JPS59154409A - Image pickup lens - Google Patents

Abstract

PURPOSE:To prevent displacement of image point due to deformation of a plastic lens caused by fluctuation of humidity by making a space ring that positions a highly hygroscopic plastic lens a hygroscopic space ring that deforms by moisture absorption. CONSTITUTION:A space ring 1 is made of metal etc. which is not influenced by humidity, and hygroscopic space rings 4, 4' are made of plastic material having humidity coefficient of linear expansion beta. When alpha represents humidity coefficient of linear expansion, (f) represents focal length at reference humidity, (l) represents thickness of collar part at reference humidity and (m) represents width of hygroscopic space rings 4, 4' at reference humidity in a highly hygroscopic plastic lens 3, change of relative humidity by a% RH in working environment changes focal length of the highly hygroscopic plastic lens and thickness of the collar part by aalphaf, aalphal respectively, and changes width of the hygroscopic space rings 4, 4' by abetam. Accordingly, by satisfying aalphaf=aalphal+abetam generation of chromatic aberration due to temperature fluctuation and change of position of image point can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Original Field 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 whose constituent lenses are plastic lenses.

[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 now used as constituent lenses for imaging lenses such as video cameras and 8mm cameras. However, on the other hand, plastic lenses, which have various advantages, have hygroscopic properties and change shape due to moisture absorption and dehumidification, resulting in changes in focal length. There is a point.

FIG. 1 is a cross-sectional view of a main part of an example of a conventional imaging lens having a plastic lens as a constituent lens, and the effects of moisture absorption and dehumidification of the plastic lens will be explained with reference to this figure. In FIG. 1, 1 is a spacer ring, and 2 and 3 are plastic lenses.

A recess 11 is provided within the lens barrel (not shown), and the flanges of plastic lenses 2 and 3 are inserted between the recess 11. That is, the plastic lenses 2 and 3 are fixed with their brim portions sandwiched between the two gaps 10. And the width of the spacing ring 10 is determined so that a predetermined spacing is set between the lenses.

By the way, suppose we use a concave lens made of polystyrene instead of the conventional flint glass lens for the plastic lens 2 to eliminate chromatic aberration, and a convex lens made of polymethyl methacrylate instead of the crown glass lens for the plus 7 lens 3. , polystyrene lenses have a radius of curvature variation of 0.015 due to relative humidity fluctuations.
X I F"/%RH, which is hardly affected by humidity, whereas polymethyl methacrylate lenses have a large fluctuation of 2.5 On the other hand, the shape of the plastic lens 3 is thick (changes).

Therefore, plastic lenses 2.3 to 1 standard humidity.
Assuming that the image point is point O, as the humidity increases, the lens surface of the plastic lens 3 deforms as shown by the dashed line, and as a result, the radius of curvature of the lens surface becomes thicker. The image point shifts to point o'' on the opposite side of the subject (to the right on the page).Conversely, when the humidity decreases,
The lens surface of the plastic lens 3 is deformed as shown by the two-dot chain line, the radius of curvature of the lens surface becomes smaller, and the image point is displaced to point O' on the object side.

In this way, when a highly hygroscopic plastic lens is used as a constituent lens, the image point position changes due to fluctuations in humidity.
The image on the imaging surface (the target or film surface of the imaging device) will become blurred.

Furthermore, while the focal length of lens 3 changes, the focal length of lens 2 hardly changes, so achromatization by lenses 2 and 3 becomes insufficient and chromatic aberration occurs.

[Purpose of the invention]

An object of the present invention is to provide an imaging system that eliminates the drawbacks of the prior art described above, prevents displacement of an image point due to deformation of a plastic lens due to humidity fluctuation, and always allows the image point to coincide with the imaging plane. We provide lenses for

[Summary of the invention]

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

[Embodiments of the invention]

Embodiments of the present invention will now be described with reference to Figure M1.

FIG. 2 is a 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 the parts corresponding to FIG. 1 are the same. It is marked with a sign.

