Can someone explain to me what various lens distortion types there are & how to recognise them. Or steer me to a article on this subject. I am aware of barrel & picushion but am not sure of.. Field curvature & linear distortion(the same thing?)

-- Melvin (bramley@nanaimo.ark.com), October 25, 2001

Answers

Aaron Sussman's Amature Photographer's Handbook has a good, detailed, yet easy to read section on this, and everything. I think it's a must have -- older, but a copy is in every library and there's always one at a used bookstore. It's got a basic solid answer for everything (excpet digital!!) with diagrams and photo exampels.

Honestly though, I've got some lenses that got 50 to near 100 years and they had thoes things figured out way back then -- the best you're gonna get to unerstand it is a diagram becase you really can't see it -- in the practical level. Dean

-- Dean Lastoria (dvlastor@sfu.ca), October 25, 2001.

Linear distortion includes ( ) barrel, and ) ( pincushion, and an unusual wavy kind -- alternating between barrel and pincushion -- which Popular Photography calls "moustache" distortion. Field curvature means that if you focus the image of a surface parallel to the film (a painting, for example) so that it is precisely in focus at the center of the film, the precisely focused part of the image will bend away from the film as you move away from the center. In other words, the exactly focused part is like the surface of a bowl, rather than lying flat along the surface of the film from edge to edge. Lenses used to copy flat subjects (e.g. a painting, document, or film slide) need to have a flat field at close focus, to maintain image sharpness from edge to edge.

-- Michael Lopez (mlopez@ers.usda.gov), October 26, 2001.

Thanks for the replies now a further question. Does distortion (barrel or pincushion) only occur to the right & left of centre of a 'horizontal' 35mm image or does it also occur to theTop & Bottom ;of centre; of a horizontal image!!

-- Melvin (bramley@nanaimo.ark.com), October 26, 2001.

I am more awake today!! The lens projects a circular image therfore the distortion will be from all sides of the frame!

-- Melvin (bramley@nanaimo.ark.com), October 27, 2001.

Melvin: There's some geometrical distortion in nearly all lenses, though not necessarily of the obvious barrel or pincushion type. There's a type of distortion where the scale of objects is rendered bigger or smaller towards the edges of the frame than at the centre. For example; if you were to photograph a checker board, all the squares would be shown with the correct right-angle corners, and with straight sides, but the outer squares would be a slightly different size from the innermost ones. Lenses for copying or graphic arts use are specifically designed to minimise this type of mapping or scale error distortion.
Having said that: Most of todays quality fixed focal length or 'prime' lenses have very little distortion, except for fisheye type wide-angles, and very compact telephoto lenses.
The same can't be said of modern zoom lenses though. Nearly every wide to tele zoom will show some barrel distortion at the wideangle end, and pincushion distortion at the long end. Only the best and most expensive zooms have a small enough distortion for this to be unobtrusive. Cheaper zooms have obvious distortion at both ends of the zooming range.

Curvature of field is not a type of distortion. It's where the plane of focus of the lens isn't flat but dished or domed in shape. This causes objects at the edge of the frame to come to a different focus from those in the centre, sometimes very noticeably so. It's most common in cheap wideangles and again, zooms.

-- Pete Andrews (p.l.andrews@bham.ac.uk), October 29, 2001.