In article <33C7C6BF.ABAEB327@undp.org>, pcote@undp.org says...

>I am planning tu buy a wide angle lens. Someting wider than my 24mm.
>What are the advantages of buying a 18mm AIS instead of the 20mm,
>knowing that those extra 2mm cost almost 100% more? Thanks, Pedro

Almost no difference... I owned both MF lenses, and kept the slightly
superior smaller, lighter, and cheaper 20mm f2.8 Nikkor since the angle
of view was virtually the same with both lenses (theoretical diagonal
angular difference notwithstanding...;-)
Hope This Helps
(David Ruether - http://www.fcinet.com/ruether )