Variable magnification copying lens system

A variable magnification copying lens system with the arrangement composed, in order from the object side, of a first lens unit (A) having a negative focal length, a lens group (B) having a positive focal length, and a second lens unit (C) having a negative focal length. The system being capable of maintaining a constant distance between the object surface and the image surface not only by changing the distance between the first lens unit (A) and the lens group (B) and that between the lens group (B) and the second lens unit (C) but also by moving the overall lens system. The first and second lens units (A) and (C) have the primary function of maintaining a constant distance between the object surface and the image surface by a substantially symmetrical movement with respect to the lens group (B). Each of the first and second lens units being simply made of a single negative lens. The lens group (B) has the primary function of zooming and a five-unit and five-element configuration composed of a center biconvex lens, a negative meniscus lens that is positioned on both sides of the biconvex lens and the concave surface of which is directed toward the center, and a positive meniscus lens that is positioned on the outer side of the negative meniscus lens and the concave surface of which is directed toward the center.

BACKGROUND OF THE INVENTION 
The present invention relates to a variable magnification copying lens 
system that is useful in the optics of a copying machine and which is 
capable of realizing both size enlargement and reduction while maintaining 
a constant distance between the object surface and the image surface. 
The conventional variable magnification lens systems capable of size 
enlargement and reduction while maintaining a constant distance between 
the object and image are classified into the following three types: 
(i) a system that is simply composed of two negative and positive, or 
positive and negative, lens groups, but which is incapable of attaining a 
zoom ratio of 2 (see, for example, Japanese Patent Publication No. 
13887/1983 and Unexamined Published Japanese Patent Application No. 
68810/1982); 
(ii) a system that is composed of three negative, positive and negative 
lens groups, or four negative, positive, positive and negative lens 
groups, but which is incapable of attaining a zoom ratio much greater than 
2 (see, for example, Unexamined Published Japanese Patent Application Nos. 
159614/1981, 67909/1982 and 67512/1984); and 
(iii) a system that is composed of three negative, positive and negative 
lens groups, or four negative, positive, positive and negative lens 
groups, and which is capable of attaining a zoom ratio as great as 4 or 9, 
but which is useful in a variable magnification lens system for 
platemaking, rather than copying, purposes because of the large 
object-to-image distance (small view angle), the large size of the overall 
lens system and the great number of lens elements incorporated (see, for 
example, Unexamined Published Japanese Patent Application Nos. 1242/1974, 
60655/1978 and 11260/1980). 
However, these conventional lens systems are either low in zoom ratios (a 
little more than 2) or bulky and are incapable of meeting the demands of 
recent versions for relatively high zoom ratios and compact arrangements. 
The conventional systems consisting of three or four lens groups use a 
master lens group (reference lens group) with six or more lens elements 
arranged in a substantially symmetrical fashion, and in some of the 
systems, the aerial distance in the center is fixed, but in most systems, 
the central aerial distance is variable and requires complicated lens 
arrangements and zooming methods. 
SUMMARY OF THE INVENTION 
The primary object, therefore, of the present invention is to provide a 
variable magnification copying lens system that consists of three 
negative, positive and negative lens groups and which attains a zoom ratio 
greater than 3. The lens system of the present invention requires a small 
object-to-image distance (wide view angle), is compact in size, uses a 
reduced number of lens elements and can be manufactured at low cost. 
The variable magnification copying lens system of the present invention 
basically has the arrangement, in order from the object side, of a first 
lens unit (A) having a negative focal length, a lens group (B) having a 
positive focal length, and a second lens unit (C) having a negative focal 
length, and is capable of maintaining a constant distance between the 
object surface and the image surface not only by changing the distance 
between the first lens unit (A) and the lens group (B) and that between 
the lens group (B) and the second lens unit (C) but also by moving the 
overall lens system. This lens system is characterized in that said first 
and second lens units (A) and (C) have the primary function of maintaining 
a constant distance between the object surface and the image surface by a 
substantially symmetrical movement with respect to the lens group (B), 
each of said first and second lens units being simply made of a single 
negative lens, said lens group (B) having the primary function of zooming 
and having a five-unit and five-element configuration composed of a center 
biconvex lens, a negative meniscus lens that is positioned on both sides 
of said biconvex lens and the concave surface of which is directed toward 
the center, and a positive meniscus lens that is positioned on the outer 
side of said negative meniscus lens and the concave surface of which is 
directed toward the center. 
