JPH0514243B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an imaging lens suitable for a video camera, an 8 mm camera, etc., and particularly to an imaging lens having a plastic lens as a constituent lens.

[Prior art] Because the lens surface is easy to mold, has excellent impact resistance, is lightweight, and is inexpensive, plastic lenses have become popular in place of glass lenses, and are used as imaging lenses for video cameras and 8mm cameras. It has also come to be used as a constituent lens.

However, although plastic lenses have various advantages, they also have hygroscopic properties and change shape due to moisture absorption and dehumidification, resulting in changes in focal length.

FIG. 1 is a cross-sectional view of a main part of an example of a conventional imaging lens that uses a plastic lens as a constituent lens.
Explain the effects of dehumidification. In FIG. 1, 1 is a spacer ring, and 2 and 3 are plastic lenses.

A spacing ring 1 is provided within the lens barrel (not shown),
Between these spacing rings 1 are plastic lenses 2, 3.
The collar is inserted. That is, the plastic lenses 2 and 3 are fixed with their brim portions sandwiched between the two spacing rings 1. and,
The width of the spacing ring 1 is determined so that a predetermined spacing is set between the lenses.

By the way, if the plastic lens 2 is replaced with a concave lens made of polystyrene in place of the conventional flint glass lens for eliminating chromatic aberration, and the plastic lens 3 is replaced with a crown glass lens with a convex lens made of polymethyl methacrylate, polystyrene The radius of curvature of the lens changes due to relative humidity fluctuations by 0.015×10 ^-5 /%RH
, and is almost unaffected by humidity, whereas polymethyl methacrylate resin is 2.5×
10 ^-5 /%RH, and for this reason, the shape of the plastic lens 3 changes greatly in response to changes in humidity.

Therefore, if the image point of the plastic lenses 2 and 3 is point 0 with respect to the standard humidity, the plastic lens 3
The lens surface deforms as shown by the two-dot chain line, and as a result, the radius of curvature of the lens surface becomes larger, and the image point moves to a point on the opposite side of the subject (towards the right on the page).
Conversely, when the humidity decreases, the lens surface of the plastic lens 3 deforms as shown by the dashed line, the radius of curvature of the lens surface becomes smaller, and the image point shifts to point 0' on the subject side. It will be displaced to .

In this way, if a highly hygroscopic plastic lens is used as a component lens, the image point position will change due to changes in humidity, and the image on the imaging surface (the target of the imaging device or the film surface) will become blurred. . Furthermore, while the focal length of lens 3 changes, the focal length of lens 2 hardly changes. Therefore, achromatization by lenses 2 and 3 is insufficient, resulting in chromatic aberration.

[Purpose of the invention]

The purpose of the present invention is to eliminate the drawbacks of the above-mentioned prior art,
To provide an imaging lens that prevents displacement of an image point due to deformation of a plastic lens due to humidity fluctuations and always allows the image point to coincide with an imaging surface.

[Summary of the invention]

In order to achieve this object, the present invention provides a spacer ring for positioning a highly hygroscopic plastic lens, which is a hygroscopic spacer ring that is dimensionally deformed by moisture absorption, and the dimensional deformation of the hygroscopic spacer ring causes the highly hygroscopic plastic lens to be positioned. The present invention is characterized in that the displacement of the highly hygroscopic plastic lens is offset by the displacement of the highly hygroscopic plastic lens due to the deformation of the highly hygroscopic plastic lens due to humidity.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a cross-sectional view of the main parts of an embodiment of the imaging lens according to the present invention, in which 4 and 4' are hygroscopic spacing rings, 5 is a spring, and parts corresponding to those in FIG. 1 have the same reference numerals. is attached.

