JPS6160406B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an optical system such as a camera, and more particularly to a lens holding device in which the distance between lenses changes depending on temperature, and preferably a lens holding device for an optical system using a plastic lens. As an example of the above-mentioned optical system, FIG. 1 shows an example of a camera lens frame incorporating a conventional lens. In FIG. 1, a first lens 1 and a second lens 2 are housed inside a front lens barrel 6, and a first retaining ring is engaged with a first barrel portion 5 by a first retaining ring fitting screw 8. Fixed by 7. A lens barrel fitting screw 12 is provided at the end of the front group lens barrel 6, whereby the rear group lens barrel 14 is meshed and fixed. The third lens 3 is fixed inside the rear group lens barrel 14 by a second retaining ring 10 that is meshed with the second barrel portion 9 and a second retaining ring screw 11 . The fourth lens 4 is connected to the third barrel portion 13 and the third
The third presser ring 1 is meshed and fitted with the presser ring fitting screw 15.
6. A plastic lens is used as the fourth lens 4. A focus adjustment screw 17 is provided on the outer periphery of the rear group lens barrel 14, and is engaged with the camera body 19, and a focus fixing screw 1 is also provided as a rotation stopper.
The position is fixed by 8. A focus plate 20 for forming an image is provided inside the camera body 19. The operation of the conventional system shown in FIG. 1 is as follows. After assembling the first lens 1 and second lens 2 into the front group lens barrel 6 and installing the third lens 3 and fourth lens 4 into the rear group lens barrel 14, the front group lens barrel 6 and the rear group lens barrel 14 are assembled. They are fitted together using cylinder fitting screws 12. Next, it is attached to the camera body 19 using the focus adjustment screw 17, and at this time, the rear group lens barrel 14 is fixed so that the focus plate 20 provided inside the camera body 19 is in focus, that is, the image is formed. , the entire lens barrel is moved back and forth along the optical axis to determine the appropriate focal length F. After the focus position is adjusted, the focus fixing screw 18 is screwed in to prevent rotation of the rear lens barrel. With the above, the optical system can be positioned in an appropriate state. Therefore, although the conventional method has no drawbacks at room temperature, it has the following drawbacks when expanding the usage range of the optical system and the usage environment of the camera. First, as the environmental temperature increases or decreases, the front group lens barrel ₆ and the rear group lens barrel 14 expand and contract in the optical axis direction. The distance f _B from the rear end of the fourth lens 4 to the focusing plate 20 cannot be maintained at an appropriate distance, resulting in an out-of-focus photograph. The longer the overall length of the lens barrel, the greater the effect. Second, even if the lens barrel itself is short and made of a material with low thermal expansion, if plastic is used as the lens material, the refractive index of the plastic will change as the temperature increases, so the distance f _B will change. The image becomes out of focus. Ordinary material plastic MMA
Then, if the temperature is lowered from room temperature to -20°C, the refractive index will increase by 450×10 ^-5 , and the distance f _B will change by more than +0.15 mm. Changes occur in the opposite direction when the temperature increases. The allowable value of the focal position movement amount changes depending on the lens, but the maximum allowable value is the value of the depth of focus. Third, when plastic is used as the lens material, thermal stress is generated between the lens barrel and the plastic lens as the temperature increases due to the difference in linear expansion coefficient between the lens barrel and the plastic lens. The radius of curvature changes, the distance between lenses changes, and the distance f _B changes, resulting in out-of-focus. When the fourth lens 4 made of plastic MMA is combined with the lens barrel of the conventional structure shown in Fig. 1, the radius of curvature of the concave side of the fourth lens 4 increases by about 0.05 if the temperature is lowered from room temperature to -20°C. However, the distance f _B changes by +0.2 mm, the distance from the third lens 3 decreases by 0.01 or more, and the change in distance f _B becomes about +0.04 mm. As mentioned above, when plastic is used as the lens material or when the lens barrel expands and contracts significantly, it is necessary to limit the practical temperature range of the lens system to a narrow range with the conventional structure. The changes in f _B due to temperature changes when using the plastic lenses described above are shown in Table 1 below.

[Table] In the lens system shown in Figure 1, as shown in the table above, if the temperature drops from room temperature to -20°C, the amount of change in coupling will be approximately 0.4 mm, resulting in blurring. Patent No. 1325936 and British Patent No. 1017775 are patents that correct for focal shift due to temperature changes.
