Everybody said dont do it. The image will be poor. I did it anyway. I just had to try the 400mm FD prime on the EOS body and well with the adaptor 1.4 factor plus the 1.6 crop on the XTI its kinda like a supertele long. Forget about anything wider than f5.6 its horrid trash and mid day light getts lots of fringing. This was the best shot late in the day when the light was not to harsh.