 
d.sub.17 = 13.95 
n.sub.10 = 1.713 
.nu..sub.10 = 53.9 
r.sub.18 = -88.093 
d.sub.18 = 0.47 
r.sub.19 = 459.95 
d.sub.19 = 13.95 
n.sub.11 = 1.72 
.nu..sub.11 = 50.2 
r.sub.20 = 199.86 
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wherein the reference symbols r.sub.1 through r.sub.20 represent radii of 
curvature on the respective surfaces of said lens elements, the reference 
symbols d.sub.1 through d.sub.19 designate the respective thicknesses of 
said lens elements and airspaces formed therebetween, the reference 
symbols .nu..sub.1 through .nu..sub.11 denote Abbe's numbers of the 
respective lens elements and the reference symbol f represents the total 
focal length of the lens system as a whole. 
This lens system comprises a front diverging lens group L.sub.1 comprising 
a first lens component 1, a second lens component 2 and a third lens 
component 3, a front converging lens group L.sub.2 comprising a fourth 
lens component 4 and a fifth lens component 5, 6, and a rear converging 
lens group L.sub.3 comprising a sixth lens component 7, 8, a seventh lens 
component 9 and an eights lens component 10. In this lens system, the 
flare stop E is arranged in the airspace d which is formed by dividing the 
lens component 4 along a flat plane into two lens elements 41 and 42 since 
the ray 1.sub.0 of the paraxial pencil intersects with the ray 1.sub.2 
within the fourth lens component 4 of the front converging lens group. The 
variation in coma obtained by the stop E arranged as described above is 
illustrated in FIG. 8A, FIG. 8B and FIG. 8C respectively. FIG. 8A shows 
coma at marginal portion of the image formed with said lens system. FIG. 
8B illustrates coma which is aggravated when the lens system does not 
comprise the stop E. FIG. 8C shows coma which is corrected when the ray 
1.sub.1 directing to the intermediate portion y.sub.1 of the image is 
limited to the ray 1 by arranging the stop E in said lens system. FIG. 9 
compares aperture efficiency between case (1) where the stop E is not 
arranged and case (2) where the stop E is arranged in said lens system. 
From these figures, it will be clearly understood that the aperture 
efficiency is not degraded so remarkably at the intermediate portion of 
the image. Image contrast at the intermediate portion y.sub.1 of the image 
is illustrated in FIG. 10A and FIG. 10B respectively. The curve (1) 
corresponds to a case where said lens system does not comprise the stop E 
and curve (2) corresponds to a case where the stop E is arranged in said 
lens system. As is clear from these curves, the stop E can effectively 
improve image contrast in low-frequency region. Especially on the 
tangential image plane, contrast is remarkably improved as shown in FIG. 
10B. 
As is proved by the foregoing descriptions, flare due to coma can be 
sufficiently corrected by the stop E though the ray 1.sub.1 of the zonal 
oblique pencil directing to the intermediate portion y.sub.1 of the image 
is limited only slightly to the ray 1 by said stop E. It will therefore be 
understood that remarkable effect is obtainable with the stop E which is 
arranged in the vicinity of the intersecting point between the ray 1.sub.0 
of the paraxial pencil and the lower ray 1.sub.2 of the marginal oblique 
pencil. 
Further, it will be possible to correct flare due to coma of the upper ray 
of the oblique pencil directing to the marginal portion of the image by 
the means similar to that for correcting flare due to coma of the lower 
ray. In other words, flare due to coma of the upper ray of the marginal 
oblique pencil can be limited by arranging a flare stop E' having an 
effective radius equal to the distance as measured from the optical axis 
to the intersecting point between the maximum aperture ray of paraxial 
pencil and upper ray of the marginal pencil at a position in the vicinity 
of said intersecting point.