In a preferred embodiment of the variable magnification copying lens system 
of the present invention, either the first lens unit (A) or the second 
lens unit (C) satisfies the following conditions: 
EQU -0.8&lt;F/F.sub.I(III) &lt;-0.35; (1) 
EQU 40&lt;.nu..sub.I(III) ; and (2) 
EQU -1.5&lt;F/f.sub.1(14) &lt;-0.8 (3) 
wherein 
the focal length of the overall system for unity magnification; 
F.sub.I(III) : the focal length of the first or second lens unit; 
.nu..sub.I(III) : the Abbe number of the negative lens in the first or 
second lens unit; and 
f.sub.1(14) : the focal length of the first or 14th surface, and the lens 
group (B) satisfies the following conditions: 
EQU 2.5&lt;F/f.sub.3(12) &lt;4.0; (4) 
EQU -4.0&lt;F/f.sub.6(9) &lt;-2.5; and (5) 
EQU 1.2&lt;F/f.sub.7(8) &lt;2.5 (6) 
wherein 
f.sub.3(12) : the focal length of the third or 12th surface; 
f.sub.6(9) : the focal length of the sixth or ninth surface; and 
f.sub.7(8) : the focal length of the seventh or eighth surface. 
This embodiment is advantageous for the purposes of providing a compact 
system configuration and compensation of aberrations. 
From the viewpoint of manufacturing costs, the system of the present 
invention features a completely symmetrical arrangement of lens elements. 
In consideration of the space within the copying machine, another preferred 
embodiment of the lens system of the present invention is such that the 
following condition is satisfied at the enlargement end: 
EQU 0.8&lt;.DELTA.D.sub.I II /.DELTA.D.sub.II III .ltoreq.1.0 (7) 
wherein .DELTA.D.sub.I II is the amount of change in the distance between 
the first lens unit (A) and the lens group (B), and .DELTA.D.sub.II III is 
the amount of change in the distance between the lens group (B) and the 
second lens unit (C), each amount of change being measured with respect to 
the lens group (B).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The conditions (1), (2) and (3) concern the first lens unit (A) or the 
second lens unit (C). 
The condition (1) concerns the power of either lens unit. If the upper 
limit for this condition is exceeded, advantages result for the purpose of 
compensating aberrations, but because of the small change that can be 
realized in the focal length of the overall system in comparison with the 
change in the distance between the first lens unit (A) and the lens group 
(B) and that between the lens group (B) and the second lens unit (C), a 
zoom ratio of 2 upward can only be attained by increasing the distance 
between the first lens unit (A) and the lens group (B) and that between 
the lens group (B) and the second lens unit (C), but then, this inevitably 
requires the use of extremely large-diameter lens elements in the first 
and second lens units. If the lower limit for the condition (1) is not 
reached, the increased power presents advantages for the purpose of 
reducing the size of the overall system, but on the other hand, a single 
lens element is insufficient to effectively compensate for variations in 
such aberrations as chromatic aberration, spherical aberration and 
curvature of field in a zooming mode. 
The condition (2) must be met in order to make up each of the first and 
second lens units (A) and (C) by a single negative lens, and if this 
condition is not met, variations in the chromatic aberration cannot be 
effectively compensated in a zooming mode. 
The condition (3) concerns the power of the surface on the object side of 
the first lens unit (r.sub.1) or the surface on the image side of the 
second lens unit (r.sub.14). By making these surfaces divergent, a 
curvature of field can be overcompensated and a positive distortion can be 
obtained, with the result that a balance is achieved between the first or 
second lens unit and the lens group (B) so as to provide a compact and 
high-performance lens system. If the upper limit for the condition (2) is 
exceeded, the power of each of the divergent surfaces, and hence, the 
power of each lens unit is so much reduced that a large lens system is 
necessary to achieve a balance with the lens group (B). If the lower limit 
for condition (3) is not reached, the power of each of the divergent 
surfaces is too great to provide a balance with the lens group (B). Taking 
an astigmatism as an example, it is undercompensated at an intermediate 
view angle whereas it is overcompensated on the periphery, thereby making 
it impossible to obtain a balance between the intermediate and peripheral 
portions. 
