Sigma AF 20mm f/1.8 EX - Review / Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

The AF 20mm f/1.8 EX showed a slight degree of barrel distortion for a lens of this focal length. In very critical scenes this may be visible but it's better than most zooms here.

Vignetting

Sigma claims this lens to feature "minimal light-fall-off with superior peripheral brightness". Well, at max. aperture this isn't really true with vignetting exceeding 1EV. Here the issue can be disturbing in some scenes. However, from f/2.8 onwards the problem is basically negligible so it's still a very decent performance (on an APS-C DSLR).

MTF (resolution & chromatic aberrations)

The lens exhibited a varying performance in the lab but this was to be expected from a lens exploring the extremities of lens design.
At f/1.8 the center resolution is already quite fine but the borders are very poor. However, at f/2.8 the performance is very respectable and it's increasing further till f/5.6-f/8 where it peaks in excellent center- and very good border quality.

The bad message is that the sample lens showed a rather hefty degree of decentering resulting in a very pronounced and disturbing drop in performance towards the right side of the image field and remember that we're talking about the APS-C crop here. The effect was disturbing up to about f/5.6. It was surely a specific bug in the tested sample but we have seen this before ...

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

Chromatic aberrations (color shadows at harsh contrast transitions towards the image borders) are quite pronounced with an average pixel width of up to 1.5 pixels. This can be slightly disturbing in critical scenes. As always it should be mentioned that CAs can be corrected via imaging tools.




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