Compact zoom lens system

A compact zoom lens system is disclosed having a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, and a fourth lens group with negative refractive power, such that the second and third lens groups are moved for the purpose of zooming, and the first lens group is moved in order to compensate for the movement of image surface, the entire system having a large zoom ratio.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
The present invention relates to a compact zoom lens system comprising four 
lens groups. 
(b) Description of the Prior Art 
Known zoom lens systems comprising four lens groups are generally composed 
of a focusing lens group having positive refractive power, a variator 
having negative refractive power, a compensator having positive refractive 
power and a relay lens group having positive refractive power. As widely 
known, such a zoom lens system is arranged to be zoomed by moving the 
variator along the optical axis and to compensate for the movement of 
image surface, which is caused at that time, by moving the compensator 
along the optical axis. The lens system is focused by moving the focusing 
lens group. 
Such known zoom lens systems comprising four lens groups have a 
disadvantage that the lens system becomes large in size due to the amount 
of movement of movable lens groups, i.e., the variator and compensator, is 
comparatively large. Especially when it is arranged to make the zoom ratio 
large, the above-mentioned amounts of movement become large. To make the 
zoom ratio large by making the abovementioned amounts of movement 
comparatively small, the power of the variator should be made large. When, 
however, power of the variator is made large, aberrations are aggravated; 
variation of aberrations caused when the variator is moved also become 
large, and it becomes considerably difficult to correct them. Moreover, to 
correct the above-mentioned aggravated aberrations and variation of 
aberrations, it is necessary to increase the number of lenses constituting 
the lens system, and, as a result, it becomes impossible to make the lens 
system compact. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide a 
compact zoom lens system comprising four lens groups, i.e., a first lens 
group, a second lens group, a third lens group and a fourth lens group, 
for which a new lens-group moving method is adopted, i.e., the second lens 
group and the third lens group are individually moved for the purpose of 
zooming and, at the same time, the first lens group is moved for the 
purpose of compensating for the movement of image surface, said compact 
zoom lens system comprising four lens groups being arranged that the 
vari-focal ratio is large, aberrations can be corrected easily and the 
telephoto ratio is small. 
FIG. 1 shows a schematic diagram illustrating respective lens group 
constituting the compact zoom lens system according to the present 
invention and the new lens-group moving method adopted for said zoom lens 
system. 
The compact zoom lens system according to the present invention comprises a 
first lens group I having positive refractive power, a second lens group 
II having negative refractive power, a third lens group III having 
positive refractive power, and a fourth lens group IV having negative 
refractive power, in order from the object side. Out of these lens groups, 
the second lens group II and the third lens group III are moved as shown 
in FIG. 1 for the purpose of zooming. In other words, the second lens 
group and the third lens group are respectively arranged as variators. 
When the second lens group II and the third lens group III are moved as 
shown in FIG. 1, the first lens group I is moved as shown in FIG. 1 in 
order to compensate for the movement of the image surface. That is, the 
first lens group I is arranged to serve as a focusing lens group and, at 
the same time, as a compensator. The fourth lens group IV is a relay lens 
group which is kept fixed during zooming and relays the image formed by 
the third lens group III to the final image surface at a certain 
magnification. 
For the zoom lens system composed as shown in FIG. 1 and arranged to move 
the lens group as shown in FIG. 1, there are relations expressed by the 
formulas (1) and (2), shown below, when the focal lengths of the first 
lens group I, the second lens group II and the third lens group III are 
respectively represented by reference symbols f.sub.1, f.sub.2 and 
f.sub.3, magnification of the second lens group II in the wide position W 
shown in FIG. 1 is represented by reference symbol .beta..sub.2W, 
magnification of the second lens group II in the teleposition T shown in 
FIG. 1 is represented by reference symbol .beta..sub.2T, magnification of 
the third lens group III in said wide position W is represented by 
reference symbol .beta..sub.3W, magnification of the third lens group III 
in said teleposition T is represented by reference symbol .beta..sub.3T, 
and magnification of the fourth lens group IV is represented by reference 
symbol .beta..sub.4, and when respective lens groups are regarded as thin 
lenses and airspaces between respective lens groups when said lens groups 
approach each other most closely are regarded as zero. 
