Canon EF 20mm f/2.8 USM - Review / Lab Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

The lens showed a slight to moderate degree of barrel distortion (~1.55%) which is quite good for this focal length class.

The chart above has a real-world size of about 120x80cm.

Vignetting

On an APS-C DSLR the full frame EF 20mm f/2.8 USM enjoys a sweet spot advantage but the usually positive effect isn't all that pronounced regarding vignetting which is rather high at 1.28EV at f/2.8. From f/4 and up the issue is already very well under control and no problem under field conditions.

MTF (resolution)

The resolution figures in the lab were something of a mixed bag. The center performance is exceptionally high straight from the max. aperture. At f/4 it is even among the highest tested to date. Unfortunately the borders can't quite follow but the quality is still good to very good here. The extreme borders (data usually only obtained for dedicated APS-C lenses) show a rather pronounced fall off at f/2.8 which recovers by f/4. At f/8 even the extreme borders are in very good territory.

The lens exhibited a rather pronounced field curvature and a slight degree of residual aberrations (focus shift when stopping down) so you need to make sure that the depth-of-field is sufficient for your scene.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

CAs (visible as color shadows at harsh contrast transitions) reach an average CA pixel width well below the 1 pixel barrier which is very good for a lens in this focal length class.




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