In article <4rufvv$a80@walters.East.Sun.COM>, dkc@troi.East.Sun.COM says...

>When a lens test indicates some type of distortion (pincushion/barrel)
>it will give the amount in a percentage. I was wondering at what percent
>will the naked eye be able to detect this distortion.
>For example: A 50mm/1.7 lens tests indicates Distortion: 1.5% barrel
>Can I see a barrel distortion of 1.5% on a print or slide
>or need it be say 5% before the naked eye can detect a barrel effect?
>The reason I ask, I see most lens vary between 1% and 5%. When should
>I worry about the number, 1,2,3,4,5%?

Much of the time, you will not be aware of lens distortions, no matter
how bad they are. Exceptions: horizons or distant large water-body
edges near the frame edge; buildings shot with straight edges running
near a large proportion of the frame edge; copy work. The distortion
is usually (but not always!) greatest near the frame edge, and it is
always zero with straight lines running through the center of the image.
There are three kinds of distortion: Barrel, in which straight lines
not running through the image center bow away from the image center; Pincushion, in which straight lines not running through the image center
bow toward the image center; and "wavy-line", which is a combination of
barrel over most of the image, switching to pincushion near the edges
or corners. Wide-angles usually show the first or third types; normals
often show the first type; telephotos sometimes show the second type;
macros usually show no distortion; zooms usually show the first and
second type, and can also show the third type of distortion. Keep in
mind that most SLR camera viewfinders have pincushion distortion, so
it is hard to evaluate lens distortion (or accomodate it, either,
unfortunately) using the camera viewfinders.
Hope This Helps