In FIG. 2, spacing ring 1 is a spacing ring made of metal or the like which is not affected by humidity, and hygroscopic spacing rings 4, 4' are spacing rings made of a plastic material having a humidity linear expansion coefficient β. The plastic lens 2 made of polystyrene is positioned with respect to the lens barrel with its brim inserted between the spacer ring 1 and the hygroscopic spacer ring 4. The distance between the plastic lens 2 and the flange is set to be sufficiently larger than the thickness of the flange portion of the plastic lens 2, and a spring 5 is provided between them, and the biasing force of the spring 5 presses the plastic lens 2 against the hygroscopic spacing ring 4. are doing. In addition, a highly hygroscopic plastic lens made of polymethyl methacrylate (hereinafter referred to as
A highly hygroscopic plastic lens (3) is inserted with its collar between the hygroscopic spacing rings 4, 4'.

In such a configuration, the image point at the reference humidity is point 0.
, and if the humidity has increased now, as mentioned earlier, the highly hygroscopic plastic cleanser 3
deforms, and the radius of curvature of the lens surface increases, resulting in a longer focal length. On the other hand, since the plastic lens 2 does not deform, the deformation of the highly hygroscopic plastic lens 3 tends to cause chromatic aberration and also causes the image point to shift in the direction of arrow A. However, as the humidity increases, the hygroscopic spacing rings 4 and 4' also undergo dimensional deformation and their respective widths increase, and the highly hygroscopic plastic lens 3 is oriented in the direction of displacement of the image point due to the dimensional deformation of the hygroscopic spacing ring 4'. The plastic lens 2 is displaced in the direction of arrow A' in the opposite direction, and as the dimensional deformation of the hygroscopic spacing ring 4 occurs, the plastic lens 2
is also displaced in the direction of arrow B.

Conversely, when the humidity decreases, the highly hygroscopic plastic lens 3 deforms and the radius of curvature of the lens surface becomes smaller, chromatic aberration tends to occur, the focal length becomes shorter, and the image point moves in the direction of arrow A'. Trying to displace. However, the hygroscopic spacing rings 4, 4' are also dimensionally deformed and their respective widths are reduced, and as a result, the highly hygroscopic plastic lens 3 is affected by the dimensional deformation of the hygroscopic spacing rings 4, 4' and the attachment of the spring 5. The plastic lens 2 is displaced in the direction of arrow A by the force, and the plastic lens 2 is also displaced in the direction of arrow B' by the biasing force of the spring 5.

This displacement of the plastic lens 2 and the highly hygroscopic plastic lens 3 due to the dimensional deformation of the hygroscopic spacing rings 4, 4' due to changes in humidity suppresses chromatic aberration and image point displacement due to changes in humidity. It acts like this. That is, the displacement of the highly hygroscopic plastic lens 3 is such that the displacement of the image point due to changes in humidity is canceled out so that the image point is always at point 0, and the displacement of the plastic lens 2 is The distance between the plastic lens 3 and the highly hygroscopic plastic lens 3 is set so as to always satisfy the achromatic condition to prevent chromatic aberration from occurring. For this purpose, the amount of dimensional deformation due to humidity of the hygroscopic spacer ring 4' is made to match the amount of displacement of the image point, and the highly hygroscopic plastic lens 3 is
Plastic lens 2 and highly hygroscopic plastic lens 3 due to displacement due to dimensional deformation of
The amount of dimensional deformation of the hygroscopic spacing ring 4 is made to match the amount of displacement of the interval between the two.

Therefore, if the focal length of the highly hygroscopic plastic lens 3 at the reference humidity is f, the refractive index is n, and the two radii of curvature are R ₁ and R ₂ , then 1/f = (n-1) (1/R ₁ -1/R ₂ ) ...(1), and these radii of curvature R ₁ and R ₂ are almost equal,
When R ₁ ≒−R ₂ =R, equation (1) becomes as follows.

