sentences:  The focal length of a zoom lens is not fixed; instead it can be varied between a specified minimum and maximum value.Modern lens technology is such that the loss of image quality in zoom lenses (relative to non-zoom lenses) is minimal, and zoom lenses have become the standard lenses for SLRs and DSLRs.This is different from the late 1980s when, due to image quality concerns, most professional photographers still relied primarily on standard non-zoom lenses.