General photographic lenses are designed to achieve an optimum balance of aberration compensation at only one commonly-used shooting distance. Thus, although aberrations are well compensated at the reference shooting distance, aberrations increase at other shooting distances (especially at close shooting distances) and cause image degradation. To prevent this from happening, a floating system is used which varies the interval between certain lens elements in accordance with the extension amount. This method is also referred to as a close-distance aberration compensation mechanism.canonglossaryeu
A lens element in a compound lens that changes its position as the lens is focused.