Wide angle zoom lens system

A wide angle zoom lens system comprising, in order from the object side, a first positive lens unit, a second positive lens unit and a third negative lens unit; and configured so as to perform variation of focal length by varying airspaces reserved between these lens units. The first lens unit consists of a negative front subunit and a rear positive subunit. The zoom lens system has a wide field angle and favorable optical performance.

BACKGROUND OF THE INVENTION 
a) Field of the Invention 
The present invention relates to zoom lens system which has a wide field 
angle. 
b) Description of the Prior Art 
In the present days, there is a remarkable trend manufacture, as commercial 
products developed with a new concept, fully automated cameras which 
incorporate zoom lens systems having high vari-focal ratios. These zoom 
lens systems having high vari-focal ratios always have wide field angles 
or high telephoto ratios. 
The conventional zoom lens systems have field angles on the order of 2 
.omega.=63.degree. at wide positions thereof. Further, out of zoom lens 
systems which have low vari-focal ratios, there are known those which have 
field angles on the order of 2 .omega.=84.degree. as exemplified by the 
zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 
2-284,109. 
Further, conventional super wide angle zoom lens systems are known only as 
interchangeable lens systems which are to be used with single-lens reflex 
cameras. However, the lens system proposed by Japanese Patent Kokai 
Publication No. Hei 2-135,312 is a compact zoom lens system which is 
usable with lens shutter cameras. 
Each of the lens systems mentioned above cannot correct aberrations 
sufficiently favorably since it was obtained simply by modifying an 
exsisting zoom lens system so as to have a wider field angle. In other 
words, each of the zoom lens systems mentioned above is a telephoto type 
zoom lens system which is modified so as to have a shorter focal length or 
a wider focal length and is insufficient in optical performance thereof. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide a zoom lens system 
which comprises three or four lens units, and is configured so as to have 
wide field angles, large apertures and aberrations corrected favorably by 
selecting an adequate composition for a first lens unit out of the lens 
units. 
The zoom lens system according to the present invention comprises, in order 
from the object side, a first lens unit having positive refractive power, 
a second lens unit having positive refractive power and a third lens unit 
having negative refractive power; is configured to perform variation of 
focal length by varying an air-space reserved between the first lens unit 
and a second lens unit as well as an airspace reserved between the second 
lens unit and a third lens unit; and is characterized in that the first 
lens unit consists of a front subunit and a rear subunit, and satisfies 
the following condition (1): 
EQU .vertline..phi..sub.f .phi..sub.1 .vertline.&lt;6.0 (1) 
wherein the reference symbols .phi..sub.f and .phi..sub.1 represent 
refractive power of the front subunit and refractive power of the first 
lens unit respectively. 
The zoom lens system according to the present invention has been obtained 
by widening the field angle and further improving optical performance of 
the lens system proposed by Japanese Patent Kokai Publication No. Hei 
2-135,312. Speaking concretely, the field angle of the lens system 
disclosed by the above-mentioned patent has been widened reasonably by 
shortening a total length of the lens system, selecting the 
above-described type of zoom lens system which comprises the three lens 
units, in order from the object side, a first lens unit having positive 
refractive power, a second lens unit having positive refractive power and 
a third lens unit having negative refractive power, and composing the 
first lens unit so as to be suited for widening the field angle of the 
lens system. 
Japanese Patent Kokai Publication No. Hei 3-208,004 proposed a zoom lens 
system as an improvement of the above-mentioned lens system to facilitate 
correction of aberrations and widen the field angle thereof by reserving 
an airspace wider than a certain value between a negative lens component 
and a positive lens component composing the first lens unit without 
modifying a composition of this lens unit itself so as to reduce an angle 
of incidence of the offaxial ray on the second lens unit having positive 
refractive power. 
In contrast, the zoom lens system according to the present invention has a 
field angle widened by adopting a composition of the first lens unit which 
is modified so as to be suited for widening the field angle of the 
preceding lens system as a whole. In other words, the zoom lens system 
according to the present invention has been obtained not on the basis of a 
fundamental composition of the above-mentioned zoom lens system having the 
widened field angles, but on the basis of a fundamental composition of the 
preceding zoom lens system which has an entire zooming range from a wide 
position to a tele position or a range of field angles which is convenient 
for designing a zoom lens system and by shifting the composition toward 
the wide position. 
