retrofocus-type (wide-angle) l

It is a lens which has the principal point behind the rear part of the entire lens. The design is like the reverse of a telephoto lens (thus called retro), and since the back focus (back focal distance) can be obtained longer than the focal length, it is used mostly for SLR wide-angle lenses. Compared to symmetrical wide-angle lenses, the brightness of the edge of the image field is great, but the distortion is bigger. From the outside, the front lens element is large and the aperture blade appears larger when seen from the front.