| 摘要 |
PURPOSE:To eliminate a variation of a radius of curvature, by making a parallel part of the outside circumferential part of a plastic lens contact with a projecting part or a ball of low friction, and sliding the lens relatively and freely by low friction against a barrel part and a holding ring in case of a variation of a working temperature. CONSTITUTION:On a barrel part 22 of a lens frame 21, a semi-spherical projection 27 is provided at equal intervals. In the same way, on a lens side surface of a holding ring 26, a semi-spherical projection 27 is provided. Also, on the outside circumference of a lens 24, a parallel part 29 is formed so that component of force in the oblique direction is not generated when the holding ring 26 has been clamped. Also, even if there is a movement in the radial direction between the plastic lens 24, and the lens frame 21 and the holding ring 26 at the time of a temperature variation, only force like a right angle is put on the holding ring 26, and there is no influence. Also, a space between the projection 27 of the barrel part 22 and the lens 24 is of a point contact in the same way, and is subjected to no influence, therefore, the lens 24 executes thermal expansion, and contraction almost freely. |
