Lens assembling structure

A lens assembling structure including a lens system and a holding member for holding each lens element, a particular lens element in the lens system including a lens function surface and an abutting surface. The abutting surface is designed such that an inclination center due to decentering is located on the optical axis and at a point where aberration deterioration due to inclination is reduced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an assembling structure when a lens is assembled 
to a holding member such as lens barrel, and more specifically to a lens 
assembling structure which is hardly adversely affected even when the lens 
is decentered from a regular position by decentering, etc. 
2. Description of the Prior Art 
When a lens L is assembled to a holding member 10, one end face of the lens 
L, as shown in FIG. 8, abuts against the holding member 10 and the other 
end face is secured to the holding member 10 by a ring screw (not shown). 
If the lens L is assembled to the holding member 10 in this manner, when 
the lens L is decentered caused by assembling errors or vibrations, the 
lens L is inclined about the center R2 of curvature of the abutting 
surface against the holding member 10. The term "decentering" when used 
herein refers to a movement perpendicular to the optical axis of the lens 
L, and the term "inclination" refers to a rotational motion of the lens L 
which is caused to move in such a manner as to have an angle between the 
optical axis of the lens L and the optical axis of a whole lens system by 
the decentering. 
However, when the lens is rotated about the center of curvature, the 
rotation of the lens acts in such a manner as to increase the generation 
of aberration. As a result, there is a high possibility that performance 
of the lens system deteriorates. 
Particularly, when the lens is formed of a material having a large 
coefficient of expansion such as plastic, it is necessary to reserve a 
large clearance in order to avoid generation of warp caused by rising of 
the internal pressure of the lens which is caused by expansion of such 
material. Therefore, a problem is caused when decentering occurs, since 
the decentering amount is large and deterioration of the lens system is 
great. 
SUMMARY OF THE INVENTION 
The present invention has been accomplished in view of the above problem. 
It is therefore the object of an invention to provide a lens assembling 
structure which is capable of rotating a lens in the direction of offset 
the adverse effect of the performance of the lens caused by the 
decentering, even when a particular lens in a lens system is decentered. 
A lens assembling structure according to the present invention is 
characterized in that an abutting surface to a holding member of a 
particular lens in a lens system is designed such that the center of 
inclination of the particular lens resulting from decentering is 
positioned on the optical axis of the lens system where aberration 
deterioration caused by inclination is less that is, the aberration 
deterioration caused by inclination is minimized.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
As is shown in FIG. 1, which is a schematic view illustrating the change of 
aberration when a lens inclines. Character X denotes the coordinate in the 
direction of an optical axis. When a lens L is inclined about a particular 
point on the optical axis, if attention is paid to a certain aberration, a 
changed amount .delta. of the aberration caused by the inclination can be 
approximated by a linear equation with respect to the x-coordinate on the 
optical axis. Therefore, by finding the changed amount .delta.(P1) of the 
aberration when the lens is inclined about a point P1 in front of the 
lens, and the changed amount .delta.(P2) of the aberration when the lens 
is inclined about a point P2 behind the lens by ray tracing, an 
inclination center (x=P0) of the lens can be found which does not cause 
certain aberration even when lens L is inclined. 
An example of the use of the linear equation will now be described. It is 
known that when inclination of the lens is small, an amount of change of 
aberration .delta. is proportional to a coordinate of a center of 
inclination. This relationship is shown by the following equation: 
EQU .delta.(x)=a(x-b).theta., 
where 
.delta.(x) is the amount of change of aberration 
a is a constant 
x is the x-coordinate of the center of inclination 
b is a constant which is equal to the distance between points O and P.sub.o 
in FIG. 1 
.theta. is the angle of inclination 
FIG. 1 shows a lens structure and an amount of change of aberration. If the 
angle of inclination .theta. is fixed, .delta.(P.sub.1) and 
.delta.(P.sub.2), which are the changes of aberration when the inclination 
center is positioned at P.sub.1 and P.sub.2, are obtained by known methods 
of ray tracking. It also may be considered that the change of aberration 
is proportional to the coordinate of the center of inclination when the 
.theta. is fixed. 
Therefore, the following two equations may be obtained. And from the 
equations, the constants a and b may be determined. 
EQU .delta.(P.sub.1)=a(P.sub.1 -b).theta. 
