Retrofocus type wide angle lens

A retrofocus type wide angle lens has, 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 negative meniscus lens having its convex surface facing the object side, a third component which is a negative meniscus lens having its convex surface facing the object side, a fourth component having a cemented surface of negative refractive power convex toward the object surface and having a negative refractive power as a whole, a fifth component which is a biconvex positive lens having a surface of sharper curvature on the object side, a sixth component which is a positive meniscus lens having its convex surface facing the image side, a stop, a seventh component which is a biconcave cemented negative lens comprising a positive meniscus lens having its convex surface facing the image side and a biconcave negative lens cemented thereto, an eighth component which is a positive meniscus lens having its convex surface facing the image side, and a ninth component which is a biconvex positive lens. The retrofocus type wide angle lens satisfies specified conditions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a compact retrofocus type wide angle lens. 
2. Description of the Prior Art 
As wide angle lenses for single lens reflex cameras, use is generally made 
of the so-called retrofocus type lenses which can provide a back focal 
length long as compared with the focal length. However, the construction 
of the retrofocus type lens is asymmetrical with respect to the stop and 
therefore, aberration correction is difficult and the degree thereof 
becomes stronger as the angle of view becomes greater. To meet the 
requirement of interchangeable lenses for compactness and greater aperture 
ratio as the recent tendency, aberration correction tends to be more 
difficult. That is, if the refractive power of the divergent group 
disposed adjacent to the object side is strengthened for the purpose of 
compactness, various aberrations such as spherical aberration and coma 
created in each lens are aggravated, and this tendency becomes stronger 
with an increase in aperture. In a wide angle lens having an angle of view 
of 94.degree., a construction in which the number of negative lenses 
constituting the divergent group is increased is known, for example, from 
British Patent No. 1,238,668 or U.S. Pat. No. 4,348,085, but this 
construction has suffered from a disadvantage that the full length of the 
lens system increases and the forward lens system becomes bulky. In 
addition, it has involved many difficulties under the influence of 
high-order aberrations to correct chromatic difference of magnification 
well over a wide angle of view, and many of the known lenses of this type 
have been insufficient because of the deterioration of the marginal image 
relative to the center of the picture plane or the blur of colors. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a compact retrofocus 
type wide angle lens having a back focal length 1.85 times or more as long 
as the focal length of the entire system and yet having an angle of view 
of the order of 95.degree. and an aperture ratio of 1:2.8 and keeping good 
balance between various aberrations. 
The present invention has found out optimum conditions for aberration 
correction in a unique constructions wherein, for the contrary 
requirements of compactness and a great aperture, an appropriate 
refractive power is provided to the divergent group and as the components 
thereof, a cemented negative lens having a relatively great center 
thickness and having its convex surface facing the object side and having 
a cemented surface of negative refractive power and having a long focal 
length is disposed and a positive lens of strong refractive power is 
disposed on the object side of a stop and a cemented negative lens having 
a great center thickness is disposed on the image side of the stop.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The retrofocus type wide angle lens of the present invention, as shown, for 
example, in FIG. 1 which shows the construction of a lens according to an 
embodiment of the present invention, has, in succession from the object 
side, a first component L1 which is a positive meniscus lens having its 
convex surface facing the object side, a second component L2 which is a 
negative meniscus lens having its convex surface facing the object side, a 
third component L3 which is a negative meniscus lens having its convex 
surface facing the object side, a fourth component L4 convex toward the 
object side and having a cemented surface of negative refractive power and 
having a negative refractive power as a whole, a fifth component L5 which 
is a cemented or single biconvex positive lens having a surface of sharper 
curvature on the object side, a sixth component L6 which is a positive 
meniscus lens having its convex surface facing the image side, a stop S, a 
seventh component L7 which is a biconcave cemented negative lens 
comprising a positive meniscus lens having its convex surface facing the 
image side and a biconcave negative lens cemented thereto, an eighth 
component L8 which is a positive meniscus lens having its convex surface 
facing the image side, and a ninth component L9 which is a biconvex 
positive lens, and satisfies the following conditions: 
(1) -0.9f&lt;f.sub.14 &lt;-0.6f 
(2) 0.35f&lt;D.sub.4 &lt;0.53f 
(3) -1.3f&lt;r15&lt;-0.9f 
(4) -0.9&lt;(r21+r20)/(r21-r20)&lt;0.2 
(5) 23&lt;.nu..sub.2 &lt;40 
where f is the combined focal length of the entire system, f.sub.14 is the 
combined focal length of the first component L1 to the fourth component 
L4, D.sub.4 is the combined center thickness of the fourth component L4, 
r15, is the radius of curvature of the object side lens surface of the 
seventh component L7, r20 and r21 are the radii of curvature of the object 
side and image side lens surfaces, respectively, of the ninth component 
L9, and .nu..sub.2 is the abbe number of the second component L2. 
