Portrait Perspective

We tend to think of lenses as magnifying subjects, but in photography most lenses reduce the subject to a much smaller image. The type of lens you use often affects the perspective of the subject in the image. A "normal" lens (50mm lens for 35mm format, 80mm lens for 6x6cm medium format, 180mm for 4x5" format) will distort subject features in close-up children pictures; shorter lenses (wide-angle) severely distort the subject, and longer lenses (telephoto) distort less. Lenses of twice the normal focal length provide a reasonably undistorted perspective (100mm for 35mm format; 150mm for medium format, 360mm for 4x5" format). This is separate from the distortion common in zoom lenses.

Most large format lenses are slightly "wide-angle" even if longer than normal, but if the camera back is zeroed and only the center of the lens area is used then the same rule applies (i.e. 360mm is the ideal portrait lens for 4x5" format). Since we are talking about very long lenses in medium/large format, a true "telephoto" lens is a convenient compromise. "Telephoto" here means the lens design isn't symmetrical, and the distance from the lens to the film plane is shorter than the focal length, which allows the use of a more compact camera. In theory, true telephoto lenses are more likely to distort than a normal/symmetrical design (lens to film distance = focal length). I haven't done any controlled tests, but in some cases the distortion of a telephoto lens may make a subject look better than in real life! (Sample)

The opposite of a telephoto lens is a true wide-angle lens: the lens elements aren't symmetrical but the lens to film distance is longer than the focal length. This allows the use of normal cameras rather than a special "short" camera. The retrofocus design of most wide-angle lenses makes them shorter and wider (more subject to flare), while a few symmetrical (more or less) wide angle lenses such as the old Zeiss Biogon and Schneiderís newer Super Symmar lenses are longer and narrower (less subject to flare). Multicoating of lenses was developed to reduce the probability of flare in modern lenses.

Photo © Andrew Nazemnov Taken with a Sony A700 from Flickr.com