Zoom lens system

A zoom lens system comprising a vari-focal system and a relay lens system wherein the vari-focal system comprises a first lens group having positive refractive power and comprising a cemented doublet, a second lens group having negative refractive power and comprising a cemented doublet, and a third lens group having positive refractive power, and the relay lens system comprises a front lens group having positive refractive power, and a rear lens group having positive refractive power, the aperture ratio thereof being large and aberrations thereof being corrected favorably.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
The present invention relates to a zoom lens system and, more particularly, 
to a zoom lens system with a focal length variable from 75 mm to 150 mm 
and large aperture ratio about F/2.8. 
(b) Description of the Prior Art 
There are many known zoom lens systems for photographing with focal lengths 
variable from about 75 mm to about 150 mm. However, most of them have 
aperture ratios about F/4 to F/3.5. This means that the F number when the 
stop is fully open is large, brightness of the lens system is low and, 
consequently, photographing conditions are limited. Moreover, as the 
brightness of the lens system is low, it is inconvenient in various points 
when deciding the composition of the photograph to be taken and focusing 
the lens system on the object by using the finder. Therefore, at present, 
users prefer bright photographic lenses with single focal length to zoom 
lenses. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide a 
zoom lens system with a large aperture ratio, i.e., F/2.8, and high 
performance. 
The zoom lens system according to the present invention comprises, in the 
order from the object side as shown in FIG. 1, a first lens group I having 
positive refractive power and comprising a cemented doublet, a second lens 
group II having negative refractive power and comprising a cemented 
doublet, a third lens group III having positive refractive power, said 
first, second and third lens groups constituting a vari-focal system, and 
a relay lens system IV which comprises a front lens group IVa having 
positive refractive power and a rear lens group IVb having positive 
refractive power, said zoom lens system according to the present invention 
fulfilling the conditions (1) and (2) shown below: 
##EQU1## 
where, reference symbol f.sub.I represents the focal length of the first 
lens group I, reference symbol f.sub.II represents the focal length of the 
second lens group II, reference symbols n.sub.I and n.sub.I ' respectively 
represent refractive indices of respective lens elements constituting the 
cemented doublet in the first lens group I, reference symbol r.sub.I 
represents the radius of curvature of the cemented surface in the first 
lens group I. 
When making the aperture ratio of a lens system large, it is difficult to 
correct spherical aberration especially. In case of the zoom lens system 
according to the present invention, spherical aberration is corrected 
favourably by adopting the above-mentioned lens composition and, at the 
same time, arranging to fulfill the conditions (1) and (2). 
In case of a zoom lens system, the lens composition varies at the time of 
focusing and zooming unlike photographic lens systems in general. 
Therefore, it is difficult to correct aberrations of the lens system as a 
whole in well balanced state unless aberrations of each lens group are 
corrected favourably to some degree. 
For example, the first lens group I is to be moved for the purpose of 
focusing. Aberrations to be caused by said lens group can be made small 
when the power of said lens group is made weak. However, when the power is 
made too weak, the focusing action becomes weak and this is not desirable. 
Therefore, the first lens group I should have a certain degree of power. 
In the present invention, by taking the fact that the first lens group I 
has positive refractive power into consideration, a cemented surface 
having negative refractive power is provided in said lens group and said 
negative refractive power is defined as shown by the condition (1) so that 
spherical aberration to be caused by the first lens group I is thereby 
prevented from occurring. As the height of paraxial marginal ray is the 
highest in the first lens group I, it is important to favourably correct 
spherical aberration to be caused by said lens group. 
If the value defined by the condition (1) becomes larger than the upper 
limit thereof, negative refractive action of the above-mentioned cemented 
surface becomes weak and, consequently, spherical aberration will be 
undercorrected. 
