Ultra Wide Angle Photography

There are two different varieties of wide-angle lens: short-focus lenses and retrofocus lenses.
Short-focus lenses are generally made up of multiple glass elements whose shapes are more or less symmetrical in front of and behind the diaphragm. As the focal length decreases, the distance of the rear element of the lens from the film plane or digital sensor also decreases. This makes short-focus wide-angle lenses undesirable for single-lens reflex cameras unless they are used with the reflex mirrors locked up. Short-focus lenses are widely used on large format view cameras and rangefinder cameras.
The retrofocus lens solves this proximity problem through an asymmetrical design that allows the rear element to be further away from the film plane than its effective focal length would suggest (Angénieux retrofocus). For example, it is not uncommon for the rear element of a retrofocus lens of 18 mm to be more than 25 mm from the film plane. This makes it possible to design wide-angle lenses for single-lens reflex cameras.