I have a Canon Rebel XT and I am looking to upgrade my current lenses. I have picked out a standard zoom lens, but I am having trouble deciding on a telephoto lens. I currently have Tamron lenses that are about 2 1/2 years old and I find that the telephoto lens is too soft, especially at 300mm. Just for some background, I generally use the telephoto lens to take pictures at conventions, so it's generally stage lighting and from a distance. I use a tripod, but sometimes there isn't enough room to really set it up properly. I've recently bought a monopod, but I haven't used it yet (I will have a chance to try it out this weekend). I am not a professional by any means and the problem could very well be with me and not the lenses. I would like to keep my budget reasonable (the zoom lens I picked out is about $230 on amazon), but I don't want to end up with a lens where I don't see a marked improvement. The two main lenses I am looking at are: Sigma 70-300mm f/4-5.6 DG APO Macro Telephoto Canon EF 75-300mm f/4-5.6 III USM Telephoto And I am also considering (but hesitant because of the reduced zoom): Canon EF-S 55-250mm f/4.0-5.6 IS Telephoto In an ideal world I would love to buy (but it's about twice what I am wanting to pay): Canon EF 70-300mm f/4-5.6 IS USM Lens Anyway, I am just looking for some advice. Thanks.