Wide-angle lens system

A wide-angle lens system which has a long back focal length comprising a first lens unit having a negative power, a second lens unit having a positive power and a third lens unit having a positive power: the first lens unit being composed of a 1-1 positive lens unit which is composed only of a positive lens component and a 1-2 negative lens unit which is composed only of negative lens components, the third lens unit being composed of a 3-1 subunit which comprises at least a negative lens component and a 3-2 positive subunit which comprises at least a negative lens component, the 3-1 subunit being composed of a first negative lens component and a second positive lens component, and the 3-2 subunit being composed of a third negative lens component and a fourth positive lens component.

BACKGROUND OF THE INVENTION 
a) Field of the Invention 
The present invention relates to a wide-angle lens system which has a field 
angle on the order of 50.degree. to 90.degree., an F number on the order 
of 4.0 or high brightness and a long back focal length, and is optimum for 
use in the electronic cameras and video cameras which use image pickup 
tubes and solid-state image pickup devices. 
b) Description of the Prior Art 
For the electronic cameras and video cameras which use image pickup tubes 
and image pickup devices, it is necessary to dispose optical members such 
as low pass filters and infrared cut filters between lens systems and 
surfaces of the image pickup devices. Therefore, lens systems to be used 
in these cameras must have back focal lengths which are long as compared 
with focal lengths thereof. 
In case of an image pickup system which uses a color separating optical 
system for picking up three colors of R, B and G with a plurality of image 
pickup devices to improve qualities of colored images, it is necessary to 
interpose optical elements such as mirrors and prisms for splitting an 
optical path in addition to the optical elements such as the low pass 
filter, whereby a lens system must have a longer back focal length. 
Lens systems disclosed by Japanese Patents Kokai Publication No. Sho 
63-149618, Kokai Publication No. Hei 1-61714 and Kokai Publication No. Hei 
4-118612 are known as conventional examples of wide-angle lens systems 
which have field angles not smaller than 50.degree. and back focal lengths 
long enough for interposing optical elements such as mirrors and prisms 
for splitting optical paths. 
In the recent years where progress has been made in manufacturing 
technologies, image pickup devices which are used in electronic cameras 
and video cameras have been shifted from image pickup tubes mainly to 
solid-state image pickup devices. Solid-state image pickup devices which 
have a large number of pixels are available. 
Accordingly, cameras which use solid-state image pickup devices are now 
usable for printing purposes though these cameras were not used 
conventionally for a reason that images obtained with these cameras were 
lower in quality than those of images obtained with silver salt cameras. 
However, it is difficult to manufacture a compact image pickup device even 
with the recent manufacturing technologies since it requires pixels in a 
number equal or larger than that of pixels specified by standards for 
televisions such as highvision which forms highly minute images. 
Therefore, electronic cameras are being developed by arranging larger 
numbers of pixels on enlarged image pickup devices without changing a size 
of each pixel. 
However, when enlarged image pickup devices can be manufactured from a 
single wafer which has a definite area and require a higher manufacturing 
prime cost. Accordingly, attempts have been made to accomplish both a 
compact configuration and a reduction of manufacturing costs at the same 
time by reducing the size of pixels and developing an image pickup device 
on which a large number of pixels are arranged. 
However, the reduction in a size of pixels to be arranged on an image 
pickup device results in enhancement of the so-called Nyquist frequency, 
thereby requiring a photographic lens system which has extremely high 
optical performance. 
On the other hand, the electronic cameras and video cameras which use 
electronic image pickup devices require an optical system having a long 
back focal length, thereby obliging to use a retrofocus type optical 
system which has a negative-positive power distribution in order from the 
object side, or a negative-positive power distribution asymmetrical with 
regard to a stop. As a result, the optical system can hardly correct 
offaxial aberrations such as distortion and astigmatism, a paraxial light 
bundle is made a diverging light bundle by a front lens group and 
spherical aberration is produced by a rear lens group, thereby making it 
difficult to obtain a bright lens system. 
Above all, a lens system which has a wider photographing field angle has a 
back focal length having a higher ratio relative to a focal length 
thereof, thereby making it necessary to strengthen powers of a negative 
lens group and a positive lens group or widening an airspace between the 
negative lens group and the positive lens group. 
The former method further aggravates aberrations, thereby making it 
difficult to maintain high optical performance of the lens system. 
Further, the latter method enlarges the lens system. In order to obtain a 
lens system which has a relatively compact size, a long back focal length 
and high optical performance, it is therefore necessary to select an 
adequate negative-positive power distribution as well as an adequate value 
of distance between principal points. 
Since problems of color reproducibility, color moire, etc. are more serious 
as an image pickup system is configured to reproduce images of higher 
qualities, a multi-plate camera represented by the so-called three-plate 
camera which picks up the three primary colors RGB with three image pickup 
devices is more desirable than the so-called single-plate camera which 
uses a single image pickup device on which color filters are arranged in a 
mosaic or stripes. 
For this reason, it is necessary to interpose a color separating optical 
system between a photographic lens system and image pickup devices, 
thereby requiring a much longer back focal length and making it more 
difficult to design a photographic lens system. 
Lens systems disclosed by Japanese Patents Kokai Publication No. Sho 
63-149618 and Kokai Publication No. Hei 1-61714 are known as conventional 
examples of photographic lens systems which are used in such a image 
pickup system. These lens systems do not sufficiently correct spherical 
aberration and astigmatism. Though a lens system which is disclosed by 
Japanese Patent Kokai Publication No. Hei 4-118612 which is known as 
another conventional example has remarkably high optical performance, this 
lens system is composed of a remarkably large number of lens elements, 
whereby the lens system is large as compared with a focal length thereof 
and is contrary to the object described above of an image pickup system 
which is configured compact by reducing a size of image pickup devices. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide a wide-angle lens 
system which has a field angle on the order of 50.degree. to 60.degree., 
an F number on the order of 4.0, a back focal length to arrange optical 
elements such as low pass filters and infrared cut filters between image 
pickup devices and the lens system as well as optical path splitting 
members for forming an image of the three primary colors of RGB with a 
plurality of image pickup devices and extremely high optical performance, 
and is optimum for use in electronic cameras, video cameras, etc. which 
use compact image pickup devices on which large numbers of pixels are 
arranged. 
The wide-angle lens system according to the present invention which has a 
first composition consists, in order from the object side, of a first lens 
unit which has a negative power as a whole, a second lens unit which has a 
positive power as a whole and a third lens unit which has a positive power 
as a whole: the first lens unit being composed, in order from the object 
side, of a 1-1 subunit which is composed only of a positive lens component 
or positive lens components and has a positive power, a 1-2 subunit which 
is composed only of a negative lens component or negative lens components 
and has a negative power, and the third lens unit being composed, in order 
from the object side, of a 3-1 subunit which comprises at least a negative 
lens component and a 3-2 subunit which comprises at least a negative lens 
component and has a positive power as a whole, the 3-1 subunit being 
composed, in order from the object side, of a first lens component which 
is composed only of a negative lens element or negative lens elements and 
has a negative power and a second lens component which has a positive 
power as a whole, and the 3-2 subunit being composed, in order from the 
object side, of a third lens component which has a negative power as a 
whole and a fourth lens component which has a positive power as a whole. 
The wide-angle lens system according to the present invention which has a 
second composition consists, in order from the object side, of a first 
lens unit which has a negative power as a whole, a second lens unit which 
has a positive power as a whole and a third lens unit which has a positive 
power as a whole: the first lens unit being composed, in order from the 
object side, of a 1-1 subunit which is composed only of a positive lens 
component or positive lens components and has a positive power and a 1-2 
subunit which is composed only of a negative lens component or negative 
lens components and has a negative power, the third lens unit being 
composed, in order from the object side, of a 3-1 subunit which comprises 
at least a negative lens component and a 3-2 subunit which comprises at 
least a negative lens component and has a positive power as a whole, the 
3-1 subunit being composed, in order from the object side, of a first lens 
component which is composed only of a negative lens component or negative 
lens components and has a negative power, and a second lens component 
which has a positive power as a whole, and the 3-2 subunit being composed, 
in order from the object side, of a third lens component which has a 
positive power as a whole, a fourth lens component which has a negative 
power as a whole and a fifth lens component which has a positive power as 
a whole. 
