The present invention provides optical systems containing an afocal variator able to impart focus means to the optical system, as well as essentially constant magnification throughout the focal translation. The present application teaches a specific formula or set of spacing requirements for the positioning of the afocal variator that can be used with any afocal variator optical lens system comprised of various lens combinations. Provided that the formula for spacing the afocal variator within the optical system is utilized, the afocal variator acts not only to focus the optical system but maintains substantially constant magnification during the translation of focus.