EQU .delta.(P.sub.2)=a(P.sub.2 -b).theta. 
Since the constants a and b are determined, the coordinate x where the 
change of aberration .delta.(x) is minimized can be found. 
Further, if there are two or more aberrations which must be paid attention, 
the center of the inclination is decided so that the balance of affections 
to any aberrations is kept after finding P0 of each aberration. 
When a lens L is decentered, and inclined about point P0, aberration due to 
the inclination does not occur. 
FIG. 2 shows the sectional view of the lens system according to the 
embodiment, and concrete numerical values are as shown in Table 1. In the 
table, the reference character FNo. denotes a F-number, f denotes a focal 
length of the lens system in which the wavelength of the light is 
substantially in the center of the useable light spectrum, M denotes a 
magnification, r denotes the radius of curvature of a surface, d denotes a 
lens thickness or a spatial distance, n denotes a refractive index in 
substantially the center of the useable light spectrum, .nu.d denotes an 
Abbe number. 
This lens system comprises a first positive lens L1, a second negative lens 
L2, a third positive lens L3 and a cover glass C. It is a reading lens 
which is often used in an optical system for a facsimile machine, etc. 
The spherical aberration, chromatic aberration, astigmatism, and distortion 
in the above-mentioned construction in which the lens is not decentered is 
shown in FIG. 3. 
TABLE 1 
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FNO. = 1:7.0 f = 29.87 
M = -0.112 
surface 
NO. r d n .nu.d 
______________________________________ 
1 8.190 3.80 1.49186 
57.4 
2 12.181 0.75 
3 -14.500 1.80 1.58547 
29.9 
4 9.471 0.37 
5 14.400 1.19 1.88300 
40.8 
6 -18.980 28.77 
7 .infin. 0.50 1.51633 
64.1 
8 .infin. 
______________________________________ 
In this embodiment, the second lens L2 is a particular lens which is 
inclined about a point of less aberration deterioration when decentering 
occurs. 
FIG. 4 is a view schematically showing an assembling structure of the 
second lens L2 to the holding member 10. The cylindrical holding member 10 
is provided with a projecting portion 11 projecting inwardly. The second 
lens L2 abuts against the projecting portion 11. A surface of the second 
lens L2 on its projecting portion 11 side comprises a lens function 
surface 21 which is the portion of the lens where light passes through, 
and an abutting surface 22 to abut against the holding member 10 and 
forming the peripheral portion of the lens surface 21. The abutting 
surface 22 is a spherical surface having a different curvature from that 
of the lens function surface 21. And this abutting surface 22 is designed 
such that when the second lens L2 is decentered, the second lens L2 is 
inclined about a point of less aberration deterioration. 
It is to be noted that the abutting surface 22 is not indispensable. Also, 
it may be of a curved surface other than the spherical surface. The 
above-mentioned lens shape can be easily obtained if the lens is formed of 
plastic. 
When the second lens L2 is disposed with its abutting surface 22 abutting 
against the projecting portion 11, the inclination center 0 of the second 
lens L2 is a point 52.45 mm away from a third surface to a subject side. 
The transverse aberration when the lens is not inclined is shown in FIG. 5. 
When the second lens L2 is decentered by 50 .mu.m about the point 0, the 
transverse aberration changes as shown in FIG. 6. 
On the other hand, if the second lens L2 is disposed with its lens function 
surface 21 abutted against the holding member 10 as the prior art of FIG. 
8, the inclination center of the second lens L2 becomes a point 14.50 mm 
away from the third surface to the subject side (center of curvature of 
the third surface). In the foregoing state, when the second lens L2 is 
decentered by 50 .mu.m, the transverse aberration is deteriorated as shown 
in FIG. 7. 
Comparison of the foregoing reveals that the amount of aberration 
deterioration with respect to the same amount of the decentering is 
significant when the lens is inclined about the center of curvature of the 
lens function surface. 
As described in the foregoing, according to the present invention, as the 
inclination center due to decentering of a lens is set to a point of less 
aberration deterioration caused by inclination of the lens, the adverse 
affection from aberration can be reduced even when the lens is inclined. 
Therefore, even when the assembling clearance to the holding member of the 
lens is formed large in order to use a plastic lens having a large 
coefficient of expansion, performance deterioration caused by decentering 
can be reduced.