EQU f.sub.1 =f.sub.2 (1-1/.beta..sub.2W) (1) 
EQU (.beta..sub.2T -1)f.sub.2 =f.sub.3 -f.sub.3 /.beta..sub.3T ( 2) 
When the focal length of the lens system as a whole in the wide position is 
represented by reference symbol f.sub.W and the focal length of the lens 
system as a whole in the teleposition is represented by reference symbol 
f.sub.T, it is possible to express f.sub.W and f.sub.T by the formulas (3) 
and (4) shown below. 
EQU f.sub.W =f.sub.1 .multidot..beta..sub.2W .multidot..beta..sub.3W 
.multidot..beta..sub.4 ( 3) 
EQU f.sub.T =f.sub.1 .multidot..beta..sub.2T .multidot..beta..sub.3T 
.multidot..beta..sub.4 ( 4) 
Therefore, the zoom ratio V of the zoom lens system becomes as shown in 
formula (5), shown below. 
##EQU1## 
In the formula (5), reference symbol V.sub.2 represents the magnification 
ratio (.beta..sub.2T)/.beta..sub.2W of the second lens group between the 
telephoto and wide positions, and reference symbol V.sub.3 represents the 
magnification ratio (.beta..sub.3T)/.beta..sub.3W of the third lens group 
between the telephone and wide positions. 
Even when the zoom ratio V is defined by formula (5), it is possible to 
select V.sub.2 and V.sub.3 arbitrarily to some degree. Therefore, when 
deciding the basic composition of a zoom lens system with a certain zoom 
range based on the formulas (1), (2) and (5), it is possible to compose a 
zoom lens system attaining the object of the present invention as far as 
the values of .beta..sub.2W, .beta..sub.3T, V.sub.2 /V.sub.3 and 
.beta..sub.4 are selected. 
For example, to make the overall length of the lens system short without 
causing aggravation of aberrations and to prevent the amounts of movement 
of movable lens groups from becoming large, magnifications of movable lens 
groups generally varies so that the ranges of variation of magnification 
contain .times.1. When this point is taken into consideration, it is 
preferable to make .beta..sub.2W about -0.5. 
Besides, when the magnification of the third lens group is made high, the 
back focal length f.sub.B becomes long. When, however, the magnification 
of the third lens group is made too low, the focal length of the third 
lens group becomes short, and this is not preferable for correction of 
aberrations. From the viewpoint described above, it is preferable to make 
.beta..sub.3T about -1.5. 
Furthermore, the zoom lens system according to the present invention is 
characterized in that the magnification thereof is varied by two lens 
groups, i.e., the second lens group and the third lens group. To 
effectively utilize this characteristic, it is preferable to establish the 
magnification ratio V.sub.2, of the second lens group approximately equal 
to the magnification ratio V.sub.3 of the third lens group. For the 
above-mentioned reason, it is preferable to arrange that V.sub.2 /V.sub.3 
becomes about 1.0. 
When the values of the above-mentioned parameters are decided, the value of 
.beta..sub.4 is approximately decided. When the above-mentioned parameters 
are set as above, the value of .beta..sub.4 becomes 1.43. 
As described above, an example of a zoom lens system which attains the 
object of the present invention may be considered as follows. 
.beta..sub.2W =0.5, .beta..sub.3T =1.5, V.sub.2 /V.sub.3 =1.0, .beta..sub.4 
=1.43 
For focal lengths of respective lens groups, i.e., for f.sub.1, f.sub.2, 
f.sub.3 and f.sub.4, values shown below may be considered as values which 
are practically possible and which are reasonable in general. 
f.sub.1 =3, f.sub.2 =-1, f.sub.3 =1, f.sub.4 =-3.5 
When a zoom lens system with V=3 is established, it becomes V.sub.2 
=V.sub.3 =1.73. 
