| 摘要 |
PURPOSE:To enable high-accuracy automatic focusing over the entire range of zooming by moving a 4th lens element monotonously from an image side to an object side when the zoom lens is focused on an infinite-distance body or from the object side to the image side when the lens is focused on a short- distance body in zooming operation from the wide-angle end to the telephoto end. CONSTITUTION:When the lens is zoomed from the wide-angle end to the telephoto end, the 4th lens element IV shifts in position monotonously from the image side to the object side for the focusing on the infinite-distance body or from the body side to the image side for the focusing on the short-distance body. Namely, the focusing operation is carried out not by a 3rd lens element III as a compensator, but by a 4th lens element IV. Therefore, a tracking curve having no extremal value can be drawn. The tracking curve has no extremal value, so the speed and acceleration of the 4th lens element IV in the zooming are invariably in the same direction. Consequently, various aberrations are compensated excellently over the entire zoom range.
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