A varifocal lens is one where the focal length of the lens can be varied. This is a fancy way of saying it is a zoom lens. Most varifocal lenses have, and in almost all circumstances should have, an
auto iris feature.
The alternative to a varifocal lens is a fixed focal length lens. A fixed focal length lens does have one advantage over a varifocal lens besides cost. A fixed focal length lens usually allows more light to pass through the lens at a given focal length than a varifocal, or zoom lens. This can be important in low light situations.