There is a number. This US patent attempts to change the lens spacing by using the difference in the linear expansion coefficient of metals, but the linear expansion coefficient of metals is extremely small, about 1 to 2 × 10 ^-5 , and the necessary A large space is required to obtain a change in lens spacing, and it cannot be incorporated into a lens system of a small camera as shown in FIG. In the case of the British patent, the space is relatively small because it uses linear expansion of plastic (expansion number 9 × 10 ^-5 in the case of POM), but the space required to incorporate it into a lens system for a small camera is still unreasonable. Yes, it cannot be used. Furthermore, since a long tubular spacer made of plastic is used, the lens holding mechanism is complicated, making it difficult to accurately set the position of the lens system, and the optical axis alignment accuracy is low. SUMMARY OF THE INVENTION An object of the present invention is to provide a lens holding device that moves the relative position of lenses in the optical axis direction in order to compensate for changes in lens spacing due to changes in power, changes in refractive index of plastic lenses, and changes in focus due to changes in curvature. There is something to do. The outline of the lens holding device according to the present invention for achieving the above-mentioned object is to fix one end of a bimetal within a lens barrel, and support a plastic lens movably in the optical axis direction between the other end of the bimetal and an elastic body. do. By setting the displacement dimension due to temperature change of the bimetal as the required displacement amount of the lens interval, movement of focus due to temperature change is corrected, and in practice, the optical system can maintain correct focus without being affected by temperature. Embodiments and drawings illustrating the present invention will be described. Figure 2 is a principle expression of the present invention, and the optical system shown in Figure 1 corrects the changes in distance f _B based on temperature changes in the curvature, lens spacing, and plastic refractive index shown in Table 1. This is an experimental value showing the optically required relative movement amount of the lens barrel with respect to temperature when the distance between the third lens and the fourth lens is changed. This experimental value is shown by the two-dot chain line in the figure. As shown in FIG. 2, this curve approximates the straight line shown by the dotted line in the figure, so if the correction amount is obtained according to this straight line, no out-of-focus will occur when the temperature changes. Of course, the numerical values shown as the amount of movement in FIG. 2 are just examples for easy understanding, and need to be determined for each optical system. The error from the theoretical value due to correction by straight line approximation is maximum
It is about 0.005mm and does not pose a practical problem. 3 and 4 show a plastic lens holding device for implementing the principle of FIG. 2 according to the invention. Although other parts of the optical system are omitted, for example, it is used in place of the rigid holding structure that holds the fourth lens 4 shown in FIG. 1, and the lens distance between it and the third lens is changed as shown in FIG. 2. composition. The lens barrel 31 is, for example, a rear lens barrel shown in FIG. The stopper 32 is a shoulder formed on the lens frame 31 or a fixed member, and the lens frame 31
A plastic lens 34 is held between the presser foot 33 and the presser foot 33 screwed into the plastic lens 34. According to the present invention, an elastic body 35 is interposed between the plastic lens 34 and the stopper 32, and a bimetal 36 is interposed between the plastic lens 34 and the presser foot 33. In the illustrated example, an annular flange 37 is formed on the left edge of the plastic lens 34.
The elastic body 35 is brought into contact with. The elastic body 35 may be an annular elastic member made of rubber, urethane, etc. around the entire circumference, or may be a spring positioned opposite the bimetal 36. As shown in FIG. 5, the bimetal 36 is fixed at one end. For this purpose, the ends of a plurality of plate-shaped bimetals shown in FIG. 4 are fixed between the shoulder 38 formed on the lens barrel 31 and the presser foot 33, or the bimetal 36 is fixed to an annular member (not shown). Then, the annular member is fixed between the shoulder portion 38 and the presser foot 33. The vicinity of the tip of the bimetal 36 is brought into contact with an annular projection 39 formed on the plastic lens 34, and the plastic lens 34 is held between the elastic body 35 and the bimetal 36. With the above-described structure, when a temperature change occurs, the bimetal 36 has its radially outer end fixed in FIG. . Therefore, the plastic lens 34 as a whole moves in the optical axis direction against the elastic body 35. As a result, the lens interval changes along the required correction straight line shown in FIG. 2, and the change in distance f _B shown in FIG. 1 is corrected. As will be described later, the displacement dimension of the bimetal 36 fixed at one end when the temperature changes has a linear relationship with the temperature, and the correction value becomes a straight line in FIG. 2. The required change, that is, the error from the two-dot chain line in Figure 2, is a maximum of 0.005 mm.
