PURPOSE: To automatically correct the rate of extension of lenses by having a cam provided with a movement regulating member for focusing lens groups on a lens barrel member and changing the acting position of the cam according to zooming operations.

CONSTITUTION: Double helicoid mechanisms are disposed between the inside and outside barrels of a stationary barrel 1 to provide a lens barrel part. A focusing cam 5-1 and a transmission pin 5-2 are provided to a focus control ring 5 held to the helicoid male 3. A focus correcting cam 6-1 is provided to a focusing ring 6 held to the barrel 1 and is engaged to the pin 5-2 to accomplish the correction of the extent of movement of the lenses. Hence, by determining the positions of the respective group lenses by the member moved by the focusing operation and the correcting member moved by the zooming operation, the rate of extension of the lenses is automatically corrected when free zooming is carried out for a focused object.