In article <51n3ql$mcq@newsbf02.news.aol.com>, philmeistr@aol.com says...

>I was reading two lens test reports in Popular Photography and I'd like
>some help on a few terms they used.
>
>First of all, about a Sigma 28-70 f/2.8-4, they said its linear distortion
>is 2.8% barrel @ 28mm, 1.7% pincushion @ 50, and 1.9% pincusion @ 70. Is
>this a lot for a general purpose zoom? Will I really notice it when I
>photograph buildings? And is one type of distortion worse than the other?

This is enough linear distortion to notice when photographing buildings
where straight lines extend across most of the frame near the edge, but
it might not be noticed in relatively short straight lines near the edge,
or with long ones near the center (there is no distortion of lines passing
through the center of the image). Barrel distortion is usually not very objectionable (we see with considerable barrel distortion, contrary to
popular opinion....;-), but pincushion does look unnatural (straight
lines bow inward toward the center).

>What does "lateral color" mean?

I defer to the optical experts on this one....

>Why does ghosting appear in backlit shots, as mentioned about the Ricoh
>55mm f/1.2? Will a lens hood help the problem?

Lens design and coatings, and element edge and barrel coatings affect
flare, ghosting, etc. caused by bright parts of the subject. If the
bright subject causing the problem is within the acceptance angle of
the shade, the shade will not eliminate the problem.

>And finally, on a different topic, I've read that the new 2-element
>"apochromat" (I think that's what they were called) closeup lenses are
>very sharp and distortion-free. Has anyone used one of these, and how
>much do they cost? Would I be better off getting an extention tube or a
>regular set of closeup lenses if the apochromat lens is too expensive?

That's "achromat", and they are generally noticeably better than single
element close-up lenses. Distortion is not much of a problem with
close-up lenses, but sharpness is. The achromats (made by Nikon, Canon,
Sigma, and others) are excellent on some lenses at smaller stops, not
so great on others, which is also true of tubes - so it is hard to
predict which will perform better on a particular lens, though,
IN GENERAL, lenses of symmetrical optical design will perform well
on tubes, and lenses of a very asymmetrical design will not (and the
achromats may then be better on these), but I have seen odd combinations
work very well (combining the Sigma achromat and tubes and/or
teleconverters makes a really excellent high magnification macro out
of the Nikkor AI/AIS 200mm f4 non-macro, something that would seem to
be unlikely...). Don't be afraid to experiment. BTW, good achromats
run about $30-50, not too bad a price.... If two powers are available,
get the stronger.
Hope This Helps