In FIG. 2, the spacing ring 1 is a spacing ring made of metal etc. that is not affected by humidity, and the hygroscopic spacing rings 4, 4
' is a spaced ring made of a plastic material with a linear expansion coefficient β. The plastic lens 2 made of polystyrene is inserted between the lens 1 and the hygroscopic spacing ring 4, and is positioned with respect to the lens barrel.
The distance between the hygroscopic spacing ring 4 and the hygroscopic spacing ring 4 is set to be sufficiently larger than the thickness of the brim portion of the plastic lens 2, and a spring 5 is provided between them, and the biasing force of the spring 5 causes the plastic lens 2 to absorb moisture. It is in pressure contact with the intersexual ring 4. In addition, a highly hygroscopic plastic lens 3 made of polymethyl methacrylate (hereinafter referred to as a highly hygroscopic plastic lens) has a hygroscopic interval of 3174.4
' is inserted between.

7) In the configuration, if the image point at the standard humidity is point O, and now the humidity increases, the highly hygroscopic plastic lens 3 will deform and its lens surface will change as described above. has a thicker radius of curvature and a longer focal length.

On the other hand, since the plastic lens 2 does not deform, in the end, due to the deformation of the highly hygroscopic plastic lens 3,
Chromatic aberration is about to occur and the image point is about to be displaced in the direction indicated by the arrow. However, as the humidity rises, the hygroscopic spacing rings 4 and 4' also undergo dimensional deformation and their respective widths increase.
The highly hygroscopic plastic lens 3 is displaced by one direction in the direction of arrow A', which is the opposite direction to the displacement direction of the image point, due to the dimensional deformation of the hygroscopic interval ring 4'. 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 its lens surface becomes smaller, causing chromatic aberration and shortening the focal length (as a result, the image point moves in the direction of arrow A'). However, the hygroscopic interval "534 + 4' is also dimensionally deformed and the respective widths are reduced. Therefore, (',:, the highly hygroscopic plastic lens 3 has a hygroscopic interval ring of 4°. Along with the dimensional deformation of lens 4', the plastic lens 2 is displaced in the direction of arrow B' due to the biasing force of spring 5, and the plastic lens 2 is also displaced in the direction of arrow B' due to the biasing force of spring 5.

This displacement of the plus deck lens 2 and the highly hygroscopic plastic lens 3 due to the dimensional deformation due to the change in humidity of the hygroscopic interval 94, +' 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 O, and the displacement of the plastic lens 2 is The gap between the lens 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, 1 dimensional deformation due to humidity of the hygroscopic spacing ring 4' 8
The plastic lens 2 and the highly hygroscopic plastic lens 3 are different from each other due to the displacement of the sieve hygroscopic plastic lens 3 due to the dimensional deformation of the hygroscopic spacing ring 4'. Therefore, regarding the highly hygroscopic plastic lens 3, the moisture K
i, the expansion coefficient is α, the focal length at the standard humidity is f, the thickness of the brim at the standard humidity is t, and the hygroscopic spacing rings 4, 4'
If the width at the standard humidity is m, then when the relative humidity layer in the usage environment changes ajiRH, the focal length of the highly hygroscopic plastic lens and the thickness of the buckle part will be aαf + aα, respectively.
t, and the width of the hygroscopic spacing rings 4, 4' is a1
Therefore, by satisfying aαf=aα7+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 linear coefficient of humidity expansion, such as β-Yeni L.

As an example, the highly hygroscopic plastic lens 3 is a polymethyl methacrylate lens (α2 2.5 x 10-5/% R
H), and if the width m of the hygroscopic spacing rings 4, 4' is 5 holes, then β = 9 x 10-5 mass% RH, and plastics with such a coefficient of humidity linear expansion include pebutide groups and urethane groups. There is a cylindrical molecule material that has only 100% in its 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 provide an imaging lens that provides a clear and clear image, and has excellent functions by eliminating the drawbacks of the above-mentioned conventional techniques.

[Brief explanation of drawings]

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... Spacing ring, 2... Plus deck lens, 3...
- Highly hygroscopic plastic lens, 4.4'...hygroscopic interval ring.

Claims (1)

[Claims] In a gymnastics lens in which several lenses are each positioned by spacer rings, and one of the lenses is a highly hygroscopic plastic lens, the spacer ring for positioning the highly hygroscopic plastic lens is used. The hygroscopic spacer ring is made of a material that undergoes dimensional deformation due to moisture absorption, and the displacement of the image point of the highly hygroscopic plastic lens due to moisture absorption can be compensated by the displacement of the highly hygroscopic plastic lens due to the dimensional deformation of the hygroscopic spacer ring. An imaging lens characterized by being configured as follows.