Hi Everybody, This is my first post to the forums. Also, I am VERY new to photography, so please excuse my ignorance if this question is simple. I have a digital Canon EOS Rebel XT with a standard EF-S 18-55mm lens. I recently purchased a 100-300mm Canon EF telephoto lense f/4.5-5.6 with USM and rear focusing which, from everything I've read about the EF-S mount being backward compatible with EF, should work fine on my camera. However, the lense has a considerable amount of blur, which seems to be worse upon zooming in on items. I'm 100% certain this is not my hands shaking, as I've set it on a stable surface with the same results. The blur is noticeable both while looking through the viewfinder and in the photos. Is there anything I'm missing regarding the use of an EF lense on an EF-S camera? Is there a setting I need to adjust? Or is it a bad lens? Any help with this issue would be great. Again I apologize if this is something obvious, this is my first real experience using a telephoto lense on my camera. Thank you all.