Formula (1) above prescribes the combined refractive power of the first to 
fourth components which together constitute a divergent lens group. To 
maintain good aberration balance while securing a predetermined back focal 
length, the lens construction of the divergent group and its appropriate 
refractive power distribution are important. In a divergent group having a 
strong negative refractive power to secure a long back focal length, coma 
tends to occur in addition to negative distortion and positive curvature 
of image field. In the present invention, as described above, the 
divergent group is comprised of a positive meniscus lens disposed as the 
first component most adjacent to the object side, and subsequently the 
second and third components which are two negative meniscus lenses, and 
the fourth component which is a negative cemented lens of relatively small 
refractive power, and enables good correction of various aberrations to be 
accomplished by the refractive power distribution as indicated in formula 
(1). If the combined focal length of the first to fourth components as the 
divergent group exceeds the upper limit of formula (1), the back focal 
length can be made longer while, on the other hand, the refractive power 
of each surface will become stronger and negative distortion and coma can 
no longer be well corrected even by the succeeding lens components. On the 
other hand, if the lower limit of formula (1) is exceeded, the burden of 
each lens component of the divergent group will be reduced and this is 
advantageous to aberration correction, but it well becomes difficult to 
secure a sufficiently long back focal length. 
Condition (2) prescribes the combined center thickness of the fourth 
component L4 as the cemented negative lens positioned most rearwardly in 
the divergent group, and is particularly for well accomplishing the 
flattening of the image plane and the correction of distortion. If the 
upper limit of this condition is exceeded, it will be advantageous to the 
flattening of the image plane and the correction of distortion, but will 
result in an increase in the aperture of the forward lens and it will 
become difficult to achieve the compactness of the lens system. On the 
other hand, if the lower limit of this condition is exceeded, it will 
result in the curvature of the sagittal image plane and at the same time, 
the correction of distortion will become insufficient. 
It is important to provide in the fourth component a cemented surface 
convex toward the object side and having a negative refractive power, and 
it is desirable to satisfy the following condition: 
(6) 0.1f&lt;(n.sub.H -n.sub.L)/r.sub.c &lt;0.4f, 
where r.sub.c is the radius of curvature of this cemented surface, and 
n.sub.H and n.sub.L are the refractive indices of the lenses on the object 
side and the image side, respectively, of this cemented surface. If the 
upper limit of this condition (6) is exceeded, spherical aberration will 
be over-corrected and coma will be aggravated, and if the lower limit of 
this condition is exceeded, the necessity of strengthening the refractive 
powers of the second and third components will arise from the relation 
with condition (1) and therefore, the correction of coma and distortion 
will tend to become difficult. 
Now, considering the convergent group succeeding to the above-described 
divergent group, the light beam caused to diverge by the divergent group 
is converged by the fifth and sixth components of strong positive 
refractive power as the forward components of the convergent group, and 
enters the seventh component which is a biconcave cemented negative lens 
disposed immediately rearwardly of the stop. Here, the action of the 
object side surface r15 of the seventh component which receives the 
converged light beam from the sixth component is important. That is, the 
seventh component lies near the stop S and has the function of correcting 
spherical aberration under-corrected by the sixth component and at the 
same time, securing the back focal length sufficiently long and assisting 
the work of the divergent group. If the value of the radius of curvature 
r15 of this surface departs from the lower limit of condition (3), the 
burden of negative refractive power will become excessively great on the 
image side surface of the seventh component and this will result in an 
increase in coma. On the other hand, if the upper limit of condition (3) 
is exceeded, spherical aberration will be over-corrected and it will be 
difficult to realize a light specification of an aperture ratio of 1:2.8. 
