"Brian Tung" wrote in message
...

Distortion is variation in magnification with off-axis angle. If object
A is twice as far from the axis as object B, then A should appear twice
as far from the center of the field of view as B. If it appears *less*
than twice as far, you have what is called barrel distortion, because if
you look at a square through such a system, the sides of the square will
be bulged outward, like a barrel.

If it appears *more* than twice as far, you have pincushion distortion.
If you look at a square through such a system, the sides of the square
will be bowed inward. I guess that reminds people of pincushions.

Thanks for the clarification... all though, I have to say, the pin cushion
in my wife's sewing kit is oblate. I also think it interesting how the
description of a curved line at the edge, gives the illusion that the bowing
out is a result of the center of the line getting thrust beyond where it
should be, rather than the corners of a square getting "sucked in" toward
the center.

You do realize of course that from your description, one might get the
impression that you have some experience putting square pegs into round
holes. g

-Stephen