>I'm looking for advice on a good piece of glass to invest in >for my D200. I currently have a Sigma 18-135mm, which was >fairly inexpensive. > >I've been told to invest in a good lens...it can make a world >of difference. > >Any recommendations? I'd like to stick to under $500, with >the same focal length, possibly a bit wider (10mm or so).
Well, you don't say what your subjects are, so it's a bit hard to give a good target lens. You also cap your budget at a price that takes nearly all decent glass off the table unless you are buying used. And even then it would be difficult.
So I'll try to offer the following advice.
Nikon's G series prime lenses are probably going to work beautifully on the D200. Any of the AF-S G series zooms are probably going to work well also. Any zoom with goes beyond a 4:1 ratio is probably not going to be much of an upgrade.
Safe bets are any zoom with a F2.8 max aperture, or primes with an F1.8 or larger max aperture. While there are a few exceptions, staying within those guidelines should put you on the path to "good glass".
Your suggestion to the OP to get a good fast prime lens is excellent, since it will be quite difficult to find a zoom lens within his budget that is significantly better than the 1ens he already has. I suggest the Nikon 50/1.8 which is inexpensive but delivers exellent results with the D200, even wide open.
for my D300 I selected two AFS Nikkor: the 12-24 and the 17-55, plus a sigma 50-150. They all perform damn well, although the 17-55 with some flaw like the well known limits in focusing at the wide end (from 17 to 21 mm). When you learn the limits of your samples you know how to employ them 100% and your results are technically excellent (as our friend KR usually says...) On the contrary, even a pro lens like the 17-55 may show faulty results. Hi people ...
well, good glas is not cheap. For all around shooting i would get 18-200mm nikon and you will be satisfied with in a day light shooting. If you want to invest more, and depending of your subject and style, you may get 70(80)-200 f2.8 in sigma... and you will not be sorry, or for little bit more nikon ....