One of the general recommendations I've seen on this forum is to base your choice on the lens system rather then the camera body, and then to put your money into the best glass you can afford. While this makes sense I'm a little confused. Because of the way things are marketed its quite easy to compare bodies, glass however seems to be more difficult to get a good reading on (at least from a newbe perspective). Also how does third party glass such as Sigma/Tokina/Tameron fit into the equation. Can some of these lenses be cost effective alternates to OEM glass without sacrificing quality. How does one search out the gems among all of the available glass?
This all depends on a lot of things.
1. Are you planning to be a professional?
2. How much do you plan to spend?
3. How long have you been doing photography on any level?
There are more questions, but let's start there. Once you determine these, here are some things to consider:
1. Glass will outlast camera bodies, so that's why people say to invest in the glass. In addition, don't spend $300 on a lens just because you think it's a bargain; think about the long run. I have lenses that cost more than $1,600 EACH, and I know that they will consistently produce superb images even when I move on to new bodies in the future. I made a huge mistake when I bought two cheap lenses early on. Those lenses ended up collecting dust before I finally sold them on eBay for next-to-nothing.
2. Some gems can be found with Sigma/Tamron. I don't believe in Tokina at all. With that said, keep in mind that some 3rd-party lenses are made obsolete by camera manufacturers in future bodies and have to be re-chipped by Sigma/Tamron/Tokina in order for them to work. WIth that said, I have a Tamron 28-75 f/2.8 that I simply love on my Canon 20D. For the money, it's the best lens I own. It's the ONLY 3rd-party lens I have, however.
3. To compare lenses, I usually surf
http://www.dpreview.com and ask questions there. A great review site for lenses is
http://www.fredmiranda.com. Don't let the high cost of lenses deter you from buying a good lens. If you plan to keep doing photography for a long time, it will pay for itself (especially if you become a pro and start making money).
Hope this helps
.