The wide-angle lens system according to the present invention which has a 
third composition consists, in order from the object side, of a first lens 
unit which has a negative power as a whole, a second lens unit which has a 
positive power as a whole and a third lens unit which has a positive power 
as a whole: the first lens unit being composed, in order from the object 
side, of a 1-1 subunit which is composed only of a positive lens component 
or positive lens components and has a positive power, a 1-2 subunit which 
is composed only of a negative lens component or negative lens components 
and has a negative power, the third lens unit being composed, in order 
from the object side, of a 3-1 subunit which comprises at least a negative 
lens component and a 3-2 subunit which comprises at least a negative lens 
component and has a positive power as a whole, the wide-angle lens system 
satisfying conditions (1) through (6) which are mentioned below: 
(1) 0.2&lt;.vertline.f.sub.1 /f.sub.3 .vertline.&lt;0.8 
(2) 0.01&lt;.vertline.f/f.sub.2 .vertline.&lt;0.7 
(3) 0.01&lt;.vertline.f.sub.1 /f.sub.1-1 .vertline.&lt;0.5 
(4) 0.7&lt;.vertline.f.sub.1-2 /f.sub.1 .vertline.&lt;1.0 
(5) .vertline.f.sub.3 /f.sub.3-1 .vertline.&lt;0.6 
(6) 0.5&lt;.vertline.f.sub.3 /f.sub.3-2 .vertline.&lt;0.95

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The wide-angle lens system according to the present invention which has a 
first composition is characterized in that it consists, in order from the 
object side as shown in FIG. 1 for example, of a first lens unit G1 which 
has a negative power as a whole, a second lens unit G2 which has a 
positive power as a whole and a third lens unit G3 which has a positive 
power as a whole: the first lens unit G1 being composed, in order from the 
object side, of a 1-1 subunit G11 which is composed only of a positive 
lens component or positive lens components and has a positive power and a 
1-2 subunit G12 which is composed only of a negative lens component or 
negative lens components and has a negative power, the third lens unit G3 
being composed, in order from the object side, of a 3-1 subunit G31 which 
comprises at least a negative lens component and a 3-2 subunit G32 which 
has at least a negative lens component and has a positive power, and the 
3-1 subunit G31 of the third lens unit G3 being composed, in order from 
the object side, of a first lens component L31 which is composed only of a 
negative lens component and has a negative power and a second lens 
component L32 which has a positive power as a whole, and the 3-2 subunit 
G32 being composed, in order from the object side, of a third lens 
component L33 which has a negative power as a whole and a fourth lens 
component L34 which has a positive power as a whole. 
When a color separating optical system such as a color separating prisms is 
used, it is general to perform color separations using a dichroic film 
which controls wavelength spectra of a light transmitted for effective use 
of rays. However, it is desirable for obtaining uniform color separations 
within a screen to nearly equalize inclinations and spreads of light 
bundles incident on the color separating optical system at different 
locations within the screen since wavelength spectra which transmit 
through the dichroic film are different dependently on inclinations of 
rays incident on the dichroic film. In other words, it is desirable that 
an offaxial principal ray is incident on the color separating optical 
system at an angle which is the same as that of an optical axis and that 
an offaxial light bundle spreads revolutionally symmetrically with regard 
to the principal ray. It is therefore desirable that a photographic lens 
system has an exit pupil nearly at infinite distance, but color signals 
which are not problematic for practical use can be obtained by disposing a 
white shading correcting means in an electric circuit after receiving the 
light bundle with an image pickup device so far as the photographic lens 
system has an exit pupil at a distance which is adequately long. 
Unlike a silver salt camera or the like, an electronic image pickup device 
allows rays to be imaged on a photoelectric conversion surface of each 
pixel through members such as a color filter disposed at an adequate 
interval from the photoelectric conversion surface and a micro lens for 
condensing rays efficiently onto the photoelectric conversion surface, and 
the photoelectric surface and each of the members are arranged so as to 
correspond at one to one to each other for each pixel. When a center ray 
of a light bundle is incident obliquely onto the image pickup device at an 
inclination angle of incidence which is large as compared with a size of 
each pixel and the interval between the member and the photoelectric 
conversion surface which are arranged as described above, a light bundle 
or a portion thereof which has passed through the member protrudes from 
the photoelectric conversion surface corresponding thereto and does not 
contribute to conversion into an electric signal or incident on another 
pixel, thereby producing non-uniformity in brightness or spurious colors. 
For correcting the defect described above, there is adopted means which 
produces an adequate deviation between the optical member and the pixel 
corresponding thereto dependently on a location of the exit pupil of the 
photographic lens system. Though this means controls angles of emergence 
of rays by locating the location of the exit pupil within an adequate 
range, a location of the exit pupil which is extremely close to an image 
surface imposes stricter restrictions on design and is undesirable for 
manufacturing an image pickup device which is to be used versatilely or 
when photographic lens systems are to be used in exchange with each other. 
For this reason, it is desirable to allow a light bundle to be incident on 
an electronic image pickup device so that its center ray has an 
inclination angle not exceeding an adequate angle. In other words, it is 
desirable that a photographic lens system has an exit pupil which is 
located at an adequately long distance. 
Due to the requisites for the color separating optical system and the 
requirements for the image pickup device described above, it is desirable 
that a photographic lens system has an exit pupil at a position which is 
located at a nearly infinite distance or an adequately long distance and 
it is necessary to adequately strengthen a positive power of a rear lens 
group in case of the retrofocus type. 
An object of the present invention is to provide a lens system which has a 
long back focal length, so that a color separating optical system which 
splits an optical path for a plurality of image pickup devices can be 
disposed between a photogaphic lens system and the image pickup devices as 
shown, for example, in FIG. 1. 
In order to increase the back focal length of a retrofocus type lens system 
which comprises a negative front lens group and a positive rear lens group 
without increasing its total length, it is therefore necessary from a 
simple paraxial formula to strengthen a positive power of a rear lens 
group. 
From viewpoints of any of the characteristics of the color separating 
optical system, the structure of the electronic image pickup device and 
the optical paraxial condition, it is necessary to strengthen a negative 
power of a front lens group and a positive power of a rear lens group in a 
retrofocus type lens system which has the negative-positive power 
distribution. 
Though the present invention has the primary object to provide an optical 
system or an image pickup system optimum for use with multi-plate type 
cameras which pick up images of high quality, or the three primary colors 
RGB with a plurality of image pickup devices, the lens system according to 
the present invention is configured, as described above, so as to have a 
composition which is suited best for use with image pickup systems in 
which optical members such as an optical path splitting prism are disposed 
on the image side of photographic lens systems for performing automatic 
focusing and automatic exposure as discussed above and image pickup 
systems in which optical members requiring long optical paths are disposed 
between photographic lens systems and image surfaces in addition to low 
pass filters and infrared cut filters such as image pickup systems in 
which optical members such as optical path splitting prisms are disposed 
on the image side of photographic lens systems for the so-called 
single-lens type view finders. 
A retrofocus type lens system such as that described above can hardly 
correct offaxial aberrations such as distortion and astigmatism since the 
lens system has a power distribution which is asymmetrical with regard to 
a stop. When a positive rear lens group has a strong power, the lens 
system allows the rear lens group to produce remarkable spherical 
aberration and can hardly correct this aberration since a negative front 
lens group diverges an axial light bundle. As the negative front lens 
group and the positive rear lens group have stronger powers, the lens 
system can correct the aberrations more hardly, aberrations, negative 
distortion which is produced by the front lens group and negative 
spherical aberration which is produced by the rear lens group in 
particular. 
For this reason, the lens system according to the present invention is 
composed, in order from the object side, of the first lens unit G1 which 
has a strong negative power, the second lens unit G2 which has a positive 
power and has a function to lower paraxial rays, and the third lens unit 
G3 which has a positive power and has a function to adjust an imaging 
point and a location of an exit pupil so that aberrations can be corrected 
favorably by the second lens unit having the positive power though the 
lens system is macroscopically retrofocus type which has a 
negative-positive power distribution. 
