In article <6funnl$jrq$1@talia.mad.ibernet.es>, stork@costabravanet.com says...

>What is pincushion distortion?

Lens linear distortion falls into four catagories:
- barrel (Subject Straight Lines are rendered bowed out away from the
image center)
- none (SSL are rendered straight throughout the image area)
- pincushion (SSL are rendered bowed inward toward the image center)
- wavy-line, or "moustache" (SSL are rendered in combination pincushion
and barrel distortion)
Lens distortion in simple cases is greatest furthest from the image
center, and is non-existant with straight subject lines running through
the image center. Often, though, a manufacture will allow fairly strong
barrel distortion to occur between the image center and the outside
edge, but then correct the distortion as the frame edge is approached
by shifting it to the wavy-line type - which may even approach zero at
the frame edge, giving the impression of low distortion. I suspect that
allowing some distortion to occur frees the lens designer to better
optimize field flatness and off-axis sharpness. I also suspect that
lens distortion is more an issue with lens designs that are not very
symmetrical front-to-back, though I have seen nearly symmetrical lenses
that showed linear distortion...
David Ruether
http://www.fcinet.com/ruether