The second lens group II serves as a variator at the time of zooming. To 
arrange the vari-focal system compactly by arranging that the amount of 
movement of said lens group at the time of zooming does not become large, 
it is unavoidable to make the power of the second lens group II 
comparatively strong. Therefore, by taking the fact that the second lens 
group II has negative refractive power into consideration, a cemented 
surface having positive refractive action is provided in said lens group, 
and positive refractive action of said cemented surface is defined so that 
spherical aberration to be caused by the second lens group II is thereby 
corrected favourably. 
By adopting the lens composition explained so far, it is possible to obtain 
a zoom lens system with a large aperture ratio and well corrected 
aberrations which is an object of the present invention. However, when it 
is arranged that the rear lens group IVb in the relay lens system IV 
comprises a plural number of lenses including at least one positive lens 
and respective lens groups fulfill the conditions (3) through (5) shown 
below, it is possible to favourably correct negative distortion which is 
liable to become conspicuous when a photograph is taken. 
##EQU2## 
In respective conditions shown in the above, reference symbol f.sub.IV 
represents the focal length of the relay lens system IV, reference symbol 
f.sub.IVb represents the focal length of the rear lens group IVb in the 
relay lens system IV, reference symbol f.sub.IVb3 represents the focal 
length of the positive lens arranged on the image side in the rear lens 
group IVb in the relay lens system IV, and reference symbol D represents 
the airspace between the front lens group IVa and rear lens group IVb in 
the relay lens system IV. 
The condition (3) relates to balance of refractive powers between the first 
lens group I and rear lens group IVb in the relay lens system IV. 
As offaxial rays are subjected to strong positive refractive action by the 
first lens group I, there is such tendency that principal rays are 
directed toward the plus side. Besides, lower rays which cause coma also 
have such tendency that they are directed toward the plus side. Therefore, 
when the rear lens group IVb is arranged to have positive refractive 
action by selecting the focal length f.sub.IVb of the rear lens group IVb 
in the relay lens system IV within the range defined by the condition (3), 
it is possible to arrange that distortion, which curves toward the puls 
side, is shifted toward the minus side. Besides, it is possible to direct 
the lower rays also toward the minus side and, therefore, it is possible 
to correct coma. 
If the value of f.sub.IVb /f.sub.I becomes smaller than the lower limit of 
the condition (3), it is impossible to effectively offset distortion, 
which is caused by the first lens group I, by means of the rear lens group 
IVb in the relay lens system IV and, consequently, positive distortion 
remains. If the value of f.sub.IVb /f.sub.I becomes larger than the upper 
limit of the condition (3), it is impossible to obtain a sufficient back 
focal length which is needed for a lens system for a single-lens reflex 
camera. 
The condition (4) is established in order to correct distortion more 
favourably in addition to the fact that the condition (3) is established. 
In the relay lens system IV, the heights of offaxial rays become the 
highest at the positive lens arranged on the image side in the rear lens 
group IVb in the relay lens system IV. Therefore, the action to shift 
distortion toward the mines side becomes strong when the focal length of 
said positive lens is made short and becomes weak when the focal length of 
said positive lens is made long. On the other hand, at said positive lens, 
the height of paraxial ray is low. Therefore, by said positive lens, it is 
possible to control offaxial rays without largely varying the paraxial 
ray. 
If the focal length of said positive lens becomes large and f.sub.IVb3 
/f.sub.IV exceeds the upper limit of the condition (4), the correcting 
action of said positive lens for distortion becomes insufficient, and 
distortion curves toward the plus side in all zoomed states from the 
teleposition to the wide position. When the focal length of said positive 
lens becomes short and f.sub.IVb3 /f.sub.IV becomes smaller than the lower 
limit of the condition (4), distortion in the wide position largely curves 
toward the minus side though distortion in the teleposition becomes 
favourable. 
The condition (5) is established in order to arrange the relay lens system 
compactly in spite of the fact that the relay lens system comprises the 
positive front lens group IVa and positive rear lens group IVb and, at the 
same time, to correct aberrations satisfactorily favourably. 