The conditions (4), (5) and (6) concern the lens group (B) and must be met 
for the purpose of achieving a balance in the compensation of aberrations 
developing in the divergent surfaces of the first and second lens units 
(A) and (C). 
The condition (4) specifies the powers of the two outermost surfaces 
(r.sub.3 and r.sub.12) of the lens group (B). These surfaces are 
convergent, and if the lower limit for the condition (4) is not reached, 
they have too small powers to achieve a balance with the divergent 
surfaces of the first and second lens units (A) and (C). If the upper 
limit for the condition (4) is exceeded, the powers of the convergent 
surfaces become so great that considerable difficulty is involved in 
compensating for spherical and coma aberrations. 
The condition (5) concerns the powers of two divergent surfaces (r.sub.6 
and r.sub.9) within the lens group (B). Although the lens group (B) is 
positive, some compensation for aberrations within this lens group is 
necessary in order to avoid excessive variations of aberrations that may 
occur in a zooming mode. If the lower limit for the condition (5) is not 
reached, the powers of the divergent surfaces (r.sub.6 and r.sub.9) become 
so great as to cause overcompensation of spherical and coma aberrations. 
If the upper limit for the condition (5) is exceeded, spherical and coma 
aberrations that will develop in the outermost surfaces of the lens group 
(B) cannot be properly compensated. 
The condition (6) concerns the powers of the two surfaces (r.sub.7 and 
r.sub.8) of the positive lens positioned in the center of the lens group 
(B). Like the surfaces r.sub.3 and r.sub.12 specified by the condition 
(4), the surfaces r.sub.7 and r.sub.8 are convergent. Both conditions (4) 
and (6) have controlling effects on spherical and coma aberrations, but 
the effect of the coma abberation on the spherical aberration is greater 
in condition (6) than in condition (4). 
The lens system of the present invention permits the use of a pair of lens 
elements of the identical shape by arranging them symmetrically with 
respect to the center line, and therefore, the overall system can be 
manufactured efficiently and at low cost. 
The condition (7), that is preferably satisfied by the lens system of the 
present invention at the enlargement end, states that the amount of 
movement of the first lens unit (A) with respect to the lens group (B) 
should be made smaller than that of the second lens unit (C). If this 
condition is met, the distance between the object and the lens system is 
sufficiently increased to ensure the mirror installation space for copying 
machines which are usually designed to accommodate a mirror on the object 
side of the lens system. 
As will be apparent from the foregoing description, the present invention 
provides a variable magnification copying lens system that uses a reduced 
number of lens elements, requires a simple zooming method, is compact in 
size and experiences minimum degrees of aberrations. 
EXAMPLES 
Examples 1 to 4 of the present invention are shown below, wherein r=the 
radius of curvature of a specific lens surface, d=the lens thickness or 
distance between lens surfaces, N=the refractive index of a specific lens 
at d-line, .nu.=the Abbe number of a specific lens, F=the focal length of 
the overall system for unity magnification, Fe=the effective F number for 
unity magnification, f.sub.B =back focus, L=the distance between the 
object and the image surface, and m=magnification range. 
______________________________________ 
Example 1 
F No. 1:8 Fe = 16 F = 135.4 f.sub.B = 195.3 - 339.5 
L = 550.9 m = -0.64 - -2 
Surface 
No. r d N .nu. 
______________________________________ 
1 -64.416 3.000 1.62299 
58.2 
2 -138.849 3.333 - 6.301(-0.64X) - 
10.600(-2X) 
3 22.710 3.861 1.62299 
58.2 
4 34.290 0.727 
5 71.747 1.200 1.60342 
38.0 
6 22.266 3.009 
7 41.545 3.991 1.67003 
47.3 
8 -41.545 3.009 
9 -22.266 1.200 1.60342 
38.0 
10 -71.747 0.727 
11 -34.290 3.861 1.62299 
58.2 
12 -22.710 3.000 - 6.297(-0.64X) - 
11.074(-2X) 
13 138.849 3.000 1.62299 
58.2 
14 64.416 258.134 
F/F.sub.I(III) = -0.694 
.nu..sub.I(III) = 58.2 
F/f.sub.1(14) = -1.309 
F/f.sub.3(12) = 3.71 
F/f.sub.6(9) = -3.67 F/f.sub. 7(8) = 2.18 
.DELTA. D.sub.I II /.DELTA. D.sub.II III = 0.90 
______________________________________ 
Example 2 
F No. 1:8 Fe = 16 F = 135.4 f.sub.B = 195.2 - 339.6 
L = 551.0 m = -0.64 - -2 
Surface 
No. r d N .nu. 
