In a retrofocus design,which is advantageously applied to wideangle lenses, the back focus is designed longerthan the lens focal length to allow clearance for the movement of the reflex-mirror(No Mirror Lock up or seperate viewing accesory attachment is required). It consistsof front diverging apd rear converging lens groups, as opposed to the telephoto design,and is therefore also called the inverted telephoto design.mirglossary
Also known as a reverse or inverted telephoto, this lens design has a diverging lens element positioned in front of the aperture and a converging element positioned at the rear. This makes the distance from the rear of the lens to the focal plane longer than the lens focal length. Retrofocus design has been adopted in wide-angle lenses so the rear of the lens does not impede the movement of an SLR camera s reflex mirror.