When the second lens unit G2 which has the positive power is disposed after 
the first lens unit G1, however, it is difficult to prolong a back focal 
length. 
Therefore, the third lens unit G3 of the lens system according to the 
present invention is composed, in order from the object side, of the 3-1 
subunit G31 which comprises at least a negative lens component, and the 
3-2 subunit G32 which comprises at least a negative lens component, and 
has a positive power. It is desirable to configure the 3-1 subunit G31 of 
the third lens unit so as to have a negative power or a positive power 
which is weaker than that of the 3-2 subunit G32 so that the lens system 
has a power distribution close to that of the retrofocus type while 
maintaining a positive power required for imaging. 
Further, the first lens unit G1 which has the negative power produces 
negative distortion and the third lens unit G3 which has the positive 
power also produces negative distortion, thereby tending to aggravate 
negative distortion in the lens system as a whole. This tendency is made 
more remarkable by strengthening the negative power of the first lens unit 
G1 and the positive power of the third lens unit G3. In order to correct 
this negative distortion, it is necessary to produce positive distortion 
by distributing a positive power in the first lens unit G1 or a negative 
power in the third lens unit G3. 
In the lens system according to the present invention which has a wide 
field angle, rays which are to form images at marginal portions of a 
screen are higher on the first lens unit G1 than those on the other lens 
unit, whereby the first lens unit G1 produces remarkable negative 
distortion. Therefore, it is possible to prevent optical performance from 
being degraded due to a relative arrangement error between the first lens 
unit G1 and the subsequent lens units by distributing a positive power in 
the first lens unit G1 to reduce distortion to be produced by the first 
lens unit G1. 
In order to configure the lens system according to the present invention so 
as to have a long back focal length, the first lens unit G1 must have a 
negative power of an adequate strength and it is required that a positive 
power to be distributed in this lens unit is relatively weak but capable 
of correcting distortion sufficiently. It is therefore desirable to 
distribute a positive power at a most object-side location in the first 
lens unit where offaxial rays are highest. 
For this reason, the first lens unit G1 of the lens system according to the 
present invention is composed, in order from the object side, of the 1-1 
subunit G11 which is composed only of a positive lens component or 
positive lens components and has the positive power, and the 1-2 subunit 
G12 which is composed only of a negative lens component or negative lens 
components and has the negative power. 
In order to obtain higher optical performance of the lens system according 
to the present invention by making compatible reservation of a long back 
focal length and correction of aberrations, it is desirable to compose the 
3-1 subunit G31 of the third lens unit G3, in order from the object side, 
of the first lens component L31 consisting only of a negative lens 
component or negative lens components and the second lens component L32 
having a positive power as a whole, and compose the 3-2 subunit G32, in 
order from the object side, of the third lens component L33 having a 
negative power as a whole and the fourth lens component L34 having a 
positive power as a whole. That is to say, it is desirable for maintaining 
a long back focal length to select a retrofocus type lens arrangement for 
each of the 3-1 subunit G31 and the 3-2 subunit G32 of the third lens unit 
G3. 
In order to correct offaxial aberrations such as astigmatism and spherical 
aberration with good balance while maintaining an extremely long back 
focal length in a lens system which has a wide field angle in particular, 
it is desirable to compose a 3-2 subunit of a third lens unit, in order 
from the object side, of a third lens component which has a positive power 
as a whole, a fourth lens component which has a negative power as a whole 
and a fifth lens unit which has a positive power as a whole so that the 
3-2 subunit has a triplet type power distribution. 
Speaking concretely, the lens system according to the present invention 
which has a second composition is characterized in that it consists, in 
order from the object side as shown in FIG. 3 for example, of a first lens 
unit G1 which has a negative power as a whole, a second lens unit G2 which 
has a positive power as a whole and a third lens unit G3 which has a 
positive power as a whole: the first lens unit G1 being composed, in order 
from the object side, of a 1-1 subunit G11 which is composed only of a 
positive lens component or positive lens components and has a positive 
power and a 1-2 subunit G12 which is composed only of a negative lens 
component or negative lens components and has a negative power, the third 
lens unit G3 being composed, in order from the object side, of a 3-1 
subunit G31 which comprises at least a negative lens component, and a 3-2 
subunit G32 which comprises at least a negative lens component and has a 
positive power as a whole, the 3-1 subunit G31 of the third lens unit 
being composed, in order from the object side, of a first lens component 
L31 which is composed only of a negative lens component or negative lens 
components and has a negative power and a second lens component L32 which 
has a positive power as a whole, and the 3-2 subunit G32 of the third lens 
unit being composed, in order from the object side, of a third lens 
component L33 which has a positive power as a whole, a fourth lens 
component L34 which has a negative power as a whole and a fifth lens 
component L35 which has a positive power as a whole. 
The lens system according to the present invention which has a third 
composition consists, in order from the object side, of a first lens unit 
G1 which has a negative power as a whole, a second lens unit G2 which has 
a positive power as a whole and a third lens unit G3 which has a positive 
power as a whole: the first lens unit G1 being composed, in order from the 
object side, of a 1-1 subunit G11 which is composed only of a positive 
lens component or positive lens components and has a positive power and a 
1-2 subunit G12 which is composed only of a negative lens component or 
negative lens components and has a negative power, and the third lens unit 
G3 being composed, in order from the object side, of a 3-1 subunit G31 
which comprises at least a negative lens component, and a 3-2 subunit G32 
which comprises at least a negative lens component and has a positive 
power, and is configured to satisfy conditions (1) through (6) which are 
listed below: 
(1) 0.2&lt;.vertline.f.sub.1 /f.sub.3 .vertline.&lt;0.8 
(2) 0.01&lt;.vertline.f/f.sub.2 .vertline.&lt;0.7 
(3) 0.01&lt;.vertline.f.sub.1 /f.sub.1-1 .vertline.&lt;0.5 
(4) 0.7&lt;.vertline.f.sub.1-2 /f.sub.1 &lt;1.0 
(5) .vertline.f.sub.3 /f.sub.3-1 .vertline.&lt;0.6 
(6) 0.5&lt;.vertline.f.sub.3 /f.sub.3-2 .vertline.&lt;0.95 
wherein the reference symbol f represents a focal length of the lens system 
as a whole, the reference symbols f.sub.1, f.sub.2 and f.sub.3 designate 
focal lengths of the first lens unit G1, the second lens unit G2 and the 
third lens unit G3 respectively, the reference symbols f.sub.1-1 and 
f.sub.1-2 denote focal lengths of the 1-1 subunit G11 and the 1-2 subunit 
G12 respectively of the first lens unit G1, and the reference symbols 
f.sub.1-3 and f.sub.3-2 represent focal lengths of the 3-1 subunit G31 and 
the 3-2 subunit G32 respectively of the third lens unit G3. 
The conditions (1) and (2) mentioned above define a power distribution 
among the first lens unit G1, the second lens unit G2 and the third lens 
unit G3 as a macroscopic power distribution. 
If .vertline.f.sub.1 /f.sub.3 .vertline. has a value which is smaller than 
the lower limit of 0.2 of the condition (1), the third lens unit G3 will 
have an insufficient power, thereby making it difficult to locate an exit 
pupil at a nearly infinite or adequately long distance, and an airspace 
between the first lens unit and the third lens unit must be widened for 
reserving a power required for the lens system as a whole, thereby 
enlarging the lens system. If .vertline.f.sub.1 /f.sub.3 .vertline. has a 
value which is larger than the upper limit of 0.8 of the condition (1), in 
contrast, the first lens unit G1 will have an insufficient negative power, 
thereby making it difficult to reserve a required back focal length in the 
lens system. 
If .vertline.f/f.sub.2 .vertline. has a value which is smaller than the 
lower limit of 0.01 of the condition (2), paraxial rays cannot be 
sufficiently lowered by the second lens unit G2, thereby making it 
difficult to correct aberrations produced by the third lens unit G3 or the 
third lens unit G3 must have a complicated composition to suppress 
aberrations to be produced by this lens unit. Further, such a small value 
of .vertline.f/f.sub.2 .vertline. makes it impossible to correct 
distortion produced by the first lens unit G1 with the second lens unit G2 
and is undesirable for correction of aberrations. 
