A couple of weeks ago I purchased the Tamron 17-50mm f/2.8 for my Digital Rebel XT / 350D, and it arrived yesterday morning. When I went out to try it, I was very disappointed as the pictures looked soft at wide apertures (2.8).
I was going to contact the seller to get a replacement (and send it all the way to Hong Kong), but the fact that at smaller apertures was incredibly sharp made me think about it. I realized it wasn't softness but a front focus problem. I had the same problem with the nifty fifty, and I always thought it was the lens, but now this was weird.
After reading the lens forum FAQ, I realized it was my camera's problem, looks like it's a common problem with this model. I thought about sending it back to Canon, but then I found this webpage (and the google translation) explaining how to fix the front focus problem with an hex key.
This morning I bought the hex keys, printed a focus test chart, and after some work... I fixed my camera!!
If you guys have problems with your XT and big aperture lenses, there is a big chance is a camera problem. I recommend you to try this out!
I'm so happy, I feel like I have a new camera and 3 new lenses!