Speaking concretely, it is possible to obtain, for example, a zoom lens 
system having a range of focal length f from 28 mm to 105 nun by designing 
first a zoom lens system having focal length f from 35 nun to 135 mm and 
equipping this zoom lens system with an attachment lens system for 
widening field angle by shortening the focal length f 0.8 .times. as long. 
When the zoom lens system which is adopted as the basis has a short total 
length f and a small outside diameter, the addition of the attachment lens 
system for widening the field angle to the zoom lens system does not 
enlarge so much the zoom lens system as a whole. 
When a single zoom lens system is composed by building the attachment lens 
system for widening field angle in the zoom lens system adopted as the 
basis, this attachment lens system can be considered as a lens unit or a 
lens component which is used for correcting aberrations. That is to say, 
the attachment lens system which is originally adopted as a separate 
afocal converter can be regarded as a lens unit or a Lens component which 
is used for widening the field angle of the zoom lens system. 
In the zoom lens system according to the present invention, this lens 
component is used as a front subunit L.sub.a in the first lens unit and 
the first lens unit of the zoom lens system adopted as the basis is used 
as a rear subunit L.sub.b of the first lens unit of the zoom lens system 
according to the present invention. That is to say, the zoom lens system 
according to the present invention is of a retrofocus type in which a 
first lens unit is composed of a wide angle afocal converter (the front 
subunit) and a positive subunit (the rear subunit), and an airspace 
reserved between these subunits is narrowed for making the zoom lens 
system compact. 
On the basis of the concept described above, the zoom lens system according 
to the present invention comprises the three or four lens units as 
described above. 
Further, in order to accomplish the object of the present invention, the 
zoom lens system according to the present invention is configured so as to 
satisfy the above-mentioned condition (1). 
The object of the present invention is to provide a wide angle zoom lens 
system which has a field angle 2 .omega., for example, of approximately 
63.degree. or larger at a wide position thereof as described above. For 
obtaining the zoom lens system having a wide field angle, the type and 
composition of the conventional zoom lens system are not modified. When 
the field angle of the conventional zoom lens system is to be widened 
without modifying the type thereof, heavier burdens for correction of 
aberrations, etc. are imposed on the third lens unit, thereby making it 
impossible to reserve a sufficient back focal length at the wide position 
of the zoom lens system. Further, a heavier burden for obtaining a 
required magnification at a tele position is imposed, thereby making it 
difficult to correct aberrations and obliging to use surfaces which can 
hardly be manufactured in practice. 
Furthermore, aspherical surfaces and graded refractive index lens elements 
cannot be manufactured easily in practice though it is conceivable to use 
aspherical surfaces and graded refractive index lens elements for 
correcting aberrations. 
In an attempt to widen a field angle of a zoom lens system, difference in 
optical path between the paraxial ray and the offaxial ray in the lens 
system makes it difficult to correct aberrations. In other words, 
curvature of field, distortion, sagittal flare due to coma, etc. are 
aggravated as a zoom lens system has a wider field angle. 
For the reason described above, the present invention selects a zoom lens 
system comprising the three lens units as the basis as already described 
above and adopts a nearly symmetrical composition wherein an aperture stop 
is arranged in the middle of a second lens unit for obtaining a zoom lens 
system which is advantageous for correction of the aberrations. Even when 
the location of the aperture stop is shifted foreward or backward, the 
fundamental composition of the zoom lens system remains unchanged though 
the zoom lens system uses a different portion of a light bundle. 
After adopting the nearly symmetrical composition which is advantageous for 
correction of the aberrations, lens components to be used for the zoom 
lens system are modified so as to be suited for widening the field angle 
of the conventional zoom lens system. Since the type selected for the zoom 
lens system according to the present invention permits shortening a back 
focal length of a lens system, the type makes it possible to shorten a 
total length and reduce an outside diameter of the lens system. That is to 
say, the zoom lens system according to the present invention is configured 
by selecting the composition which is symmetrical with regard to an 
aperture stop as the basis and using additional lens component(s) which is 
adopted for shortening the field angle of the conventional zoom lens 
system. In the zoom lens system according to the present invention, the 
front subunit serves for compensating for functions of the rear subunit in 
the first lens unit. Judging from rays passing through the first lens unit 
and the third lens unit in the zoom lens system according to the present 
invention, offaxial aberrations are largely influenced by the second lens 
unit and the third lens unit, whereas axial aberrations, mainly spherical 
aberration, are greatly influenced by the second lens unit. 
