There is a problem with simple wide angle
lenses on a 35mm SLR camera. Lets say you want to have a 28mm lens (a common wide angle length) or a 16mm fisheye
. For a simple lens, this means that the lens is placed 28mm or 16mm away from the film focal plane
. This causes a problem because the mirror gets in the way of the lens (or the lens gets in the way of the mirror as it tries to flip up).
The solution to this is much the same as the solution to the telephoto lens. For a 25mm lens the field of view is 51 degrees. With retrofocus design, a 35mm lens is used giving clearance to the mirror. The light into the 35mm lens is focused by a retrofocus element in front of the lens (telephoto has the element behind the lens) that has a 51 degree field of view, thus giving an effective focal length of 25mm even though the actual focal length of the lens is longer.
Retrofocus elements are often seen as attachments on digital cameras (the largest set of consumer point and shoot cameras today) where a wide angle screw on lens (often a fisheye) is placed in front of the lens making it a retrofocus lens. Many times, this is coupled with panoramic stitching software. Most zoom lenses are retrofocus lenses with a prime lens group in the back, a zoom group in the middle and then a retrofocus group in front of it all.
For a range finder camera, there is no mirror to move up allowing for wide angle lenses without having to employ retrofocus design.
The first commercial retrofocus lens for photography was developed by Angenieux. The term 'retrofocus' was the trade name for the Angenieux lens. The first retrofocus lens released was in 1950 with a 35mm f/2.5 lens to be followed in 1953 by a 28mm f/3.5 and again in 1957 with a 24mm f/3.5 lens.