PURPOSE: To compensate extermely small magnification by focusing through elec tric signal processing by converting a speed command signal which commands the speed of zooming into pulses, and counting them and generating a zooming position command signal.

CONSTITUTION: The zooming speed command signal from a potentiometer 8 is set to an oscillation circuit 9, which converts the signal into proportional pulses, which are counted by an up/down counter 12. The output of the up/down counter 12 is sent as the zooming position command signal to an arithmetic circuit 14, which detects the difference between the zooming position command signal from a D/A converting circuit 13 and a zooming position detection signal from a potentiometer 5 to move a zoom part; and variation in extremely small image forming magnification when a lens group for focusing moves is calculated from the information of the focus position detection signal from a potentiometer 7 to make a correction.