This is a sufficient correction for practical purposes. Next, the feasibility of implementing the configurations shown in FIGS. 3 and 4 will be explained using the optical system shown in FIG. 2 as an example. It is assumed that the elastic body 35 is a spring. bimetal 36
Under the spring force, a total deflection of 0.09 mm as shown in Figure 2 is obtained in the range of -30°C to +50°C, and it is confirmed that the force acting on the bimetal at this time does not exceed the allowable stress of the bimetal. Considering the bimetal 36 as a cantilever beam with one end fixed as shown in Fig. 5, the spring constant of the elastic body 35 in Fig. 3 is k.
Kg/mm, and when the tip of the bimetal is displaced while receiving pressure from an elastic body, the amount of displacement of the tip with respect to temperature is given by the following equation. D=KL ² Ebt ² /Ebt ³ +4L ³ k×△T Here, K is the curvature coefficient (/℃) L is the actuation amount (mm) E is the elastic modulus (Kg/mm ² ) b is the width of the bimetal ( mm) t is the bimetal thickness (mm) k is the spring constant (Kg/mm) D is the displacement (mm) △T is the temperature change (℃) As shown in the above formula, the displacement D is the temperature change △ It becomes a value proportional to T, and becomes a straight line shown by the dotted line in FIG. The slope of the dotted line in Figure 2 is 0.09/80 = 1.125×10 ^-3 (m
Determine each constant so as to obtain (m/°C). K=14.5×10 ^-6 /°C L=5mm E=1700Kg/mm ² b=5mm K=0.01Kg/mm D=0.09mm △T=80°C If this is inserted, t=0.32mm. In this way, necessary corrections can be made within the practicable size range. Regarding allowable stress, the maximum value of the reaction force of the elastic body is 0.01Kg when the reaction force of the elastic body is minimum, that is, at the lowest operating temperature, and then it moves 0.09mm, so 0.01×0.09=9 ×10 ^-4 Kg, therefore, the maximum value of the reaction force is 0.01 + 0.0009 = 0.0109 Kg. The maximum stress (Kg/mm ² ) acting on the bimetal in this case is σ=6PL/bt ² In this case, P=0.0109Kg/mm ² L=5mm b=5mm t=0.32mm Therefore, σ=0.64Kg/mm ^2The allowable stress of the bimetal used is 100℃ or less.
Since it is 17Kg/mm ² , there are no problems in use. The effects of the configuration of the present invention are as follows. Since it uses bimetal, it has an extremely simple mechanism, which is extremely easy to manufacture and assemble, and does not affect other parts. Compared to known documents, the precision of the positional relationship of the lenses is better, and the tilt movement of the core is comparable to that of glass lenses, eliminating the occurrence of out-of-focus. Although the focus shift due to temperature change was 0.4 mm at the maximum value of f _B , the correction mechanism of the present invention can reduce the shift of f _B to a maximum of 0.05 mm, which causes no effect of temperature in practical use. do not have.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a camera lens holding device as an example of a known optical system, FIG. 2 is a graph showing the relationship between temperature and the amount of lens distance movement required to illustrate the principle of the present invention, and FIG. A sectional view of the lens holding device according to the present invention, FIG. 4 is a right end view of FIG. 3, and FIG. 5 is a diagram showing displacement of the bimetal. 1, 2, 3, 4... Lens, 5, 9, 13... Barrel attachment part, 6, 14... Lens barrel, 7, 10, 16... Holding ring,
20... Focusing plate, 31... Lens frame, 32... Stopper, 33... Presser, 34... Plastic lens, 3
5... Elastic body, 36... Bimetal, 37, 39... Annular protrusion, 38... Shoulder.

Claims (1)

[Claims] 1. In a lens holding device having a plurality of lenses, a position side surface of the outer peripheral part of a desired lens among the plurality of lenses is locked to a fixing part of a lens frame by interposing an elastic body, and the outer peripheral part and other parts of the lens are locked. 1. A lens holding device, characterized in that the lens is supported movably in the optical axis direction by locking the side surface of the lens at the other end of a bimetal whose one end is fixed to a fixed portion of a lens frame.