Floating or constant?

A lens’s maximum aperture is the widest it is capable of opening, but while some zooms’ maximum aperture remains constant through the zoom range, on others this shrinks at the telephoto end. Constant aperture lenses also tend to have a wider maximum aperture in the first place, so a 24-70mm f/2.8 standard zoom is a stop faster than an 18-55mm f/4-5.6 at the wideangle end, but two stops faster at the telephoto end. Constant-aperture lenses are bigger, heavier and more expensive, but generally offer superior image quality.