Also, it is effective to increase the combined center of thickness of the 
seventh component in order to reduce the burden of the both refracting 
surfaces of the seventh component, but increasing said center thickness 
excessively would be not preferably in the working of the lens and in 
addition, would increase the coma in the sagittal direction. Therefore, 
when the center thickness of the positive meniscus lens and the biconcave 
negative lens which together constitute the seventh component are d15 and 
d16, respectively, it is desirable that the sum of them satisfy the 
following condition: 
(7) 0.2f&lt;d15+d16&lt;0.4f 
Condition (4) prescribes the shape of the ninth component as the biconvex 
positive lens positioned most adjacent to the image side, and is important 
for well maintaining the balance between the aberrations of the on-axis 
light beam and the off-axis light beam. That is, if the upper limit of 
this condition is exceeded, r20 and r21 will become equal to each other 
and this is disadvantageous to secure the back focal length sufficiently 
and at the same time, distortion will increase in the negative direction. 
On the other hand, if the lower limit of this condition is exceeded, the 
burden of the image side surface of the ninth component will increase and 
therefore, spherical aberration will be under-corrected. 
In addition to the above-described construction, to well correct chromatic 
aberration, particularly, chromatic difference of magnification, it is 
necessary to give heed to the following point. In a retrofocus type wide 
angle lens, the chromatic difference of magnification for g-line 
(.omicron.=436 nm) is generally under-corrected in the intermediate zone 
of the picture plane and over-corrected in the most marginal zone of the 
picture plane and has the so-called bending of chromatic difference of 
magnification, and this tendency becomes more remarkable as the angle of 
view becomes greater. Ths inventor has found that it is appropriate to 
form the first component, which is a positive meniscus lens, of low 
dispersion glass of great Abbe number in order to eliminate this 
disadvantage, and in addition, use the high dispersion glass as indicated 
by condition (5) above for the Abbe number of the second component as a 
negative meniscus lens. That is, the bending of chromatic difference of 
magnification can be corrected by greatly under-correcting the high-order 
chromatic difference of magnification created by the first component of 
positive refractive power, by the second component of negative refractive 
power which is next to the first component in the incidence height of the 
oblique light beam. In that case, the low-order chromatic difference of 
magnification is also under-corrected and therefore, in order to well 
correct the on-axis aberration and chromatic difference of magnification 
in the entire system, according to the present invention, at least one of 
the fourth and fifth components which are low in the incidence height of 
the oblique light beam is made into a cemented lens. If the upper limit of 
condition (5) is exceeded, the correction of the bending of chromatic 
difference of magnification will become insufficient, and if the lower 
limit of condition (5) is exceeded, chromatic difference of magnification, 
including the low-order one, will become under-corrected and it will 
become difficult to keep balance in the entire system. 
To correct chromatic difference of magnification better, it is desirable 
that the cemented surface in the seventh component L7 adjacent to the 
image side of the stop be made convex toward the image side and the radius 
of curvature r16 of this cemented surface satisfy the following condition: 
(8) -1.0f&lt;r16&lt;-0.5f 
If the upper limit of this condition is exceeded, spherical aberration and 
coma of g-line will become over-corrected, and if the lower limit of this 
condition is exceeded, it will become difficult to correct the bending of 
chromatic difference of magnification. 
In each of the first to fourth embodiments, as shown in FIG. 1, the fourth 
component is comprised of a biconvex positive lens, a biconcave negative 
lens and a positive lens cemented together, the seventh component is 
comprised of a positive meniscus lens and a biconcave negative lens 
cemented together, and each of the remaining components is comprised of a 
single lens. 
Tables 1 to 4 below show the numerical data of the first to fourth 
embodiments, respectively. In each of these tables, the numbers at the 
left end represent the order from the object side, and the refractive 
indices and Abbe numbers are for d-line (.lambda.=587.6 nm). Bf represents 
the back focal length. 
TABLE 1 
______________________________________ 
(First Embodiment) 
Focal length f = 100 F-number 2.8 
Angle of view 2.omega. = 94.degree. 
Radius of Center thickness 
Refractive 
Abbe 
curvature & spacing index number 
No. r d n .nu. 
______________________________________ 
1 194.441 27.481 1.62041 60.4 L.sub.1 
2 496.499 0.491 
3 135.475 6.870 1.78470 26.1 L.sub.2 
4 54.154 19.629 
5 95.250 5.889 1.71300 54.0 L.sub.3 
6 50.692 18.648 
7 375.014 15.703 1.75520 27.6 
8 -147.513 6.379 1.77279 49.4 L.sub.4 
9 58.690 24.536 1.62588 35.6 
10 .infin. 0.491 
11 161.670 16.194 1.67270 32.2 L.sub.5 
12 -462.413 7.852 
13 -441.788 15.703 1.58267 46.5 L.sub.6 
14 -80.115 16.194 
15 -112.362 22.083 1.77279 49.4 
16 -69.443 5.398 1.78470 26.1 L.sub.7 
17 264.223 4.034 
18 -389.780 10.796 1.62041 60.4 L.sub.8 
19 -85.215 0.491 
20 472.174 13.740 1.62041 60.4 L.sub.9 
21 -153.303 
______________________________________ 
Bf = 186.3 
f.sub.14 = -0.737 f 
TABLE 2 
______________________________________ 
(Second Embodiment) 
Focal Length f = 100 F-number 2.8 
Angle of view 2.omega. = 96.degree. 