As the overall length L of the lens system becomes as shown by the formula 
##EQU2## 
values of .beta..sub.2T, .beta..sub.3W, etc. become as follows from the 
above-mentioned formula, formulas (1) through (5), shown before, and the 
above-mentioned given values of parameters: 
.beta..sub.2T =0.866, .beta..sub.3W =-0.866, L.sub.W =4.2, L.sub.T =4.94, 
f.sub.W =1.857, f.sub.T =5.573. 
where, reference symbols L.sub.W and L.sub.T respectively represent overall 
lengths of the lens system in the wide position and teleposition. 
As it is evident from the example shown above, it is possible to obtain the 
basic composition of the desired zoom lens system through practice of the 
presently disclosed invention. 
For the zoom lens system as described above, it is preferable to arrange 
the aperture stop in front of the third lens group because it is then 
possible to keep the diameter of the front lens small. In that case, the 
aperture stop may be fixed with respect to the third lens group so as to 
move together with the third lens group, or may be fixed in a space in 
front of the third lens group. 
For respective lens groups according to the present invention described 
above, the following may be considered. It is preferable that the first 
lens group comprises a negative lens and two positive lenses, or comprises 
a cemented doublet, which consists of a negative lens and a positive lens; 
and a positive lens, the second lens group comprises four lenses including 
at least two negative lenses; and the third lens group comprises five 
lenses including at least one negative lens which are arranged to form a 
triplet type lens group. The third lens group is arranged as a variator 
according to the present invention. To keep a certain airspace with 
respect to the second lens group and to increase the magnification of the 
third lens group, it is necessary to shift the principal point of the 
third lens group as forward as possible. 
When the first, second and third lens groups are arranged as described 
above, the fourth lens group is established with negative refractive 
power, and comprises at least one positive lens and at least one negative 
lens so that the lateral chromatic aberration will not be largely caused 
by the fourth lens group. 
For a zoom lens system employing the lens-group moving method according to 
the present invention, the establishment of the magnification ratio 
V.sub.2 of the second lens group and magnification ratio V.sub.3 of the 
third lens group is an important point for balancing the variation of 
aberrations caused at the time of zooming. As the third lens group has 
positive power, the overall length of the lens system becomes long if the 
magnification ratio V.sub.3 of the third lens group is made too large, and 
this is not desirable. If the magnification ratio V.sub.2 of the second 
lens group is made too large, the variation of coma and distortion caused 
by the diverging action of the second lens group becomes large. 
Due to the above, it is preferable to select the magnification ratio 
V.sub.2 of the second lens group about one half to twice of the 
magnification ratio V.sub.3 of the third lens group. That is, it is 
preferable to arrange the following relationship: 0.5&lt;V.sub.2 /V.sub.3 
&lt;2.0. Then, it is necessary that the magnification .beta..sub.4 of the 
fourth lens group fulfills the condition shown below to give the 
correcting action for curvature of image to the fourth lens group, and to 
keep the focal length of the lens system, as a whole, long. 
1.5&gt;.beta..sub.4 &gt;1 
If the magnification .beta..sub.4 becomes smaller than 1, The correcting 
action for curvature of image becomes weak. If .beta..sub.4 becomes larger 
than 1.5, aberrations caused by the first through the third lens groups 
becomes large. Moreover, the back focal length of the lens system becomes 
too long, and this is not preferable for making the overall length of the 
lens system short. 
When it is desired to focus the zoom lens system according to the present 
invention described in the above on an object at an extremely short 
distance (at the time of macro-photographing), focusing becomes possible 
when the third lens group is moved toward the object side. That is, when 
the third lens group is moved toward the object side being shifted from 
the position of .times.1, especially in the wide position, the rear focal 
point of the lens system as a whole is shifted toward the object side. 
Therefore, it becomes possible to focus the lens system on an object at an 
extremely short distance. 
In the same manner, when the fourth lens group is moved toward the image 
side, the magnification of the fourth lens group decreases and, therefore, 
it becomes possible to focus the lens system on an object at an extremely 
short distance. 
It is also possible to simultaneously perform the above-mentioned focusing 
by movement of the third lens group and focusing by movement of the fourth 
lens group. That is, it is possible to focus the lens system on an object 
at an extremely short distance by moving the third lens group toward the 
object side and moving the fourth lens group toward the image side at the 
same time.