Most variable spherical aberration generators have differential moving components or floating elements, much like a regular zoom lens but with insignificantly small overall magnification change over the "zoom" range. The main purpose of the differential movements is to generate a variable amount of spherical aberration (SA). Such aberration generators can be used as standalone systems for aberration control or as subsystems for aberration compensation. The theory can also be applied to macro focusing mechanism as well as soft-focus mechanism. This paper presents a survey of different variable SA generation mechanisms. Examples of some applications of the theory are presented.