by taustin on Sat Jan 17, 2009 12:14 am
There are a number of companies that make lenses that will work. The better known ones are Tamron and Sigma, but there are others, too. I have a Tamron telephoto zoom that works just fine. They will generally say they work on an XTi (which is the same mount as the XT, and I believe the same as the newer XSi). The actual designation you want to look for is the Canon EF or EF-S mount (the -S is image stabilized, IIRC).
However. Generally speaking, Canon uses good quality glass. Which is to say, the actual lenses are better than cheaper brands. My Tamron lens is OK, but my Canon zoom is a lot better. In addition, pay attention to whether or not the lens you're looking at is an image stabilized lens, and whether or not you need image stabilization. If generally you shoot outdoors, in good light, or with a tripod, you probably don't need an IS lens. However, if you shoot indoors, or in low light, hand held, it can save your ass. The most noticable effect of IS is to let you shoot at slower shutter speeds, and this can make a major difference. When I was in high school, on the yearbook staff, and at the top of my form, I could manage a 1/30th second shutter speed hand held. Now, in my 40s, and shooting only occasionally, I'm happy if I can manage 1/60th. However, with my XTi, in a convention hall with crappy light with no flash, I got a shot for Marina Sirtis at 1/15th of a second, hand held, that was so sharp and clear, it looked like it was shot in a studio. The after market companies (most of them, anyway) make IS lenses, but the cheapie lenses you see generally are not, where the expensive Canon lenses you see usually are. The other major consideration is the speed of the lens - the maximum f/stop it can manage. This can also really, really save your ass in low light. Again, the expensive Canon lenses you often see are usually significantly better on this point, but again, Canon makes cheaper (and lower quality) lenses, and the aftermarket companies make better and more expensive lenses.
So, in the end, you get what you pay for with lenses. Don't just compare focal lengths and assume that's the only difference. Do you need IS? Do you need a fast lens (with a low f/stop)? Figure out what you need, then make sure you're comparing lenses with the same features. You'll find that the aftermarket lenses usually are a little cheaper, but for the same features, not by a lot. And it's harder to tell the quality of the glass on aftermarket lenses (where Canon is pretty consistenly very good), unless you have a local shop you can try them out at.