If .vertline.f/f.sub.2 .vertline. has a value which is larger than the 
upper limit of 0.7 of the condition (2), in contrast, the second lens unit 
G2 will have too strong a power, thereby making it difficult to reserve a 
long back focal length in the lens system. 
Further, it is desirable to use at least two negative lens components each 
having a surface which has a stronger negative power on the image side in 
the 1-2 subunit G12 of the first lens unit, and these two lens components 
may be negative meniscus or biconcave lens components. It is desirable to 
select this composition for the 1-2 subunit G12 so that it has a function 
to cancel astigmatism produced by the 1-1 subunit G11. 
Furthermore, it is desirable to distribute powers between the 1-1 subunit 
and the 1-2 subunit of the first lens unit of the lens system according to 
the present invention so as to satisfy the conditions (3) and (4) 
mentioned above. 
If .vertline.f.sub.1 /f.sub.1-1 .vertline. has a value which is smaller 
than the lower limit of 0.01 of the condition (3), the 1-1 subunit G11 has 
too weak a positive power and undercorrects distortion, whereby remarkable 
negative distortion remains in the first lens unit. If .vertline.f.sub.1 
/f.sub.1-1 .vertline. has a value which is larger than the upper limit of 
0.5 of the condition (3), in contrast, the 1-1 subunit will have too 
strong a positive power, whereby the first lens unit G1 can hardly have a 
negative power which is sufficient for reserving a required back focal 
length. 
If .vertline.f.sub.1-2 /f.sub.1 .vertline. has a value which is smaller 
than the lower limit of 0.7 of the condition (4), the 1-2 subunit G12 will 
have too strong a negative power, thereby making it difficult to correct 
distortion favorably even with the 1-1 subunit G11. If .vertline.f.sub.1-2 
/f.sub.1 .vertline. has a value which is larger than the upper limit of 
1.0 of the condition (4), it will be difficult to obtain a negative power 
of the first lens unit G1 which is sufficient for reserving the required 
back focal length. 
Moreover, it is desirable to distribute powers between the 3-1 subunit G31 
and the 3-2 subunit G32 of the third lens unit of the lens system 
according to the present invention so as to satisfy the conditions (5) and 
(6) mentioned above. 
If .vertline.f.sub.3 /f.sub.3-1 .vertline. has a value which is larger than 
the upper limit of 0.6 of the condition (5), most of a positive power 
required for the third lens unit G3 will be shared by the 3-1 subunit G31 
when the 3-1 subunit G31 has a positive power, thereby making it difficult 
to reserve the required back focal length in the lens system. When the 3-1 
subunit G31 has a negative power, paraxial rays will be high on the 3-2 
subunit G32, thereby undesirably aggravating spherical aberration. 
If .vertline.f.sub.3 /f.sub.3-2 .vertline. has a value which is smaller 
than the lower limit of 0.5 of the condition (6), the 3-2 subunit G32 will 
have a weak positive power and the 3-1 subunit G31 must have a strong 
positive power for maintaining a power required for imaging, thereby 
making it difficult to reserve the required back focal length and 
undesirably aggravating curvature of a meridional image surface. If 
.vertline.f.sub.3 /f.sub.3-2 .vertline. has a value which is larger than 
the upper limit of 0.95 of the condition (6), in contrast, the 3-2 subunit 
G32 will have too strong a positive power, thereby making it difficult to 
balance a back focal length with a location of an exit pupil, and 
degrading imaging performance due to aggravation of spherical aberration 
and coma. 
In addition, it is desirable to adjust a location of an exit pupil with a 
lens component LP which is composed only of a positive lens element or 
positive lens elements and is disposed on the image side in the 3-2 
subunit G32 and configure this lens component LP so as to have a focal 
length f.sub.P which satisfies the following condition (7): 
(7) 0.3&lt;.vertline.f.sub.P /f.sub.3 .vertline.&lt;1.5 
If .vertline.f.sub.P /f.sub.3 .vertline. has a value which is larger than 
the upper limit of 1.5 of the condition (7), the lens component LP will 
have a weak positive power, thereby making it difficult to locate an exit 
pupil at a nearly infinite or adequately long distance. If 
.vertline.f.sub.P /f.sub.3 .vertline. has a value which is smaller than 
the lower limit of 0.3 of the condition (7), in contrast, the lens 
component LP will have too strong a positive power, thereby making it 
difficult to balance a back focal length with a location of an exit pupil, 
and degrading imaging performance due to aggravation of spherical 
aberration and coma. 
It is needless to say that the lens system according to the present 
invention which has the first composition or the second composition can 
have more favorable optical performance when it is configured so as to 
satisfy the conditions (1) and (2), the conditions (3) and (4), the 
conditions (5) and (6) or the condition (7). 
In order to prevent enlargement of the lens system according to the present 
invention which has the first composition, the second composition or the 
third composition, it is desirable to dispose the first lens unit G1, the 
second lens unit G2 and the third lens unit G3 so as to satisfy the 
following conditions (8) and (9): 
(8) 0.5&lt;.vertline.e.sub.12 /f.vertline.&lt;3.5 
(9) 3&lt;.vertline.e.sub.23 /f.vertline.&lt;10 
wherein the reference symbol e.sub.12 and e.sub.23 represent a distance 
between principal points of the first lens unit G1 and the second lens 
unit G2, and a distance between the second lens unit G2 and the third lens 
unit G3 respectively. 
If .vertline.e.sub.12 /f.vertline. has a value which is smaller than the 
lower limit of 0.5 of the condition (8), it will be necessary to 
strengthen the negative power of the first lens unit G1 for reserving the 
required back focal length, thereby undesirably aggravating distortion. If 
.vertline.e.sub.12 /f.vertline. has a value which is larger than the upper 
limit of 3.5 of the condition (8), paraxial rays will be too high on the 
second and subsequent lens units, thereby making it difficult to correct 
remarkable negative spherical aberration. 
If .vertline.e.sub.23 /f.vertline. has a value which is smaller than the 
lower limit of 3 of the condition (9), it will be difficult to obtain a 
wide photographic field angle. If .vertline.e.sub.23 /f.vertline. has a 
value which is larger than the upper limit of 10 of the condition (9), in 
contrast, the lens system will undesirably be enlarged. 
