A zoom lens assembly for use with a photographic camera having a capability of functioning two different modes one at a time. In one mode, the lens assembly is of a type having its aperture ratio variable with change in focal length, and in the other mode, it is of a type wherein change in aperture ratio resulting from the change in focal length is compensated for to give a uniform aperture ratio at all focal length settings. For this purpose, a unique change-over mechanism is built in the lens assembly for switching the lens assembly from one of the different modes onto the other of the different modes at the will of a photographer.