Radius of Center thickness 
Refractive 
Abbe 
curvature & spacing index number 
No. r d n .nu. 
______________________________________ 
1 161.771 20.589 1.59319 67.9 L.sub.1 
2 435.296 0.490 
3 131.378 4.412 1.79504 28.6 L.sub.2 
4 51.473 16.667 
5 96.904 4.412 1.71700 48.1 L.sub.3 
6 51.255 17.158 
7 490.216 14.216 1.74077 27.6 
8 -117.652 7.353 1.74810 52.3 L.sub.4 
9 52.943 25.001 1.61293 37.0 
10 -637.281 0.980 
11 150.986 17.648 1.64831 33.8 L.sub.5 
12 -833.367 6.618 
13 -562.581 17.648 1.62041 60.3 L.sub.6 
14 -80.395 14.706 
15 -109.318 20.589 1.79668 45.5 
16 -83.337 5.392 1.78472 25.8 L.sub.7 
17 229.911 4.902 
18 -455 901 11.275 1.60311 60.7 L.sub.8 
19 -82.580 0.490 
20 647.085 13.726 1.62041 60.4 L.sub.9 
21 -156.504 
______________________________________ 
Bf = 190.6 
F.sub.14 = -0.756 f 
TABLE 3 
______________________________________ 
(Third Embodiment) 
Focal length f = 100 F-number 2.8 
Angle of view 2.omega. = 96.degree. 
Radius of Center thickness 
Refractive 
Abbe 
curvature & spacing index number 
No. r d n .nu. 
______________________________________ 
1 158.340 22.060 1.58913 61.2 L.sub.1 
2 409.746 0.490 
3 127.456 4.412 1.80518 25.4 L.sub.2 
4 52.943 16.667 
5 98.876 4.412 1.71300 54.0 L.sub.3 
6 49.861 17.158 
7 441.194 15.197 1.74000 28.3 
8 -105.396 6.373 1.74810 52.3 L.sub.4 
9 52.453 25.001 1.62004 36.3 
10 -980.431 0.980 
11 149.516 17.648 1.62004 36.3 L.sub.5 
12 -490.215 6.373 
13 -378.198 17.648 1.61720 54.0 L.sub.6 
14 -79.366 17.648 
15 -110.298 17.158 1.78797 47.5 
16 -78.434 5.392 1.78470 26.1 L.sub.7 
17 229.911 4.902 
18 -441.194 13.236 1.56384 60.8 L.sub.8 
19 -80.886 0.490 
20 416.683 13.726 1.59319 67.9 L.sub.9 
21 -149.978 
______________________________________ 
Bf = 188.8 
F.sub.14 = -0.751 f 
TABLE 4 
______________________________________ 
(Fourth Embodiment) 
Focal length f = 100 F-number 2.8 
Angle of view 2.omega. = 95.degree. 
Radius of Center thickness 
Refractive 
Abbe 
curvature & spacing index number 
No. r d n .nu. 
______________________________________ 
1 196.079 24.510 1.62280 57.0 L.sub.1 
2 509.128 0.490 
3 135.784 5.882 1.78472 25.8 L.sub.2 
4 54.412 18.137 
5 94.921 5.882 1.71300 54.0 L.sub.3 
6 50.735 18.137 
7 357.843 15.686 1.7400 28.3 
8 -117.647 6.863 1.76684 46.8 L.sub.4 
9 54.902 24.510 1.64831 33.8 
10 4411.767 0.490 
11 160.784 16.176 1.68893 31.1 L.sub.5 
12 -441.177 7.353 
13 -418.058 16.176 1.62041 60.3 L.sub.6 
14 -82.787 13.235 
15 -111.765 23.039 1.77279 49.4 
16 -79.902 6.863 1.78472 25.8 L.sub.7 
17 256.373 3.922 
18 -367.647 10.784 1.62041 60.4 L.sub.8 
19 -84.101 0.490 
20 563.726 13.725 1.62041 60.4 L.sub.9 
21 -149.932 
______________________________________ 
Bf = 185.5 
F.sub.14 = -0.744 f 
In a fifth embodiment of the present invention, as shown in FIG. 2, the 
fourth component L.sub.4 is comprised of a negative meniscus lens having 
its convex surface facing the object side and a positive lens cemented 
thereto, the fifth component L.sub.5 is comprised of a negative meniscus 
lens having its convex surface facing the object side and a positive lens 
cemented thereto, and the seventh component L.sub.7 is constructed as a 
cemented lens similar to that in the first embodiment. 