______________________________________ 
1 -64.331 3.000 1.62041 
60.3 
2 -142.200 3.000 - 5.887(-0.64X) - 
10.069(-2X) 
3 22.798 3.973 1.62041 
60.3 
4 34.206 0.709 
5 69.778 1.200 1.60342 
38.0 
6 22.297 3.010 
7 41.497 3.988 1.67003 
47.3 
8 -41.497 3.010 
9 -22.297 1.200 1.60342 
38.0 
10 -69.778 0.709 
11 -34.206 3.973 1.62041 
60.3 
12 -22.798 3.000 - 6.208(-0.64X) - 
10.853(-2X) 
13 142.200 3.000 1.62041 
60.3 
14 64.331 257.960 
F/F.sub.I(III) = -0.707 
.nu..sub.I(III) = 60.3 
F/f.sub.1(14) = -1.306 
F/f.sub.3(12) = 3.68 
F/f.sub.6(9) = -3.66 F/f.sub.7(8) = 2.19 
.DELTA. D.sub.I II /.DELTA. D.sub.II III = 0.90 
______________________________________ 
Example 3 
F No. 1:8 Fe = 16 F = 135.3 f.sub.B = 191.5 - 334.0 
L = 548.6 m = -0.64 - -2 
Surface 
No. r d N .nu. 
______________________________________ 
1 -70.160 3.000 1.51633 
64.1 
2 -168.593 3.000 - 6.861(-0.64X) - 
12.469(-2X) 
3 26.790 4.420 1.57135 
53.0 
4 43.621 2.240 
5 113.000 1.200 1.60342 
38.0 
6 26.470 2.980 
7 45.985 3.520 1.72000 
50.2 
8 -45.985 2.980 
9 -26.470 1.200 1.60342 
38.0 
10 -113.000 2.240 
11 -43.621 4.420 1.57135 
53.0 
12 -26.790 3.333 - 7.623(-0.64X) - 
13.853(-2X) 
13 168.593 3.000 1.51633 
64.1 
14 70.160 255.226 
F/F.sub.I(III) = -0.577 
.nu..sub.I(III) = 64.1 
F/f.sub.1(14) = -0.996 
F/f.sub.3(12) = 2.89 
F/f.sub.6(9) = -3.08 F/f.sub.7(8) = 2.12 
.DELTA. D.sub.I II /.DELTA. D.sub.II III = 0.90 
______________________________________ 
Example 4 
F No. 1:8 Fe = 16 F = 136.0 f.sub.B = 189.0 - 327.7 
L = 552.8 m = -0.64 - -2 
Surface 
No. r d N .nu. 
______________________________________ 
1 -87.366 3.000 1.70154 
41.2 
2 -138.841 3.000 - 9.442(-0.64X) - 
18.894(-2X) 
3 31.604 4.441 1.69680 
55.5 
4 63.302 2.151 
5 257.538 1.334 1.59551 
39.2 
6 30.099 4.402 
7 64.386 3.024 1.74400 
44.7 
8 -64.386 4.402 
9 -30.099 1.334 1.59551 
39.2 
10 -257.538 2.151 
11 -63.302 4.441 1.69680 
55.5 
12 -31.604 3.000 - 10.157(-0.64X) - 
20.659(-2X) 
13 138.841 3.000 1.70154 
41.2 
14 87.366 255.904 
F/F.sub.I(III) = -0.397 
.nu..sub.I(III) = 41.2 
F/f.sub.1(14) = -1.092 
F/f.sub.3(12) = 3.00 
F/f.sub.6(9) = -2.69 F/f.sub.7(8) = 1.57 
.DELTA. D.sub.I II /.DELTA. D.sub.II III = 0.90 
______________________________________