Tested with a Canon EOS 5D

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As always with the Canon EOS 5D, the first thing I noticed with the test images was vignetting - darkening in the corners - at full aperture. Levels are fairly average for the Canon EOS 5D and worst at the 300mm end of the zoom range, but the good news is that one stop down it’s barely noticeable. Unfortunately it’s not all good news though - there is fairly obvious CA throughout the range at full aperture and the lens does suffer the usual 70-300 drop in performance at the 300 end. Central sharpness remains good all the way to 300mm, but corners which are slightly soft at 70mm are very soft by 300mm. Stopping down to f8 improves matters greatly at the 70mm end but corners are still poor at 300mm. Much the same is true for the CA - Chromatic Aberration which shows up as purple fringes around strong dark shapes near the edge of the frame. This is most obvious with shots taken at full aperture, but less noticeable one stop down. How much of a problem this is depends a lot on the subject being photographed - Before checking out all the test frames, I’d looked at lots of very sharp, normal images, taken with this lens using difficult stage lighting, but with the main subject near the center of the frame - they looked great, with good sharpness and lots of detail, so I was expecting great things from this lens. In balance the Canon 70-300mm f4-5.6 IS USM is a better than average 70-300 zoom but still a compromise in terms of pure optical quality. Interestingly the results from the Canon EOS 20D test are much better - answering the “who is this lens made for?” question quite clearly.

Canon claim a 3 stop gain in steadiness with the IS system in this lens, which to me means I should be able to hand hold shots at 300mm down to about 1/60 - my normal limit is about 1/500 second. In practice the figures for sharp images were 1/250 - 90% sharp 1/125 - 80% and at 1/60 only 10% were pin sharp although most would look OK as 6x4 inch or 15x10cm prints. In general then I would rate the IS as giving about a 2 stop gain in steadiness.