For the lens system according to the present invention which has any of the 
compositions described above, it is more desirable to replace the 
conditions (1) through (4) and the conditions (6) through (9) with the 
following conditions (1-1) through (4-1) and conditions (6-1) through 
(9-1) respectively: 
(1-1) 0.3&lt;.vertline.f.sub.1 /f.sub.3 .vertline.&lt;0.7 
(2-1) 0.03&lt;.vertline.f/f.sub.2 .vertline.&lt;0.6 
(3-1) 0.05&lt;.vertline.f.sub.1 /f.sub.1-1 .vertline.&lt;0.2 
(4-1) 0.8&lt;.vertline.f.sub.1-2 /f.sub.1 .vertline.&lt;0.9 
(6-1) 0.6&lt;.vertline.f.sub.3 /f.sub.3-2 .vertline.&lt;0.85 
(7-1) 0.5&lt;.vertline.f.sub.P /f.sub.3 .vertline.&lt;1.25 
(8-1) 0.5&lt;.vertline.e.sub.12 /f.vertline.&lt;2.8 
(9-1) 4&lt;.vertline.e.sub.23 /f.vertline.&lt;8 
Further, for the lens system according to the present invention which has 
any one of the compositions described above and a relatively narrow field 
angle of 50.degree. to 70.degree. in particular, it is more desirable to 
replace the condition (1), condition (2) and the conditions (5) through 
(7) with the following condition (1-2), condition (2-2) and conditions 
(5-2) through (7-2): 
(1-2) 0.25&lt;.vertline.f.sub.1 /f.sub.3 .vertline.&lt;0.5 
(2-2) 0.2&lt;.vertline.f/f.sub.2 .vertline.&lt;0.7 
(5-2) .vertline.f.sub.3 /f.sub.3-1 .vertline.&lt;0.4 
(6-2) 0.7&lt;.vertline.f.sub.3 /f.sub.3-2 .vertline.&lt;0.85 
(7-2) 0.4&lt;.vertline.f.sub.P /f.sub.3 .vertline.&lt;0.7 
In order to obtain a lens system which has more favorable optical 
performance, it is more desirable to replace the above-mentioned 
conditions (2-2) and (5-2) with the following conditions (2-3) and (5-3) 
respectively: 
(2-3) 0.35&lt;.vertline.f/f.sub.2 .vertline.&lt;0.6 
(5-3) .vertline.f.sub.3 /f.sub.3-l .vertline.&lt;0.35 
For the lens system according to the present invention which has any one of 
the compositions described above and a relatively wide field angle on the 
order of 70.degree. to 90.degree. in particular, it is desirable to 
replace the condition (1), the condition (2) and the conditions (5) 
through (8) with the following condition (1-4), condition (2-4) and 
conditions (5-4) through (8-4) respectively: 
(1-4) 0.5&lt;.vertline.f.sub.1 /f.sub.3 .vertline.&lt;0.75 
(2-4) 0.01&lt;.vertline.f/f.sub.2 .vertline.&lt;0.20 
(5-4) 0.2&lt;.vertline.f.sub.3 /f.sub.3-1 .vertline.&lt;0.6 
(6-4) 0.5&lt;.vertline.f.sub.3 /f.sub.3-2 .vertline.&lt;0.75 
(7-4) 0.55&lt;.vertline.f.sub.P /f.sub.3 .vertline.&lt;1.3 
(8-4) 1.5&lt;.vertline.e.sub.12 /f.vertline.&lt;3.5 
In order to obtain a lens system which has more favorable performance, it 
is desirable to satisfy, in place of the condition (2-4), the following 
condition (2-5): 
(2-5) 0.01&lt;.vertline.f/f.sub.2 .vertline.&lt;0.10 
Further, it is desirable to use the lens system according to the present 
invention as a photographic lens system which has a back focal length 
f.sub.B (optical path length from an image side surface of the lens system 
to an image surface) and a field angle 2.omega. satisfying the following 
conditions (10) and (11) respectively: 
(10) 2.0&lt;f.sub.B /f&lt;6.0 
(11) 50.degree.&lt;2 .omega.&lt;95.degree. 
If the lower limit of the condition (10) or (11) is exceeded, the lens 
system will be enlarged. If the upper limit of the condition (10) or (11) 
is exceeded, aberrations will be unbalanced for reserving the required 
back focal length, thereby making it impossible to maintain high optical 
performance. 
The lens system according to the present invention is configured to have an 
exit pupil at a nearly infinite distance or an adequately long distance. 
Speaking concretely, it is desirable to configure the lens system so as to 
satisfy the following condition (12): 
(12) .vertline.f/E.sub.X .vertline.&lt;0.1 
wherein the reference symbol E.sub.X represents a length from an image side 
surface of the lens system to an exit pupil of the lens system. 
If the upper limit of 0.1 of the condition (12) is exceeded, angles of rays 
incident on a dichroic film and spectral characteristics will be different 
between the center and a margin of a screen, when a color separating 
prism, for example, is disposed between a lens and an image pickup device, 
thereby making the shading more remarkable. When the color separating 
prism is not disposed, rays incident on the image pickup device are 
inclined, thereby producing non-uniformity in brightness and spurious 
colors or degrading universality. 
The lens system according to the present invention which has been described 
above is configured as a retrofocus type lens system in which an aperture 
stop is disposed between the second lens unit and the third lens unit or 
in the third lens unit so that a lens group disposed on the object side of 
the aperture stop has a negative power, a lens group disposed on the image 
side has a positive power and the lens system has a long back focal 
length. Moreover, the back focal length is sufficiently prolonged and 
optical performance of the lens system is enhanced since aberrations are 
corrected favorably by disposing the second lens unit which has the 
positive power in the lens units corresponding to the front lens group of 
the retrofocus type lens system. 
Now, preferred embodiments of the present invention will be described in a 
form of numeral data which is shown below: 
______________________________________ 
Embodiment 1 
f = 20.000, F number = 4.29, 2.omega. = 59.26.degree. 
______________________________________ 
r.sub.1 = 131.6229 
d.sub.1 = 3.452 n.sub.1 = 1.64419 
.nu..sub.1 = 34.48 
r.sub.2 = -398.0699 
d.sub.2 = 2.500 
r.sub.3 = 165.6431 
d.sub.3 = 3.000 n.sub.2 = 1.81264 
.nu..sub.2 = 25.43 
r.sub.4 = 20.4518 
d.sub.4 = 4.960 
r.sub.5 = -107.2113 
d.sub.5 = 2.500 n.sub.3 = 1.71615 
.nu..sub.3 = 53.84 
r.sub.6 = 32.6641 
d.sub.6 = 16.187 
r.sub.7 = 106.9357 
d.sub.7 = 5.404 n.sub.4 = 1.74435 
.nu..sub.4 = 52.68 
r.sub.8 = -38.9972 
d.sub.8 = 44.352 
r.sub.9 = .infin. (stop) 
d.sub.9 = 3.541 
r.sub.10 = -437.5753 
d.sub.10 = 3.000 
n.sub.5 = 1.73234 
.nu..sub.5 = 54.68 
r.sub.11 = 23.5478 
d.sub.11 = 1.866 
r.sub.12 = 26.5396 
d.sub.12 = 11.670 
n.sub.6 = 1.81264 
.nu..sub.6 = 25.43 
r.sub.13 = 97.6341 
d.sub.13 = 0.100 
r.sub.14 = 102.6840 
d.sub.14 = 4.000 
n.sub.7 = 1.48915 
.nu..sub.7 = 70.20 
r.sub.15 = -53.1815 
d.sub.15 = 14.333 
r.sub.16 = 293.5778 
d.sub.16 = 6.250 
n.sub.8 = 1.48915 
.nu..sub.8 = 70.20 
r.sub.17 = -19.1241 
d.sub.17 = 2.000 
n.sub.9 = 1.80642 
.nu..sub.9 = 34.97 
r.sub.18 = 53.5620 
d.sub.18 = 0.325 
r.sub.19 = 55.3178 
d.sub.19 = 8.150 
n.sub.10 = 1.49845 
.nu..sub.10 = 81.61 
r.sub.20 = -24.8127 
d.sub.20 = 2.070 
r.sub.21 = 44.5534 
d.sub.21 = 5.231 
n.sub.11 = 1.48915 
.nu..sub.11 = 70.20 
r.sub.22 = -144.7822 
d.sub.22 = 1.860 
r.sub.23 = .infin. 
d.sub.23 = 18.500 
n.sub.12 = 1.51825 
.nu..sub.12 = 64.15 
r.sub.24 = .infin. 
d.sub.24 = 1.000 
r.sub.25 = .infin. 
d.sub.25 = 50.500 
n.sub.13 = 1.69979 
.nu..sub.13 = 55.52 
r.sub.26 = .infin. 
.vertline.f.sub.1 /f.sub.3 .vertline. = 0.369, .vertline.f/f.sub.2 
.vertline. = 0.513, .vertline.f.sub.1 f.sub.1-1 .vertline. = 0.110 
.vertline.f.sub.1-2 /f.sub.1 .vertline. = 0.851, .vertline.f.sub.3 
/f.sub.3-1 .vertline. = 0.269 
.vertline.f.sub.3 /f.sub.3-2 .vertline. = 0.827, .vertline.f.sub.p 
/f.sub.3 .vertline. = 0.536 
.vertline.e.sub.12 /f.vertline. = 1.041, .vertline.e.sub.23 /f.vertline. 
= 4.457, f.sub.B /f = 2.336 
E.sub.x /f = 0.015 
______________________________________ 
______________________________________ 
Embodiment 2 
f = 20.267, F number = 3.90, 2.omega. = 58.46.degree. 
