JPS57118203A - Automatic focusing variable magnification optical system - Google Patents

Abstract

PURPOSE:To realize a variable magnification optical system which is capable of focusing with high accuracy without using a mechanical cam, by making use of an automatic focusing mechanism which performs an automatic adjustment for the position of image formation of an optical system as a part of the variable magnification mechanism of a variable magnification optical system. CONSTITUTION:In case when a specific subject on an optical axis is corectly focused to a main image pickup element 5, a waveheight value signal P for high frequency component produced from the element 5 has a large value. At the same time, the signals Q and R produced from two secondary pickup elements 31 and 41 which are shifted equidistantly at the front and the back from the correct focused position have an equal value as W=U=Q-R=O. Thus a driving mechanism 61 has no operation, and a lens group 3 is kept in the correct focused state. Then a lens group 2 is shifted right, and accordingly the element 41 is comparatively close to the correct focused point with the element 31 shifted largely to the front from the focused point as long as the lens group 3 does not change its position. Thus Q<R and W=U=K(Q-R)<O are satisfied. Accordingly the lens group 3 moves to the right and stops when Q=R is satisfied, i.e. in the focused state.