In a sixth embodiment of the present invention, as shown in FIG. 3, the 
fourth component L.sub.4 is comprised of a negative meniscus lens having 
its convex surface facing the object side, a biconvex lens and a negative 
meniscus lens having its convex surface facing the image side, these three 
lenses being cemented together, and the construction of the seventh 
component L.sub.7 is similar to the construction of the seventh component 
in the first embodiment. 
Tables 5 and 6 below show the numerical data of the fifth and sixth 
embodiments, respectively. 
TABLE 5 
______________________________________ 
(Fifth Embodiment) 
Focal length f = 100 F-number 2.8 
Angle view 2.omega. = 95.degree. 
Radius of Center thickness 
Refractive 
Abbe 
curvature & spacing index number 
No. r d n .nu. 
______________________________________ 
1 193.628 27.451 1.59319 67.9 L.sub.1 
2 495.588 0.490 
3 134.804 6.373 1.74950 35.2 L.sub.2 
4 53.431 19.608 
5 94.608 6.373 1.71700 48.1 L.sub.3 
6 50.490 18.627 
7 377.451 17.157 1.74443 49.5 
8 53.922 32.353 1.62606 39.2 L.sub.4 
9 .infin. 0.490 
10 168.137 4.902 1.69350 53.8 
11 72.549 17.157 1.68893 31.1 L.sub.5 
12 -514.706 5.882 
13 -500.351 11.765 1.56384 60.8 L.sub.6 
14 -76.961 17.157 
15 -100.000 20.098 1.74810 52.3 
16 -75.000 5.392 1.78472 25.8 L.sub.7 
17 248.530 3.922 
18 -392.157 11.275 1.67025 57.6 L.sub.8 
19 -90.686 0.490 
20 357.843 13.235 1.56384 60.8 L.sub.9 
21 -132.401 
______________________________________ 
Bf = 186.0 
F.sub.14 = -0.773 f 
TABLE 6 
______________________________________ 
(Sixth Embodiment) 
Focal length f = 100 F-number 2.8 
Angle of view 2.omega. = 95.degree. 
Radius of Center thickness 
Refractive 
Abbe 
curvature & spacing index number 
No. r d n .nu. 
______________________________________ 
1 159.297 18.628 1.61720 54.0 L.sub.1 
2 403.963 0.490 
3 128.712 4.412 1.78470 26.1 L.sub.2 
4 52.053 16.667 
5 98.213 4.412 1.69680 55.6 L.sub.3 
6 50.694 17.158 
7 539.236 14.706 1.77279 49.4 
8 50.492 28.923 1.62588 35.6 L.sub.4 
9 -78.434 4.902 1.59319 67.9 
10 -612.769 0.980 
11 156.869 16.177 1.64831 33.8 L.sub.5 
12 -689.600 7.353 
13 -489.629 16.177 1.61266 44.4 L.sub.6 
14 -79.234 15.197 
15 -102.945 22.060 1.77279 49.4 
16 -69.611 5.392 1.78472 25.8 L.sub.7 
17 230.401 4.412 
18 -445.107 10.295 1.60311 60.7 L.sub.8 
19 -82.241 0.490 
20 784.344 13.726 1.59319 67.9 L.sub.9 
21 -134.003 
______________________________________ 
Bf = 188.5 
f.sub.14 = -0.800 f 
It is apparent that any of these embodiments, in spite of having a wide 
angle of view of 94.degree. to 96.degree. and a long back focal length, 
has various aberrations corrected well and has a very excellent imaging 
performance. In each of the above-described embodiments, the deterioration 
of aberrations such as astigmatism and curvature of image field occurring 
when the lens is focused to a short distance object can be well corrected 
by moving the entire lens system forwardly and at the same time, moving 
the first to fifth components as a unit so as to reduce the spacing d12 
with respect to the succeeding lens component, as disclosed in Japanese 
Patent Publication No. 39875/1970.