______________________________________ 
r.sub.1 = 184.0707 
d.sub.1 = 4.500 n.sub.1 = 1.64419 
.nu..sub.1 = 34.48 
r.sub.2 = -377.2177 
d.sub.2 = 3.629 
r.sub.3 = 111.4484 
d.sub.3 = 3.000 n.sub.2 = 1.81264 
.nu..sub.2 = 25.43 
r.sub.4 = 20.7973 
d.sub.4 = 5.55.8 
r.sub.5 = -129.4211 
d.sub.5 = 2.500 n.sub.3 = 1.71615 
.nu..sub.3 = 53.84 
r.sub.6 = 35.1818 
d.sub.6 = 17.551 
r.sub.7 = 93.4131 
d.sub.7 = 6.000 n.sub.4 = 1.74435 
.nu..sub.4 = 52.68 
r.sub.8 = -44.8401 
d.sub.8 = 46.407 
r.sub.9 = .infin. (stop) 
d.sub.9 = 3.000 
r.sub.10 = -141.5465 
d.sub.10 = 4.500 
n.sub.5 = 1.73234 
.nu..sub.5 = 54.68 
r.sub.11 = 23.7493 
d.sub.11 = 2.500 
r.sub.12 = 28.4971 
d.sub.12 = 12.417 
n.sub.6 = 1.81264 
.nu..sub.6 = 25.43 
r.sub.13 = 207.1116 
d.sub.13 = 0.100 
r.sub.14 = 352.3790 
d.sub.14 = 4.100 
n.sub.7 = 1.48915 
.nu..sub.7 = 70.20 
r.sub.15 = -49.0109 
d.sub.15 = 13.992 
r.sub.16 = 336.8573 
d.sub.16 = 6.101 
n.sub.8 = 1.48915 
.nu..sub.8 = 70.20 
r.sub.17 = -19.4027 
d.sub.17 = 2.000 
n.sub.9 = 1.80642 
.nu..sub.9 = 34.97 
r.sub.18 = 66.8676 
d.sub.18 = 0.150 
r.sub.19 = 68.9921 
d.sub.19 = 6.213 
n.sub.10 = 1.49845 
.nu..sub.10 = 81.61 
r.sub.20 = -24.3434 
d.sub.20 = 1.000 
r.sub.21 = 44.4620 
d.sub.21 = 4.589 
n.sub.11 = 1.48915 
.nu..sub.11 = 70.20 
r.sub.22 = -200.7663 
d.sub.22 = 1.860 
r.sub.23 = .infin. 
d.sub.23 = 19.000 
n.sub.12 = 1.51825 
.nu..sub.12 = 64.15 
r.sub.24 = .infin. 
d.sub.24 = 6.380 
r.sub.25 = .infin. 
d.sub.25 = 50.000 
n.sub.13 = 1.69979 
.nu..sub.13 = 55.52 
r.sub.26 = .infin. 
.vertline.f.sub.1 /f.sub.3 .vertline. = 0.407, .vertline.f/f.sub.2 
.vertline. = 0.489, .vertline.f.sub.1 /f.sub.1-1 .vertline. = 0.096 
.vertline.f.sub.1-2 /f.sub.1 .vertline. = 0.864, .vertline.f.sub.3 
/f.sub.3-1 .vertline. = 0.171 
.vertline.f.sub.3 /f.sub.3-2 .vertline. = 0.789, .vertline.f.sub.p 
/f.sub.3 .vertline. = 0.558 
.vertline.e.sub.12 /f.vertline. = 1.106, .vertline.e.sub.23 /f.vertline. 
= 4.397, f.sub.B /f = 2.523 
E.sub.x /f = 0.031 
______________________________________ 
______________________________________ 
Embodiment 3 
f = 20.080, F number = 3.90, 2.omega. = 58.87.degree. 
______________________________________ 
r.sub.1 = 242.8886 
d.sub.1 = 4.557 n.sub.1 = 1.64419 
.nu..sub.1 = 34.48 
r.sub.2 = -285.1603 
d.sub.2 = 3.544 
r.sub.3 = 139.4667 
d.sub.3 = 3.000 n.sub.2 = 1.81264 
.nu..sub.2 = 25.43 
r.sub.4 = 20.7402 
d.sub.4 = 6.082 
r.sub.5 = -155.9618 
d.sub.5 = 2.500 n.sub.3 = 1.71615 
.nu..sub.3 = 53.84 
r.sub.6 = 36.3231 
d.sub.6 = 17.551 
r.sub.7 = 96.9952 
d.sub.7 = 6.000 n.sub.4 = 1.74435 
.nu..sub.4 = 52.68 
r.sub.8 = -43.2623 
d.sub.8 = 46.407 
r.sub.9 = .infin. (stop) 
d.sub.9 = 3.000 
r.sub.10 = -132.7582 
d.sub.10 = 4.500 
n.sub.5 = 1.73234 
.nu..sub.5 = 54.68 
r.sub.11 = 23.2931 
d.sub.11 = 2.500 
r.sub.12 = 27.6303 
d.sub.12 = 13.594 
n.sub.6 = 1.81264 
.nu..sub.6 = 25.43 
r.sub.13 = 160.5212 
d.sub.13 = 0.100 
r.sub.14 = 326.1577 
d.sub.14 = 4.687 
n.sub.7 = 1.48915 
.nu..sub.7 = 70.20 
r.sub.15 = -51.5120 
d.sub.15 = 13.992 
r.sub.16 = 128.2926 
d.sub.16 = 6.542 
n.sub.8 = 1.48915 
.nu..sub.8 = 70.20 
r.sub.17 = -18.6776 
d.sub.17 = 2.000 
n.sub.9 = 1.80642 
.nu..sub.9 = 34.97 
r.sub.18 = 89.2487 
d.sub.18 = 0.150 
r.sub.19 = 85.5876 
d.sub.19 = 5.820 
n.sub.10 = 1.49845 
.nu..sub.10 = 81.61 
r.sub.20 = -25.1540 
d.sub.20 = 1.000 
r.sub.21 = 61.5066 
d.sub.21 = 4.256 
n.sub.11 = 1.48915 
.nu..sub.11 = 70.20 
r.sub.22 = -112.1569 
d.sub.22 = 1.860 
r.sub.23 = .infin. 
d.sub.23 = 19.000 
n.sub.12 = 1.51825 
.nu..sub.12 = 64.15 
r.sub.24 = .infin. 
d.sub.24 = 6.380 
r.sub.25 = .infin. 
d.sub.25 = 50.000 
n.sub.13 = 1.69979 
.nu.13 = 55.52 
r.sub.26 = .infin. 
.vertline.f.sub.1 /f.sub.3 .vertline. = 0.400, .vertline.f/f.sub.2 
.vertline. = 0.490, .vertline.f.sub.1 /f.sub.1-1 .vertline. = 0.089 
.vertline.f.sub.1-2 /f.sub.1 .vertline. = 0.875, .vertline.f.sub.3 
/f.sub.3-1 .vertline. = 0.102 
.vertline.f.sub.3 /f.sub.3-2 .vertline. = 0.809, .vertline.f.sub.p 
/f.sub.3 .vertline. = 0.605 
.vertline.e.sub.12 /f.vertline. = 1.147, .vertline.e.sub.23 /f.vertline. 
= 4.463, f.sub.B /f = 2.547 
E.sub.x /f = 0.038 
______________________________________ 
______________________________________ 
Embodiment 4 
f = 20.084, F number = 3.90, 2.omega. = 58.86.degree. 
