Konishiroku Photo Industry Co., Ltd.

Tadashi Kojima

Shozo Ishiyama

Norikazu Arai

Makoto Banno

Kunio Sakurai

Hiroshi Itoh

When a zoom lens system is focused by moving a lens other than the foremost lens group, the advancing amount of the focusing lens varies according to the variation in the focal length even whyen focusing on an object at the same object distance. When a zoom lens system is zoomed to a certain focal length, the object distance and advancing amount of the focusing lens has a relationship expressed by a function resembling a logarithmic function. The present invention enables one to focus a zoom lens system using only one movement amount curve by arranging to move the focusing lens along the movement amount curve, utilizing different portions of the movement amount curve according to the amount of zooming.

Focusing means for a zoom lens group

A large-aperture single lens with aspherical surfaces to be used as a pickup lens for video disks, especially for tracking the video disk by directly moving the pickup lens. It is preferable that the single lens is made of plastic material in order to make it compact and light in weight. Both refractive surfaces of the single lens are arranged to have positive refractive powers in order to make the working distance of the lens long. Spherical aberration of the single lens is corrected to the degree that the diameter of the circle of confusion thereof is decided approximately by diffraction of light. Both refractive surfaces of the single lens are formed as aspherical surfaces and the shapes of the refractive surfaces are determined so as to correct aberrations including the sine condition to the required range, by taking the error to be caused at the time of manufacture into consideration.

Large-aperture single lens with aspherical surfaces

The mask blank is provided with a transparent coating layer interposed between a transparent substrate and a mask metal layer. The transparent coating layer is made of material which is resistant to an etching solution.

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Hino, Japan

Tadashi Kojima