In article <4h3nkf$ilc@newton.fgg.eur.nl>, Dieleman@cid.fgg.eur.nl says...

>The general impression was that the 2.8/20 is a far better lens than
>the 4/20. The comments lead me to the decision to buy a 2.8/20.
>Meanwhile, I took some test pictures with it. The results correspond
>very well with the opinions expressed:
[rest of interesting post on findings with 20mm f2.8 deleted...]

Having used several of the Nikkor 20mm f2.8's, I would add that I
have seen slight sample variations in performance. The best ones are
quite crisp and even to the corners by f5.6 with color slides, though they can use a bit more stopping down when using B & W film (when not
using color filters) for best corner performance. As with the 24mm
f2.8 Nikkor, performance seems similar at f2.8 and f4 (neither aperture is one I would use with either lens, if possible), but jumps up
abruptly in quality around f5.6 (though corner performance is generally
a bit better in the 20mm compared with the excellent 24mm Nikkor).
Hope This Helps