ABSTRACT
Symmetrical wide-angle lenses and their derivatives as conventional short focus types in that as equivalent focal length decreases the back focal distance also decreases, so that there may only be a very small clearance between the vertex of the rear element and the focal plane. The combination of large aperture, low vignetting, flat field and wide angle is most useful. A lens of short focal length requires only a few millimetres of axial movement to focus close, typically to an unaided near limit of 0.3 m. The focusing of retrofocus lenses, with their greater depth of field than standard or long focus lenses, may be easier using a split-image rangefinder focusing aid in the viewfinder. For photography, really large aperture retrofocus lenses are uncommon. Due to the asymmetry of the configuration, early retrofocus lenses suffered from noticeable barrel distortion. In 1929 H. Lee of Taylor Hobson applied his design experience in the correcting of distortion of telephoto lenses to improve retrofocus performance.
