On Lens Distortion Types

Many (most these days...? ;-) lenses have linear distortion, which is the failure of the lens to render straight lines in the subject that are off center in the lens image as straight lines on the film. Note that most camera viewfinders also have linear distortion, so it is hard to judge lens linear distortion by simply comparing the straightness of a subject line with the dark viewfinder edge (which in addition to being rendered not straight by the viewfinder optics, may itself not be straight...;-). Grid-type viewing screens are, however, useful for evaluating lens distortion, though not when the distortion is near the frame edges where it is generally the greatest. The normally encountered lens distortion types are:

"barrel", in which subject straight lines bow out away from the image center. This type is most commonly seen in SLR wide angle primes and at the short end of most zooms.
"pincushion", in which straight subject lines bow inward toward the image center. This type is most commonly seen at the long end of most zooms (and in most SLR viewfinders...).
"wavy-line" or "moustache", in which the lens shows barrel distortion over much of the frame, changing to pincushion near the corners. This type is most commonly seen in retrofocus wide angles for SLRs and in some zooms which include wide angle at the short end.

I think wavy-line and pincushion are the ugliest distortion types, but I have no problem with barrel distortion, which actually is present in our own vision, popular misconceptions notwithstanding...
(For more, see "On Seeing and Perspective",
"On Lens Perspective Types",and "On Lens Angles Of View,
Magnification, And Perspective".)

"Hope This Helps"
David Ruether (d_ruether@hotmail.com)

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