Moreover, in order to obtain a zoom lens system having a wide field angle, 
it is necessary to correct aberrations such as lateral chromatic 
aberration and distortion having an inflection point which are inherent in 
a wide angle lens system. That is to say, it is necessary for the zoom 
lens system according to the present invention to pay attention to 
compositions of the first lens unit and the third lens unit in order to 
correct the aberrations inherent in a zoom lens system having a wide field 
angle and pay attention to composition of the second lens unit in order to 
obtain a high aperture ratio. 
For the wide position of the zoom lens system according to the present 
invention, it is not advantageous to compose the thrid lens unit of a 
remarkably increased number of lens components. Since a power distribution 
which locates a rear principal point after a lens system is selected for 
the zoom lens system according to the present invention, reservation of a 
required back focal length is more difficult than the correction of the 
aberrations for the zoom lens system according to the present invention, 
thereby lowering freedom of the correction of the aberrations for this 
zoom lens system. Accordingly, it is preferable to compose the third lens 
unit of a small number of lens components by using an aspherical surface. 
The use of the aspherical surface makes it possible to reduce aberrations 
remaining in the third lens unit itself and correct aberrations of high 
orders produced in the first lens unit, thereby permitting correcting 
aberrations with good balance in the zoom lens system as a whole. Further, 
the use of the aspherical surface makes it possible to reserve a required 
back focal length reasonably, prevent a rear lens element from having a 
prolonged diameter, and reduce flare to be produced by an inside surface 
between a film surface and the rear lens element. 
When the points described above are taken into consideration, it is 
desirable to contrive the first lens unit so as to correct the lateral 
chromatic aberration and distortion of high orders which are inherent in 
the wide angle zoom lens system. 
In the vicinity of the wide position of the zoom lens system wherein 
particularly the offaxial ray is high, it is difficult to correct the 
residual aberrations of high orders by the second lens unit and the third 
lens unit when the aberrations are not corrected sufficiently in the first 
lens unit. This fact is apparent from tables of aberration correction 
coefficients shown below. Checks of balance between aberrations of high 
orders and aberrations of low orders is especially significant. 
Tables 1 and 2 shown below clarify conditions of the aberrations corrected 
by aberration coefficients which are selected in a first embodiment of the 
present invention to be described later. 
TABLE 1 
__________________________________________________________________________ 
Wide position 
__________________________________________________________________________ 
K SA3 CMA3 AST3 DIS3 PTZ3 
__________________________________________________________________________ 
1 -0.00001 
-0.00060 
-0.00634 
-0.68756 
-0.01525 
Lf 2 0.00393 
-0.00302 
0.00026 
-0.01807 
0.07043 
3 -0.00388 
-0.00784 
-0.00176 
-0.03585 
-0.05142 
4 0.00011 
0.00380 
0.01529 
0.35334 
0.01401 
5 0.00000 
0.00004 
-0.01272 
-0.66454 
0.01346 
6 0.01626 
0.00255 
0.00004 
0.00360 
0.06885 
Lb 7 -0.01313 
-0.01030 
-0.00090 
-0.01394 
-0.05240 
8 0.00030 
0.00926 
0.03159 
0.29145 
-0.00313 
9 -0.00643 
-0.01924 
-0.00639 
-0.06723 
-0.06106 
10 -0.00041 
0.01420 
-0.05513 
0.88004 
-0.02043 
(1) 
Lf 0.00016 
-0.00765 
0.00744 
-0.38814 
0.01777 
Lb -0.00341 
-0.00348 
-0.04350 
0.42938 
-0.05471 
(2) -0.00568 
-0.00308 
0.11297 
-0.99783 
-0.10148 
(3) 0.00164 