When the airspace D between the front lens group IVa and rear lens group 
IVb becomes smaller than the lower limit of the condition (5), it is 
effective for making the relay lens system compact. However, the heights 
of offaxial rays become low and, consequently, the effect to be obtained 
by giving positive power to the rear lens group IVb in the relay lens 
system IV becomes small. When the value of D becomes larger than the upper 
limit of the condition (5), the heights of offaxial rays become high and, 
therefore, it is possible to correct offaxial aberrations satisfactorily. 
However, it is impossible to make the lens system compact. Moreover, as 
the back focal length becomes short, it is impossible to obtain a lens 
system suitable for a single-lens reflex camera. 
For respective lens groups constituting the zoom lems system according to 
the present invention, it is preferable to adopt the lens configuration as 
shown in FIG. 1. That is, it is preferable to arrange that the first lens 
group I comprises a positive single-lens and a positive cemented doublet 
consisting of a positive and negative lens elements, the second lens group 
II comprises a negative single lens and a negative cemented doublet 
consisting of a negative and positive lens elements, the third lens group 
III comprises a positive cemented doublet consisting of a positive and 
negative lens elements, the front lens group IVa in the relay lens system 
IV comprises a positive single lens and a negative cemented doublet 
consisting of a positive and negative lens elements, and the rear lens 
group IVb in the relay lens system IV comprises a positive single lens, a 
negative single lens and a positive single lens. 
By arranging that the first lens group I comprises a positive single lens 
and a cemented doublet consisting of a positive and negative lens 
elements, it is so arranged that spherical aberration, longitudinal 
chromatic aberration, distortion, astigmatism, etc. are corrected 
favourably. 
In the first lens group I, the heights of paraxial rays is the highest. 
Therefore, if spherical aberration, longitudinal chromatic aberration, 
etc. are caused by the first lens group, it is difficult to offset them by 
the lenses constituting the second lens group II and on. Besides, the 
heights of offaxial principal rays are also high in the first lens group 
I. Therefore, if distortion, astigmatism, longitudinal chromatic 
aberration, etc. are caused by the first lens group, it is difficult to 
offset them by the lenses constituting the second lens group II and on. 
By the fact that the first lens group I is composed as described in the 
above, it is possible to give sufficient refractive power as the first 
converging lens group to the first lens group I and, at the same time, to 
make aberrations to be caused by said lens group small. That is, by 
providing the positive single lens, sufficient converging action required 
at the first lens group is given to said positive single-lens and, at the 
same time, by providing the cemented doublet consisting of the positive 
and negative lens elements, diverging action is given to the cemented 
surface thereof so as to offset aberrations caused by said positive single 
lens. Undercorrected spherical aberration caused by the positive 
single-lens is corrected by the cemented surface of the cemented doublet 
consisting of the positive and negative lens elements. Besides, 
longitudinal chromatic aberration is also reduced by the correcting action 
of said cemented doublet to the degree that it can be satisfactorily 
corrected by the second lens group II. As for the offaxial principal rays, 
strong positive distortion caused by the positive single lens can be 
corrected by said cemented surface and astigmatism can be also corrected 
in the same way. 
Now, as the second lens group II is to be largely moved at the time of 
zooming, aberrations of said lens group itself should be corrected 
satisfactorily. Besides, it should be arranged that a sufficient zoom 
ratio is obtained without moving said lens group so largely. Therefore, 
the negative single lens is provided in order to give sufficient action as 
a negative lens group to the second lens group II. Besides, by providing 
the negative cemented doublet, variation in chromatic aberration to be 
caused at the time of zooming is reduced and, by means of converging 
action of the cemented surface, spherical aberration is prevented from 
largely curving toward the overcorrection side. 
The third lens group III is arranged as a cemented doublet consisting of a 
positive and negative lens elements due to the following reason. 
