PURPOSE:To automatically correct an image formation shift when a focusing lens group is updated to optional focal length after focusing by removing the condition of equal-quantity movement of a zoom lens and specifying a shift in focusing position at each subject distance. CONSTITUTION:A bit of focusing position information Sx is outputted in proportion to the position (movement quantity) of a focusing lens group F in the optical-axis direction, so D=(C0.Zp+C1).Sx+C2, where D is an object distance. Here, C0, C1 and C2 are set constants determined at the time of designing operation and Zp is a bit of focal length information. Namely, Zp and Sx in the equation are controlled so that the object distance D does not vary, and then the movement quantity of the focusing lens group F capable of eliminating (compensating) focus movement at the time of variable power operation is not equal-quantity movement. Consequently, even when the variable power lens group is moved from a 1st focal length to a 2nd focal length to update the focal length of the whole system, a shift in image formation position characteristic to variable focus lenses is corrected instantaneously to hold a focusing state substantially at all times.
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