0.00491 
-0.07868 
0.54632 
0.13018 
__________________________________________________________________________ 
K SA5 CMA5 AST5 DIS5 PTZ5 SA7 
__________________________________________________________________________ 
1 0.00000 
0.00000 
0.00089 
0.19925 
0.00537 
0.00000 
Lf 2 0.00005 
0.00010 
-0.00019 
-0.04197 
-0.02465 
0.00000 
3 -0.00006 
-0.00030 
0.00309 
0.05851 
0.02059 
0.00000 
4 0.00000 
0.00001 
-0.00278 
-0.17371 
-0.00664 
0.00000 
5 0.00000 
0.00000 
0.00140 
0.29779 
-0.00517 
0.00000 
6 0.00045 
0.00065 
-0.00128 
-0.06874 
-0.03299 
0.00001 
Lb 
7 -0.00038 
-0.00086 
0.00385 
0.06245 
0.02784 
-0.00001 
8 0.00001 
0.00015 
-0.00667 
-0.24618 
-0.00634 
0.00000 
9 -0.00013 
-0.00038 
0.00605 
0.10153 
0.03024 
0.00000 
10 -0.00001 
0.00027 
-0.00851 
-0.05951 
0.00708 
0.00000 
(1) 
Lf -0.00001 
-0.00019 
0.00100 
0.04207 
-0.00532 
0.00000 
Lb -0.00006 
-0.00017 
-0.00516 
0.08734 
0.02066 
0.00000 
(2) 0.00215 
0.00210 
0.02325 
0.11002 
0.04238 
0.00044 
(3) -0.00011 
-0.00182 
-0.01764 
-0.23535 
-0.04339 
-0.00001 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Tele position 
__________________________________________________________________________ 
K SA3 CMA3 AST3 DIS3 PTZ3 
__________________________________________________________________________ 
1 -0.00012 
-0.00129 
-0.00153 
-0.02522 
-0.00533 
2 0.07407 
-0.15959 
0.03821 
-0.04511 
0.02461 
3 -0.07297 
0.12950 
-0.02554 
0.02573 
-0.01796 
4 0.00198 
0.00570 
0.00182 
0.00645 
0.00489 
5 0.00000 
0.00011 
-0.00449 
-0.02568 
0.00470 
6 0.30610 
-0.62100 
0.13998 
-0.11093 
0.02405 
7 -0.24715 
0.48029 
-0.10370 
0.07903 
-0.01831 
8 0.00567 
0.01211 
0.00287 
0.00127 
-0.00109 
9 -0.12109 
0.19893 
-0.03631 
0.03156 
-0.02133 
10 0.00765 
0.05210 
-0.03943 
0.10570 
-0.00714 
(1) 
Lf 0.00296 
-0.02568 
0.01292 
-0.03815 
0.00621 
Lb -0.06413 
0.12254 
-0.04108 
0.08095 
-0.01912 
(2) -0.04759 
-0.01905 
0.04255 
-0.06852 
-0.03545 
(3) 0.10867 
-0.07190 
-0.01371 
0.05029 
0.04548 
__________________________________________________________________________ 
K SA5 CMA5 AST5 DIS5 PTZ5 SA7 
__________________________________________________________________________ 
1 0.00000 
0.00000 
0.00003 
0.00058 
0.00013 
0.00000 
Lf 
2 0.00378 
-0.01177 
0.00036 
0.00004 
-0.00079 
0.00023 
3 -0.00467 
0.01264 
-0.00018 
-0.00005 
0.00061 
-0.00035 
4 0.00003 
-0.00003 
0.00001 
-0.00043 
-0.00024 
0.00000 
5 0.00000 
-0.00002 
-0.00015 
0.00139 
-0.00003 
0.00000 
6 0.03670 
-0.11819 
0.00751 
-0.00661 
-0.00013 
0.00506 
Lb 
7 -0.03092 
0.09610 
-0.00552 
0.00477 
0.00010 
-0.00437 
8 0.00066 
-0.00059 
-0.00011 
-0.00036 
-0.00021 
0.00008 
9 -0.01079 
0.03259 
-0.00184 
0.00146 
0.00039 
-0.00114 
10 -0.00095 
0.00630 
-0.00199 
0.00210 
0.00044 
-0.00014 
(1) 
Lf -0.00086 
0.00083 
0.00021 
0.00014 
-0.00028 
-0.00012 
Lb -0.00530 
0.01618 
-0.00209 
0.00276 
0.00056 
-0.00052 
(2) 0.06153 
0.00426 
0.00182 
-0.00011 
0.00112 
0.03223 
(3) -0.05301 
-0.04412 
-0.00037 
0.00050 
-0.00148 
-0.03257 
TOTAL 0.00235 
-0.02284 
-0.00043 
0.00329 
-0.00008 
-0.00097 
__________________________________________________________________________ 
Table 1 lists values of the aberrations at the wide position of the zoom 
lens system according to the present invention, whereas Table 2 lists 
values of the aberrations at the tele position thereof. Further, 
aberration coefficients of the lens units other than the first lens unit 
are listed as total sums of aberration coefficients characteristic of 
surfaces of these lens units. The reference symbols k(1-4) and L.sub.f 
represent the front subunit, the reference symbols k(5-10) and L.sub.b 
designate the rear subunit, and the reference numerals (2) and (3) denote 
the second lens unit and the third lens unit respectively. 