The third lens group III is to be moved at the time of zooming by forming a 
curve concave toward the image side in order to prevent variation in the 
image position caused when the second lens group II is moved for the 
purpose of zooming. The third lens group should be arranged as a cemented 
doublet in order to reduce variation in chromatic aberration and variation 
in spherical aberration to be caused by zooming. 
It is due to the reason described below that the relay lens system IV is 
composed of the front lens group IVa having positive refractive power and 
rear lens group IVb having positive refractive power and it is so arranged 
that the front lens group IVa comprises a positive single lens and a 
negative cemented doublet consisting of a positive and negative lens 
elements and the rear lens group IVb comprises a positive single lens, a 
negative single lens and a positive single lens. 
The relay lens system IV is arranged so that it does not move even at the 
time of zooming and aberrations are corrected in well balanced state at 
all focal lengths. 
The relay lens system IV is composed of the positive front lens group IVa 
and positive rear lens group IVb because it is then possible to give 
sufficient power to the relay lens system IV though it is disadvantageous 
for making the lens system compact. Especially when the rear lens group 
IVb is arranged to have positive power, it is possible to correct offaxial 
aberrations caused by the first lens group I. That is, distortion caused 
by the first lens group I which curves toward the plus side can be shifted 
toward the minus side by means of the rear lens group IVb and, therefore, 
it is possible to reduce positive distortion which is comparatively liable 
to become conspicuous. 
For the zoom lens system according to the present invention, it is possible 
to correct aberrations more favourably when it is arranged to fulfill the 
conditions (6) through (9) shown below at the same time as adopting the 
lens configuration described so far. 
##EQU3## 
In respective conditions shown in the above, reference symbols n.sub.1 
represents the refractive index of the positive single lens in the first 
lens group I, reference symbol n.sub.2 represents the refractive index of 
the positive lens elements constituting the cemented doublet in the first 
lens group I, reference symbol n.sub.9 represents the refractive index of 
the positive lens arranged on the object side in the front lens group IVa 
in the relay lens system IV, reference symbols n.sub.10 and n.sub.11 
respectively represent refractive indices of respective lens elements 
constituting the cemented doublet in the front lens group IVa in the relay 
lens system IV, reference symbols .nu..sub.5 and .nu..sub.6 respectively 
represent Abbe's numbers of respective lens elements constituting the 
cemented doublet in the second lens group II, and reference symbol 
r.sub.17 represents the radius of curvature of the cemented surface of the 
cemented doublet in the front lens group IVa in the relay lens system IV. 
The condition (6) is established in order to make refractive indices 
n.sub.1 and n.sub.2 of positive lenses in the front lens group I large so 
as to thereby make radii of curvature of surfaces having positive 
refractive power large and to reduce negative spherical aberration to be 
caused. If the value defined by the condition (6) is made smaller than the 
lower limit thereof, spherical aberration to be caused becomes large, and 
this is not desirable. 
The condition (7) is established in order to prevent variation in chromatic 
aberration to be caused by zooming, and it serves to minimize chromatic 
aberration to be caused by the second lens group II, which is moved 
largely at the time of zooming. If the value defined by the condition (7) 
is made smaller than the lower limit thereof, variation in chromatic 
aberration becomes large. 
The condition (8) is established so that refractive indices of positive 
lenses in the front lens group IVa in the relay lens system IV having 
positive refractive power is made large in order to thereby make radii of 
curvature of surfaces of said positive lenses large and to reduce negative 
spherical aberration to be caused. If the value defined by the condition 
(8) is made smaller than the lower limit thereof, spherical aberration 
will be undercorrected. 
The condition (9) relates to the front lens group IVa in the relay lens 
system IV. That is, a cemented doublet is provided in said lens group, and 
the condition (9) is established in order to give negative refractive 
power to the cemented surface thereof and to thereby correct spherical 
aberration, which is liable to be undercorrected. If the value defined by 
the condition (9) becomes larger than the upper limit thereof, spherical 
aberration will be undercorrected.