Studio Tests - APS-C format
The 70-200mm F2.8 IS struggles a little at maximum aperture on the smaller APS-C format, with some softness towards the corners, and never quite hitting the heights with regard to resolution. However in all other respects it puts in a decent performance; chromatic aberration is very low, and falloff and distortion negligible (the latter both advantages of the 'sweet-spot' effect of using a full-frame lens on the APS-C).
|Resolution||Sharpness on APS-C is certainly very good, if not quite outstanding. As usual, the lens is weakest at the extremes of the zoom range, and best in the middle. Naturally it also tends to be softest wide open, and gives optimum results at apertures of F5.6-F11 before diffraction softening starts to degrade the image significantly (F32 is best avoided completely.)|
|Chromatic Aberration||Chromatic aberration is very low, with just a little visible at the extremes of the zoom range. At 70mm there's some green/magenta fringing, which increases slightly on stopping down (due to an increase in the red/cyan component), but in the middle of the range (100mm and especially 135mm) CA is almost completely eliminated. At 200mm a little red/cyan fringing returns, but again it's kept extremely low. Overall a very impressive performance.|
|Falloff||We consider falloff to become perceptible when the corner illumination falls to more than 1 stop less than the centre. There's simply nothing to see here, move along please.|
|Distortion||Distortion is extremely low, from 0.57% barrel at 70mm, through neutral at 100mm, to 0.39% barrel at 200mm. This is absolutely nothing to worry about at all.|
Specific image quality issuesAs always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests.
Softness wide open
The major issue highlighted by our studio tests is that the 70-200mm F2.8 L IS USM gives low MTF50 values when used wide open on APS-C, especially at the extremes of the focal length range, and this is borne out in real-world shooting. However, examination of the images shows that the issue is fundamentally one of low contrast, and a lot of detail is actually being resolved. This means that the softness can be mitigated to some extent by a simple levels or curves adjustment, or better yet judicious use of the unsharp mask tool. (Of course it's also worth noting that depth of field is extremely shallow at F2.8, and this can add significantly to the perception of softness; accurate focusing is also absolutely essential.)
The samples below illustrate the image quality which can be expected from out-of-camera JPEGs at the extremes of the zoom range and F2.8 (as shot with the 12Mp EOS 450D); not bad, but lacking in the biting sharpness and contrast which characterizes the Nikon AF-S VR 70-200mm F2.8G on a DX camera, for example.
|70mm F2.8, EOS 450D||100% crop, centre of frame|
|200mm F2.8, EOS 450D||100% crop, centre of frame|