A zoom lens is a mechanical assembly of lens elements for which the focal length can be varied, as opposed to a fixed focal length (FFL) lens. A true zoom lens, also called a parfocal lens, is one that maintains focus when its focal length changes. A lens that loses focus during zooming is more properly called a varifocal lens. One sacrifice of a zoom lens is wide open aperture image quality and maybe other sacrifices, because there is no perfect lens.