1/f=(n-1)・2/R Therefore, f=R/2(n-1)...(2) In this highly hygroscopic plastic lens 3, the relative humidity in the usage environment changes by a%RH. As a result, the refractive index n changes to (n+Δn), the radius of curvature R changes to (R+ΔR), and the focal length f
If becomes (f+Δf), then from equation (2), f+Δf=R+ΔR/2 {(n+Δn)-1} = R+ΔR/2(n-1) {1+(Δn/n-1)}
...(3) becomes. Here, since 1≫Δn/n-1, equation (3) becomes f+Δf≒R+ΔR/2(n-1)(1-Δn/n-1
)=R/2(n-1)+ΔR/2(n-1)-R・Δn/
2(n-1) ² -ΔR・Δn/2(n-1) ² =R/2(n-1)+R/2(n-1)・ΔR/R-
R/2(n-1)・Δn/n・n/n-1-R/2(n
-1) ・ΔR/R・Δn/n・n/n−1 ...(4) On the other hand, if the humidity linear expansion coefficient is α and the humidity refractive index change rate is γ, then ΔR=a・α・R, Δn=a・γ・f ...(5) Therefore, equations (2) and (5) ), formula (4) becomes f+Δf=f+aαf−aγf·n/n−1 −a ² αγf·n/n−1 (6). Here, highly hygroscopic plastic lens 3
In the case of lenses made of polymethyl methacrylate, which is most commonly used, the humidity refractive index change rate γ is extremely small. Therefore, the third and fourth terms on the right side of equation (6) above
term can be ignored. Therefore, from this and equation (6),
The amount of change in focal length Δf is Δf=aαf (7). That is, the relative humidity in the usage environment is a%
When RH fluctuates, highly hygroscopic plastic lens 3
The focal length of changes by aαf.

By the way, the thickness l of the brim portion of the highly hygroscopic plastic lens 3 at the reference humidity, the hygroscopic interval ring 4,
If the width at the reference temperature of 4' is m, then when the relative humidity in the usage environment changes by a%RH, the thickness of the brim of the highly hygroscopic plastic lens 3 will be
The width of the hygroscopic spacing rings 4, 4' changes by aβm.

Therefore, by satisfying aαf=aαl+aβm, it is possible to suppress the occurrence of chromatic aberration and the positional fluctuation of the image point due to humidity fluctuations. In order to satisfy this condition, the hygroscopic spacing rings 4, 4' are formed of a plastic material having a humidity linear expansion coefficient of .beta.=.alpha.(fl)/m.

As an example, as a highly hygroscopic plastic lens 3, the focal length f is 20 mm, and the thickness l of the brim portion is 2 mm.
polymethyl methacrylate lens (α=2.5
×10 ^-5 /%RH), using a hygroscopic interval ring 4,4'
When the width m is 5 mm, β=9×10 ⁻⁵ /%RH, and examples of plastics having such a humidity linear expansion coefficient include polymeric materials having only peptide groups and urethane groups in the main chain.

[Function and effect of the invention]

As explained above, according to the present invention, even if the plastic lens is deformed due to changes in humidity, the displacement of the image point can be suppressed, and the subject image can always be formed on the imaging surface. Accordingly, it is possible to obtain an image that is clear and clear, and to provide an imaging lens that has excellent functions by eliminating the drawbacks of the prior art described above.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a main part of an example of a conventional imaging lens, and FIG. 2 is a sectional view of a main part of an embodiment of an imaging lens according to the present invention. 1... Interval ring, 2... Plastic cleanse, 3
...Highly absorbent plastic lens, 4,4'...
Hygroscopic interval ring.

Claims (1)

[Claims] 1. An imaging lens in which a plurality of lenses are each positioned in a lens barrel by spacer rings, and at least one of the lenses is a highly hygroscopic plastic lens whose radius of curvature changes depending on the humidity. The spacer ring for positioning the highly hygroscopic plastic lens is a hygroscopic spacer ring made of a material whose dimensions change due to moisture absorption, and the image point displacement due to the change in the radius of curvature of the highly hygroscopic plastic lens is calculated as follows: An imaging lens characterized in that it is configured to be able to compensate for displacement of the highly hygroscopic plastic lens due to dimensional deformation.