Retrofocus is a lens design in which a negative lens element is placed in front of the diaphragm and a positive element behind it. This configuration increases the distance from the rear of the lens to the focal plane, making it longer than the lens’s focal length.
Retrofocus designs are commonly used in wide-angle lenses for SLR cameras, allowing sufficient space for the reflex mirror to move freely without obstruction. This design ensures that photographers can enjoy wide-angle perspectives while maintaining full compatibility with SLR mechanics, making retrofocus lenses an essential choice for architectural, landscape, and interior photography.