______________________________________ 
r.sub.1 = 163.9541 
d.sub.1 = 4.500 n.sub.1 = 1.60891 
.nu..sub.1 = 43.73 
r.sub.2 = -355.8608 
d.sub.2 = 1.451 
r.sub.3 = 117.2582 
d.sub.3 = 3.000 n.sub.2 = 1.81264 
.nu..sub.2 = 25.43 
r.sub.4 = 20.5687 
d.sub.4 = 5.821 
r.sub.5 = -132.7176 
d.sub.5 = 2.500 n.sub.3 = 1.69979 
.nu..sub.3 = 55.52 
r.sub.6 = 35.7624 
d.sub.6 = 17.551 
r.sub.7 = 94.4328 
d.sub.7 = 5.989 n.sub.4 = 1.75844 
.nu..sub.4 = 52.33 
r.sub.8 = -44.1953 
d.sub.8 = 46.407 
r.sub.9 = .infin. (stop) 
d.sub.9 = 3.000 
r.sub.10 = -92.9723 
d.sub.10 = 4.500 
n.sub.5 = 1.73234 
.nu..sub.5 = 54.68 
r.sub.11 = 24.3949 
d.sub.11 = 2.500 
r.sub.12 = 28.8043 
d.sub.12 = 15.663 
n.sub.6 = 1.81264 
.nu..sub.6 = 25.43 
r.sub.13 = 234.0087 
d.sub.13 = 0.100 
r.sub.14 = 904.5932 
d.sub.14 = 4.063 
n.sub.7 = 1.48915 
.nu..sub.7 = 70.20 
r.sub.15 = -54.8414 
d.sub.15 = 13.992 
r.sub.16 = 83.9289 
d.sub.16 = 7.405 
n.sub.8 = 1.48915 
.nu..sub.8 = 70.20 
r.sub.17 = -20.8595 
d.sub.17 = 2.000 
n.sub.9 = 1.80642 
.nu..sub.9 = 34.97 
r.sub.18 = 69.5276 
d.sub.18 = 0.150 
r.sub.19 = 68.3979 
d.sub.19 = 5.544 
n.sub.10 = 1.49845 
.nu..sub.10 = 81.61 
r.sub.20 = -28.7633 
d.sub.20 = 1.000 
r.sub.21 = 63.1563 
d.sub.21 = 4.656 
n.sub.11 = 1.48915 
.nu..sub.11 = 70.20 
r.sub.22 = -80.5138 
d.sub.22 = 1.860 
r.sub.23 = .infin. 
d.sub.23 = 19.000 
n.sub.12 = 1.51825 
.nu..sub.12 = 64.15 
r.sub.24 = .infin. 
d.sub.24 = 6.380 
r.sub.25 = .infin. 
d.sub.25 = 50.000 
n.sub.13 = 1.69979 
.nu..sub.13 = 55.52 
r.sub.26 = .infin. 
.vertline.f.sub.1 /f.sub.3 .vertline. = 0.419, .vertline.f/f.sub.2 
.vertline. = 0.497, .vertline.f.sub.1 /f.sub.1-1 .vertline. = 0.100 
.vertline.f.sub.1-2 /f.sub.1 .vertline. = 0.870, .vertline.f.sub.3 
/f.sub.3-1 .vertline. = 0.014 
.vertline.f.sub.3 /f.sub.3-2 .vertline. = 0.823, .vertline.f.sub.p 
/f.sub.3 .vertline. = 0.620 
.vertline.e.sub.12 /f.vertline. = 1.137, .vertline.e.sub.23 /f.vertline. 
= 4.485, f.sub.B /f = 2.547 
E.sub.x /f = 0.020 
______________________________________ 
______________________________________ 
Embodiment 5 
f = 12.047, F number = 3.90, 2.omega. = 86.10.degree. 
______________________________________ 
r.sub.1 = 120.6202 
d.sub.1 = 7.705 n.sub.1 = 1.60548 
.nu..sub.1 = 60.68 
r.sub.2 = 363.6060 
d.sub.2 = 0.724 
r.sub.3 = 52.2624 
d.sub.3 = 3.000 n.sub.2 = 1.79013 
.nu..sub.2 = 44.18 
r.sub.4 = 31.4104 
d.sub.4 = 12.966 
r.sub.5 = 83.4217 
d.sub.5 = 2.500 n.sub.3 = 1.83945 
.nu..sub.3 = 42.72 
r.sub.6 = 32.7194 
d.sub.6 = 14.202 
r.sub.7 = 382.5602 
d.sub.7 = 3.000 n.sub.4 = 1.59143 
.nu..sub.4 = 61.18 
r.sub.8 = 39.4514 
d.sub.8 = 12.466 
r.sub.9 = -57.8220 
d.sub.9 = 5.192 n.sub.5 = 1.67158 
.nu..sub.5 = 33.04 
r.sub.10 = -38.5128 
d.sub.10 = 19.941 
r.sub.11 = .infin. 
d.sub.11 = 18.616 
n.sub.6 = 1.69979 
.nu..sub.6 = 55.52 
r.sub.12 = 61.0055 
d.sub.12 = 22.176 
r.sub.13 = .infin. (stop) 
d.sub.13 = 3.000 
r.sub.14 = 66.7736 
d.sub.14 = 4.500 
n.sub.7 = 1.81264 
.nu..sub.7 = 25.43 
r.sub.15 = -104.3420 
d.sub.15 = 2.500 
r.sub.16 = -43.6843 
d.sub.16 = 13.094 
n.sub.6 = 1.60548 
.nu..sub.8 = 60.68 
r.sub.17 = 60.6588 
d.sub.17 = 0.0 
r.sub.16 = 56.8576 
d.sub.18 = 3.587 
n.sub.9 = 1.48915 
.nu..sub.9 = 70.20 
r.sub.19 = -45.1710 
d.sub.19 = 14.933 
r.sub.20 = 82.8383 
d.sub.20 = 6.866 
n.sub.10 = 1.48915 
.nu..sub.10 = 70.20 
r.sub.21 = -29.1204 
d.sub.21 = 2.000 
n.sub.11 = 1.80642 
.nu..sub.11 = 34.97 
r.sub.22 = 39.5527 
d.sub.22 = 0.147 
r.sub.23 = 38.8645 
d.sub.23 = 5.097 
n.sub.12 = 1.49845 
.nu..sub.12 = 81.61 
r.sub.24 = -37.3627 
d.sub.24 = 0.150 
r.sub.25 = 56.3748 
d.sub.25 = 3.459 
n.sub.13 = 1.48915 
.nu..sub.13 = 70.20 
r.sub.26 = -78.1421 
d.sub.26 = 1.860 
r.sub.27 = .infin. 
d.sub.27 = 19.000 
n.sub.14 = 1.51825 
.nu..sub.14 = 64.15 
r.sub.28 = .infin. 
d.sub.28 = 6.380 
r.sub.29 = .infin. 
d.sub.29 = 50.000 
n.sub.15 = 1.71615 
.nu..sub.15 = 53.84 
r.sub.30 = .infin. 
.vertline.f.sub.1 /f.sub.3 .vertline. = 0.669, .vertline.f/f.sub.2 
.vertline. = 0.078, .vertline.f.sub.1 /f.sub.1-1 .vertline. = 0.086 
.vertline.f.sub.1-2 /f.sub.1 .vertline. = 0.842, .vertline.f.sub.3 
/f.sub.3-1 .vertline. = 0.343 
.vertline.f.sub.3 /f.sub.3-2 .vertline. = 0.633, .vertline.f.sub.p 
/f.sub.3 .vertline. = 0.672 
.vertline.e.sub.12 /f.vertline. = 2.464, .vertline.e.sub.23 /f.vertline. 
= 6.312, f.sub.B /f = 4.219 
E.sub.x /f = 0.039 
______________________________________ 
______________________________________ 
Embodiment 6 
f = 12.029, F number = 3.90, 2.omega. = 86.21.degree. 
