Nikon 300mm f4 or sigma 170-500mm ?
 

Hello i was thinking about purchasing a better lens for bird photography i have a 70-300mm right now and i am looking for somthing faster, sharper, and better focal length. i have heard that the 300mm afs works great with a TC is this true? and i have also heard good things about the 170-500mm i have a nikon d50 and my budget is only about $400- maby $500
any suggestions. thanks

I can't comment on the Nikon lens but the Sigma is pretty reasonable quality (see my recent Godwit - which one? photo) and a zoom is more flexible. I find that I can still get auto focus with the Sigma and a Kenko 1.4 converter on my Canon 20D but it is a bit slow both focusing wise and f stop wise with the converter on.

But if thinking of a 170-500 be sure to try it first. I had one for two days before returning it - not so much the quality more the physical length and general awkwardness of the lens. My own current set up is a Sigma 100-300mm f4 HSM coupled with a Sigma 1.4 when needed. The HSM makes the lens very fast to focus - adding the tcon does'nt slow it down that much. Although this would be a more expensive option new, it's usually possible to pick up second hand [got mine for under half price]. Quite heavy but easy to hold as it stays the same length throughout the zoom range. I have used a Nikkor 300mm f4 - again, much easier to hold than the 170-500mm. Also consider ther 135-400mm.

I have a Nikon 300 f4 and it is truly a great lens for the money. I had a Tamron 200-500 and there is just no comparison between both lenses. The Tamron was good, don't get me wrong I took some good shots with it - have a look at http://www.mejhogan.com - both the Fulmar and Glendalough - but its sweet spot was not the easiest to find and I would presume the Sigma will be the same. The Nikon is always crisp and clean even with a 1.4x converter on it. I'd go for the 300.

The Sigma is certainly not for handholding nor would it match the Nikon optically, it's advantage is greater reach for a comparatively modest price. It's f6.3 at 500 so its not fast either but it is a budget lens which is fine on a tripod or on a hide-clamp. My thought was you already have a lens up tp 300 so you may be after extending your range. As always the question is why do you need something different and the answer to that will help you decide what criteria to look for to meet your needs and hence which lens to buy.