It will be understood from the tables shown above that spherical aberration 
which is one of the axial aberrations is little in the first lens unit. 
On the other hand, it will be understood that the astigmatism and 
distortion which are the offaxial aberrations are corrected favorably in 
the first lens unit. That is to say, the front subunit effectively 
overcorrects the astigmatism which is to be undercorrected by the rear 
subunit and undercorrects the distortion which is to be overcorrected by 
the rear subunit in the first embodiment of the present invention. This 
fact or the compensation of the functions of the rear subunit by the front 
subunit can be seen from FIG. 3 illustrating a diagram wherein an offaxial 
ray which is incident at a large angle on the front subunit has a small 
angle of incidence on the rear subunit. 
Considering the fact that it is difficult to widen a field angle of a zoom 
lens system by using the composition of the first lens unit of the 
conventional zoom lens system, the present invention has modified the 
angle of incidence on the first lens unit to a value which facilitates the 
correction of the aberrations by arranging additional lens component(s) 
having weak refractive power before the first lens unit so as to widen the 
field angle of the conventional zoom lens system as described above. The 
additional lens component(s) and the first lens unit of the conventional 
zoom lens system are used as the front subunit and the rear subunit 
respectively which compose the new first lens unit to be used in the zoom 
lens system according to the present invention. Accordingly, the new first 
lens unit has the composition which facilitates the correction of the 
aberrations even when a zoom lens system has a widened field angle, and 
makes it possible to obtain a zoom lens system which has a wide field 
angle and favorably corrected aberrations. The front subunit of the first 
lens unit having the new composition may be composed of a single negative 
lens component, but it will be effective for correcting the aberrations 
more favorably to compose the front subunit of a negative lens component 
and a positive lens component. 
On the other hand, in order that the first lens unit has a small outside 
diameter and that the zoom lens system has an entrance pupil at a location 
which is not too far from the first surface thereof, it is desirable to 
configure the zoom lens system according to the present invention so as to 
satisfy the conditions to be described below. 
It is desirable that the front subunit of the first lens unit comprises at 
least one negative lens component and that the rear subunit comprises at 
least one positive lens component and one negative lens component. 
Further, it is desirable that the zoom lens system according to the present 
invention satisfies the conditions (2) through (4) mentioned below: 
EQU 0.1&lt;.phi..sub.1 /.phi..sub.W &lt;1.25 (2) 
EQU 1.1&lt;.phi..sub.12W /.phi..sub.W &lt;3.0 (3) 
EQU 1.5&lt;.beta..sub.3T /.beta..sub.3W &lt;4.0 (4) 
wherein the reference symbol .phi..sub.W represents refractive power of the 
zoom lens system as a whole at the wide position thereof, the reference 
symbol .phi..sub.1 designates refractive power of the first lens unit, the 
reference symbol .phi..sub.12W denotes total refractive power of the first 
lens unit and the second lens unit at the wide position, and the reference 
symbols .beta..sub.3W and .beta..sub.3T represent magnifications of the 
third lens unit at the wide position and the tele position respectively. 
