PURPOSE: To reduce the deviation of focus over a whole area from a wide angle end to a telephoto end by adjusting focus at three points among the wide angle end, the telephoto end and the intermediate point in a zoom lens system used in a video camera, etc.

CONSTITUTION: A variable power lens group is set at the five points zp1-zp5 from a wide angle side to the telephoto side. Zp3 is set in the vicinity of a position where the moving quantity of the focus lens group becomes max. Then, the focal positions fp1-fp5 of the focus lens group for an object in a specific distance at the respective five points are detected. After that, by prescribed procedure, difference Δp2-Δp4 between the focal positions fp2-fp4 of the focus lens group at the variable power lens positions zp2-zp4 and the focus lens group positions in design at the respective points, the wide angle end position and the telephoto end position are obtained. These are used in the zoom lens system so that the focus lens group positions for the detected variable power lens group positions are calculated, the focus lens group position is moved to the position and focus is corrected.