The Canon EF 100-300mm f4.5-5.6 USM is a fairly lightweight but reasonably well made two ring tele zoom lens with a slightly smaller zoom range than the usual 70-300. Smaller zoom ranges usually mean better performance, and in my test with the Canon 20D results overall were pretty good - Canon 100-300 - EOS 20D test
As always with the Canon EOS 20D, and other Canon digital SLR’s with the smaller sensor, image quality benefits from forming the image only from the center of the field of what was designed as a 35mm film camera lens, but the full frame sensor Canon EOS 5D will be a tougher test
Another benefit of a reduced zoom range is usually less vignetting at the extremes of the range and the first thing I noticed with the test frames was that this was the case with the Canon 100-300 and the 5D. Although present, vignetting with this lens is far less noticeable at full aperture than most others I’ve tested with the Canon EOS 5D. It’s barely noticeable at 100mm, and gradually increases up to 300mm where it’s still better than most.
Central image sharpness at full aperture is very good at 100mm improving to excellent at f8. Corners are pretty soft at full aperture and even at f8 the extreme corners are still much the same. Like most other tele zooms the performance gradually falls off as you zoom out. The pattern is much the same with good to excellent performance in the center of the frame, but fairly poor corner performance even at f8. Although there’s very little color fringing at 100mm it’s very noticeable in the corners by 300mm, even stopped down.
Overall then this lens doesn’t really deliver the sort of performance the Canon EOS 5D deserves, with relatively poor corner sharpness, even at f8 and very noticeable color fringing at the long end of the zoom range.
Not a bad choice for a Digital Rebel or 300D though.