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.
A loss that zoom lenses suffer, is in wide open aperture image quality and maybe other losses, since there is no perfect lens. All lenses are a compromise of features-size, weight, price, focal length range, image quality, autofocus performance, build quality and other features.
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