In article <432jko$acm@titan.np.ac.sg>, tph2@titan.np.ac.sg says...
>Anyone like to comment on the Sigma 15mm f2.8 as compare to Nikon
>16mm f2.8 in term of the optical performance. Nikon is probably costs
>about 2.5 times more Sigma. Is it worth to pay for that.

I have seen some variation in performance in several samples of the
16mm f2.8 Nikkor, but good samples are excellent, especially stopped-
down to mid or smaller apertures. The older (mid-style, small) 16mm Sigma I tried was very poor, perhaps defective. The best full-frame
fisheye that I have seen (and it may be within your budget) is the
exceptional older Nikkor 16mm f3.5. It is VERY sharp and contrasty
to the corners of the frame with moderate stopping-down (with very
good performance even wide-open), very difficult to flare, light and
small, and has built-in filters. You will need to look for it used,
and it was produced as both non-AI and AI (very worth AI'ing).
I will go so far as to say that this is the best wide-angle of any
kind that I have seen (and that is saying something!) - even though
it will cover about 170 degrees (on the diagonal). Also, by f5.6 it
is about equally superb on the Nikkor TC14A converter - which makes
a super-wide that is kind to people, and tends to keep (distant)
building verticals vertical. Heck, what more could anyone want????
Hope this helps.