Yuhong Wang wrote in message <349E450B.5200@nt.com>...

>I am trying to find a decent wide angle (prime or zoom)lens for my Nikon
>bodies, 20 to 28 mm is my preferred range. So far the best I got is the
>Nikkor 28/3.5 AI, very decent in sharpness and has uniform illumination.
>But it gets some wavy distortion at the edges. My Tokina 28-70/2.8 ATX
>is sharper in the center @ 28mm than the Nikkor but the distortion is
>much worse. I tried the Sigma 24/2.8 AF but got rid of it quickly
>because of the distortion(it's a very sharp lens indeed). I never tried
>the 20/2.8 Nikkor, but from the pictures taken by other people, it seems
>to get barrel distortion too. Is there any Nikon wide angle that will be
>as sharp as(or sharper than) the 28/3.5 and has better correction?
>28/2.8 AIS? 24/2.8? I doubt any zoom will be better than my 28/3.5 in
>this regard. Both 50/1.8 E and 50/1.8 AIS that I have are very good in
>distortion correction(but not the 50/1.4 AIS), I just need something
>wider than that. I heard the Olympus 28/2.0 has excellent reputaion for
>little distortion (somewhere at 0.1%?!!)I hate to see the walls become
>curves in the pictures! Thanks in advance!

Retrofocus wide-angle lenses generally have some linear distortion
as one of the compromises in their design. You may already have one
of the better wides in this respect, without going to extremes (the
Nikkor 15mm f5.6 [not the f3.5...] was very low in linear distortion,
as was the non-retro 21mm f4 [high flare, poor illumination evenness,
minimum 3' focus, and fits only the F and F2 bodies - with mirror
locked up...]). The 24 f2.8 Nikkor is low in distortion along the
frame edge, but the distortion increases noticeably a bit away from
the frame edge. I kinda wish that lens designers didn't feel that
they must go to the "wavy-line" type of distortion to minimize the
percentage rating, and would just let the lens have stronger barrel
distortion - if it is smoothly curved, barrel is the least obnoxious
type of distortion (and is similar to what is present in our own
vision... [see "On seeing and Perspective" on my web page, under
"I babble" for more on that...;-]). Another alternative is to use
a PC lens - the greater coverage results in fairly low (barrel)
distortion when unshifted, and the frame corners are quite sharp,
since you are using only the center part of the lens coverage
(I like PC's for rendering texture uniformly sharp everywhere in
the frame, though your 28mm f3.5 AI Nikkor is unusually good in
this respect also [when used at f8-16] - Nikon sometimes sacrifices
some center sharpness to pull up the edge and corner performance,
which is valuable to me [a lens that is not sharp most everywhere
in the frame at a particular stop is not sharp at that stop, in
my opinion, no matter how sharp the center is...]).
--
David Ruether
http://www.fcinet.com/ruether
ruether@fcinet.com