In article <49njnp$geu@agate.berkeley.edu>, sper@cs.berkeley.edu says...
> (most deleted) By the way, I used to think that field curvature is >the same as linear distortion, but Pop Photo reports "excess field >curvature, very little distortion". Can somebody clarify the
>difference ?

They are unrelated - field curvature refers to the inability of the
lens to focus on all parts of a subject plane at the same time (One
effect might be that a horizon could not be exactly focused over the
whole length imaged on film - you could get the center sharp, some
mid distance from center to edges sharp, the edges sharp, or the
corners sharp [if the horizon is at the top or bottom of the frame,
or the horizon is run between oposite corners], but not all at the
same focus setting on the lens.) - distortion refers to the inability
of the lens to render straight lines in the subject straight on film
(they may bend in toward, or out away, from the center - or there
could be compound curvature resulting in "wavey line" distortion).
Field curvature can sometimes be detected using a good SLR viewfinder
(though many viewfinders show poor edge sharpness, especially with
short lenses, making this difficult). Linear distortion can also
be detected using a good SLR viewfinder, but MOST viewfinders have
considerable linear distortion of their own, making this difficult,
too (though a grid screen can help).
Hope this helps.