______________________________________ 
r.sub.1 = 101.5704 
d.sub.1 = 9.973 n.sub.1 = 1.59143 
.nu..sub.1 = 61.18 
r.sub.2 = 277.3720 
d.sub.2 = 2.571 
r.sub.3 = 52.7789 
d.sub.3 = 3.000 n.sub.2 = 1.77620 
.nu..sub.2 = 49.66 
r.sub.4 = 31.2510 
d.sub.4 = 12.070 
r.sub.5 = 102.3965 
d.sub.5 = 2.500 n.sub.3 = 1.77620 
.nu..sub.3 = 49.66 
r.sub.6 = 33.9403 
d.sub.6 = 13.502 
r.sub.7 = -3871.0510 
d.sub.7 = 3.000 n.sub.4 = 1.59143 
.nu..sub.4 = 61.18 
r.sub.8 = 42.2567 
d.sub.8 = 13.610 
r.sub.9 = -42.0548 
d.sub.9 = 3.640 n.sub.5 = 1.67158 
.nu..sub.5 = 33.04 
r.sub.10 = -33.9941 
d.sub.10 = 29.595 
r.sub.11 = 148.3722 
d.sub.11 = 8.500 
n.sub.6 = 1.69979 
.nu..sub.6 = 55.52 
r.sub.12 = 78.1920 
d.sub.12 = 28.006 
r.sub.13 = .infin. (stop) 
d.sub.13 = 6.500 
r.sub.14 = 80.3751 
d.sub.14 = 5.500 
n.sub.7 = 1.81264 
.nu..sub.7 = 25.43 
r.sub.15 = 64.8302 
d.sub.15 = 2.722 
r.sub.16 = -27.9019 
d.sub.16 = 4.950 
n.sub.8 = 1.60548 
.nu..sub.8 = 60.68 
r.sub.17 = 63.7468 
d.sub.17 = 0.150 
r.sub.18 = 101.2640 
d.sub.18 = 4.662 
n.sub.9 = 1.48915 
.nu..sub.9 = 70.20 
r.sub.19 = -29.1035 
d.sub.19 = 9.582 
r.sub.20 = 879.3216 
d.sub.20 = 7.155 
n.sub.10 = 1.48915 
.nu..sub.10 = 70.20 
r.sub.21 = -19.9873 
d.sub.21 = 0.406 
r.sub.22 = -20.7867 
d.sub.22 = 2.000 
n.sub.11 = 1.80642 
.nu..sub.11 = 34.97 
r.sub.23 = 45.4694 
d.sub.23 = 4.500 
n.sub.12 = 1.48915 
.nu..sub.12 = 70.20 
r.sub.24 = -34.8495 
d.sub.24 = 0.150 
r.sub.25 = 69.8589 
d.sub.25 = 3.516 
n.sub.13 = 1.49845 
.nu..sub.13 = 81.61 
r.sub.26 = -38.6182 
d.sub.26 = 1.860 
r.sub.27 = .infin. 
d.sub.27 = 19.000 
n.sub.14 = 1.51825 
.nu..sub.14 = 64.15 
r.sub.28 = .infin. 
d.sub.28 = 6.380 
r.sub.29 = .infin. 
d.sub.29 = 50.000 
n.sub.15 = 1.71615 
.nu..sub.15 = 53.84 
r.sub.30 = .infin. 
.vertline.f.sub.1 /f.sub.3 .vertline. = 0.621, .vertline.f/f.sub.2 
.vertline. = 0.054, .vertline.f.sub.1 /f.sub.1-1 .vertline. = 0.098 
.vertline.f.sub.1-2 /f.sub.1 .vertline. = 0.811, .vertline.f.sub.3 
/f.sub.3-1 .vertline. = 0.481 
.vertline.f.sub.3 /f.sub.3-2 .vertline. = 0.664, .vertline.f.sub.p 
/f.sub.3 .vertline. = 1.204 
.vertline.e.sub.12 /f.vertline. = 2.565, .vertline.e.sub.23 /f.vertline. 
= 7.244, f.sub.B /f = 4.231 
E.sub.x /f = 0.052 
______________________________________ 
Each of the first through fourth embodiments has a composition illustrated 
in FIG. 1, wherein the lens system consists, in order from the object 
side, of a first lens unit G1 which has a negative power as a whole, a 
second lens unit G2 which has a positive power and a third lens unit G3 
which has a positive power as a whole: the first lens unit Gl being 
composed, in order from the object side, of a 1-1 subunit G11 which is 
composed of a positive lens component and has a positive power, and a 1-2 
subunit G12 which is composed of two negative lens components and has a 
negative power, the second lens unit G2 being composed of a single 
positive lens component, and the third lens unit G3 being composed, in 
order from the object side, of a stop S, a 3-1 subunit which is composed 
of a negative lens component L31 and two positive lens components L32, and 
a 3-2 subunit G32 which is composed of a negative cemented lens component 
L33 consisting of a positive lens element and a negative lens element, and 
a positive lens component L34 consisting of two positive lens elements. 
Each of these first through fourth embodiments corrects negative distortion 
remarkably produced by the first lens unit G1 by distributing a positive 
power to the 1-1 subunit G11 of the first lens unit G1. The second lens 
unit G2 functions to lower paraxial rays and correct negative distortion 
remaining in the first lens unit G1. Further, the third lens unit G3 is 
composed of the 3-1 subunit which is composed of the negative lens 
component L31 and the positive lens component L32 so as to have a 
negative-positive composition, and the 3-2 subunit which is composed of 
the negative lens component L33 and the positive lens component L34 so as 
to have a negative-positive composition, and adopt a power distribution 
wherein a positive power stronger than that of the 3-1 subunit G31 is 
distributed to the 3-2 subunit G32, thereby making it possible to obtain a 
long back focal length. 
Further, a lens component LP which is composed of the two positive lens 
elements disposed on the image side in the third lens unit G3, or the lens 
component L4 serves for locating an exit pupil at a nearly infinite or 
adequately long distance. 
In these embodiments, plane parallel glass plates disposed on the image 
side in the lens system (image side of the third lens unit G3) represent a 
low pass filter, an infrared cut filter, a color separating prism or an 
optical path splitting prism, a trimming filter and so on. 
The fifth embodiment has a composition illustrated in FIG. 2 and a 
photographing field angle which is wider than that of any one of the first 
through fourth embodiments. 
Differently from the first embodiment, the lens system preferred as the 
fifth embodiment uses a 1-2 subunit G12 of a first lens unit G1 which 
consists of three negative meniscus lens components having a concave 
surface on the image side, a second lens unit G2 which consists of a 
positive meniscus lens component having a concave surface on the object 
side, and a 3-1 subunit G31 of a third lens unit G3 which consists, in 
order from the object side, of a negative lens component L31 having a weak 
power, a stop S, and a positive lens component L32 consisting of a 
positive lens element, a negative lens element and a positive lens 
element. 
The sixth embodiment has a composition illustrated in FIG. 3 and has a wide 
field angle like the fifth embodiment. 
Differently from the fifth embodiment, the lens system preferred as the 
sixth embodiment uses a 3-2 subunit of a third lens unit G3 which 
consists, in order from the object side, of a positive lens component L33 
and a negative cemented lens component L34 consisting of a negative lens 
element and a positive lens element, and a positive lens component L35. 
Owing to this composition, the sixth embodiment suppresses offaxial 
aberrations such as coma and astigmatism more effectively than the other 
embodiments. 
The fifth and sixth embodiments are lens systems which have extremely wide 
photographing field angles of approximately 90.degree., back focal lengths 
long enough to dispose optical members such as low pass filters, infrared 
cut filters and color separating prisms or optical path splitting prisms, 
and optical performance favorably enough to use image pickup devices on 
which small pixels are arranged. 
In each of the first through fourth embodiments and the fifth embodiment, 
both the 3-1 subunit G31 and the 3-2 subunit G32 of the third lens unit G3 
have negative-positive retrofocus type power distributions, or are 
composed of the first negative lens component L31 and the second positive 
lens component L32, and the third negative lens component L33 and the 
fourth positive lens component L34 respectively, thereby prolonging a back 
focal length f.sub.B. Further, the positive lens component LP is disposed 
on the image side in the third lens unit G3, thereby locating the exit 
pupil at a nearly infinite or adequately long distance. 
In the sixth embodiment, the 3-1 subunit G31 of the third lens unit G3 is 
composed of the first negative lens component L31 and the second positive 
lens component L32, or has a negative-positive power distribution, whereas 
the 3-2 subunit G32 is composed of the third positive lens component L33 
and the fourth negative lens component L34 and the fifth positive lens 
component L35, or has a positive-negative-positive triplet type power 
distribution, thereby correcting offaxial aberrations such as coma and 
astigmatism favorably as compared with the other embodiments. Also in the 
third lens unit of the sixth embodiment, a lens component LP is disposed 
on the image side in the third lens unit for locating an exit pupil at a 
nearly infinite distance.