Full aperture results with the Canon 75-300 f4-5.6 USM II are OK to pretty good. This is a budget lens and both build quality and results reflect that. Central performance varies from good at 75mm to very good at 300mm. Edges are a fair way behind and pretty poor at the short end - especially around 135mm. As with most lenses with the Canon EOS 5D, vignetting is fairly obvious at full aperture, right through the range.
Stopped down to f5.6 or f8 results improve a great deal at 75mm with good sharpness extending most of the way to the corners. 135mm results are sharper now in the center but still poor in the corners, and at 300mm and f8 quality is now very good overall - excellent in the center.
Overall for a budget lens this is quite a good performer but sharpness in the corners in general isn’t great, and around 135mm edge performance is poor. So is this a Canon EOS 5D result only? Does the Canon 75-300 USM suit the smaller sensor canon EOS 20D better?
Not Recommended for the Canon EOS 5D