The condition (2) defines the refractive power of the first lens unit. If 
the upper limit of the condition (2) is exceeded, it will be difficult to 
correct the aberrations. If the lower limit of the condition (2) is 
exceeded, it will be necessary to move the lens units for long distances 
for zooming, thereby producing a disadvantage for compact design of the 
zoom lens system according to the present invention. 
The condition (3) is required for shortening total lengths of the first 
lens unit and the second lens unit, and determining refractive power which 
is desirable for the favorable correction of the aberrations. If the upper 
limit of the condition (3) is exceeded, it will be impossible to obtain 
sufficiently favorable optical performance of the zoom lens system 
according to the present invention though the lens system can be made 
compact. If the lower limit of the condition (3) is exceeded, the zoom 
lens system according to the present invention will be enlarged. 
The condition (4) relates to a vari-focal ratio. If the lower limit of the 
condition (4) is exceeded, it will practically be impossible to obtain a 
zoom lens system which has a high vari-focal ratio. 
Further, when attention is paid to an axial ray and an offaxial ray, as 
considered according to the paraxial theory, which pass through the first 
lens unit, it is desirable that the zoom lens system according to the 
present invention satisfies the following conditions (5) and (6): 
EQU 0.5&lt;h.sub.B /h.sub.F &lt;1.5 (5) 
EQU 0.2&lt;AB/AF &lt;2.0 (6) 
wherein the reference symbol h.sub.F represents a height of the axial ray 
incident on the front subunit of the first lens unit at the wide position, 
the reference symbol h.sub.B designates a height of the offaxial ray 
incident on the rear subunit of the first lens unit at the wide position, 
the reference symbol AF denotes a height of the axial ray incident on the 
front subunit of the first lens unit at the wide position and the 
reference symbol AB represents a height of the offaxial ray incident on 
the rear sub-unit of the first lens unit at the wide position. 
The condition (5) means that weak refractive power is to be imparted to the 
front subunit of the first lens unit even when the field angle of the zoom 
lens system is to be widened by designing the first lens unit as an 
inverted telephoto type with negative refractive power imparted to the 
front subunit or when positive refractive power is imparted to the front 
subunit. 
If the lower limit of the condition (5) is exceeded, the front subunit will 
have a weak function for correcting the offaxial aberrations at the wide 
position. If the upper limit of the condition (5) is exceeded, the front 
subunit will have too weak refractive power and must comprise an increased 
number of lens components for correcting the aberrations, thereby 
enlarging the zoom lens system as a whole. 
The condition (6) has significance which is similar to that of the 
condition (5). If the lower limit of the condition (6) is exceeded, the 
front subunit will have strong refractive power. If the upper limit of the 
condition (6) is exceeded, it will be difficult to correct the aberrations 
at the wide position. 
The conditions (5) and (6) are to be satisfied for facilitating the 
correction of the aberrations by narrowing the angle of incidence of the 
offaxial ray on the first lens unit. In other words, when these conditions 
are not satisfied, the aberration correcting functions will be effective 
so far as the zoom lens system has a field angle at the wide position 
which is close to that of an ordinary telephoto type lens system but the 
offaxial aberrations will be corrected with lens effect at the super wide 
angle of the zoom lens system according to the present invention. 
Though description has been made above of a concept for composing the zoom 
lens system which consists of the three lens units, this concept is 
applicable also to a zoom lens system which comprises four lens units. 
That is to say, it is possible to obtain a zoom lens system accomplishing 
the object of the present invention by: composing the zoom lens system, in 
order from the object side, of a first lens unit having positive 
refractive power, a second lens unit having positive refractive power, a 
third lens unit having positive refractive power and a fourth lens unit 
having negative refractive power as shown in FIG. 2; configuring the lens 
system so as to perform variation of focal length by varying an airspace 
reserved between the first lens unit and the second lens unit, an airspace 
reserved between the second lens unit and the third lens unit, and an 
airspace reserved between the third lens unit and the fourth lens unit; 
and composing the first lens unit of a front subunit and a rear subunit so 
as to permit widening a field angle of the lens system and satisfy the 
condition (1). 
Refracting conditions of the thin lens component and thick lens components 
arranged in the first lens unit are illustrated in FIGS. 4, 5 and FIGS. 6 
and 7 respectively.