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 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. All lenses is a compromise of features-size, weight, price, focal length range, image quality, autofocus performance, build quality and other features. Wikipedia [ - ]

A zoom lens is a mechanical assembly of lens elements for which the focal length can be varied,... [ + ]