Thomas G. Hineman wrote in message <638tlk$3g7@bgtnsc01.worldnet.att.net>...

>In reading some lens reviews I have seen references to "pin cushion effect".
>Could someone please enlighten me as to what is meant by pincushion effect
>and how might I know what to look for.


Many (most these days...? ;-) lenses have linear distortion, which is the failure to
render straight lines in the subject off center 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 unstraight by the VF
optics, may itself be unstraight...;-). 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
ends of 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 SLR's and in some zooms including
wide angle at the short end. I think it and pincushion are the ugliest, but I have no
problem with barrel distortion, which actually is present in our own vision, popular
misconceptions notwithstanding...;-)
David Ruether
http://www.fcinet.com/ruether
ruether@fcinet.com