Hi ryder from my own personal experience the Sigma 120-300 f2.8 is
optically as good as the 300 f2.8 nikkor AFS when I was looking
into purchasing a something up to 300 mm for handheld work I hired
the nikkor and borrowed the Sigma.
For the work I wanted to use it for birds in-flight the Sigma
would have been preferable I am not the only person who thinks this
I have read several tests in the UK press regarding the Sigma and
its optical quality most of the testers were amazed how sharp this
lens is some said even better than the nikkor.
In the end I settled on the 100-300 f4 HSM as I already have the
500 and the 800 HSM’s and I use the nikkor 1.4 and 2x AFS telly
converters on my Sigma lenses. The Sigma telly converters are of
very high optical quality I use my 1.4 and 2x occasionally on my
100-300 the only reason I do not use them all the time is that
they switch the AF function off when using my 500 and 800 and the
nikkor's don't.
Phillip.
D1x 800m @ f5.6