Archived posting to the Leica Users Group, 1998/02/02
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]> To answer the several questions about perspective distortion: according >to the >books, it is an effect that is dependent on focal length and camera position >only. However, try this practical test: cut out a circle of paper about the >size of a basketball. From ten feet away, put it in the corner of the frame >with a non-retrofocus lens like the Leica 35mm Summicron-M. Then do the same >thing with a good retrofocus lens such as the Canon EF 35mm f/2. Compare the >images. What you will see is that both lenses will distort the circle into an >oblong, ovoid shape, but the Leica lens (in this example) will distort the >circle slightly more. I can't explain this, and "perspective distortion" may Perspective distortion is caused by the way an (ideal) lens images objects; ie, plane to plane. Because of a whole lot of factors, both physiological and psychological, we do not see or process images the same way as a lens does, and this effect is strange to us. In an ideal lens, the only factor governing perspective distortion is the angular distance of the object/image from the lens axis. It has nothing to do with the type of construction, or how far the various elements are from the film plane. However, since in practice symmetrical wideangles of standard (non-retrofocus) construction have little or no distortion, and retrofocus lenses tend to have barrel distortion over most of the field (with pincushion distortion in the corners of the field), the barrel distortion to some degrees mitigates the perspective distortion, and causes it to be less noticeable than in a lens with better distortion correction. In practice, for very wide angles of view, distortion of one kind or another is inevitable. Poorly corrected retrofocus lenses, symmetrical wideangle lenses, fisheye lenses, panoramic rotating lens cameras and other even more esoteric imaging systems all work; they just each produce their own type of distortion. * Henning J. Wulff /|\ Wulff Photography & Design /###\ mailto:henningw@archiphoto.com |[ ]| http://www.archiphoto.com