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Sharpness
As part of our test of the Canon EF 300mm f/2.8 IS II USM we've shot our SFR Plus chart at every aperture to check for sharpness and fringing across the frame. The resulting images are then fed through the Imatest Master software.
The graph below shows the results of the test, with each of the coloured lines depicting the lenses performance at the centre (in blue), and an average of the results for the middle of the frame (pink) and the image edge (yellow).
The graph shows that the Canon EF 300mm f/2.8L IS USM performs well across the aperture range, with the best results for sharpness achieved between f/4 and f/7.1.
As usual the lens performs best at the centre, but at most apertures the edge performance is not far behind.
Fringing
This graph shows that signs of fringing are low across the aperture range. Despite the expected rise at the edge of the frame the amount of fringing is still very low.