Sigma AF 12-24mm f/4.5-5.6 EX DG HSM - Review / Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

The Sigma AF 12-24mm EX showed an exemplary performance regarding distortion with only slight (12mm) to marginal (24mm) barrel distortion. The remaining distortion has a slightly wavy characteristic but this remains truly baffling for an ultra-wide zoom lens.

12mm:

18mm:

24mm:

The chart above has a real-world size of about 120x80cm or 40x the focal length. Expect more distortion towards closer focus distances.

Vignetting

Typical for many ultra-wide zooms the Sigma also suffers from vignetting problems which are most pronounced at the 12mm setting where they exceed 1EV so even the APS-C crop factor doesn't help here. Stopping down to f/5.6 and beyond helps to control the issue. At 18mm and 24mm vignetting is no significant problem anymore.

MTF (resolution)

The lens produced a good to very-good performance in the lab. As to be expected the resolution figures are worst at 12mm - less so in the center but the borders are somewhat soft. The performance increases steadily till f/11 to a very good center and good borders. It is worth to mention that the lens suffered from rather extreme field curvature here - an effect which isn't really compatible to (flat) test charts. It was necessary to choose a much longer focus distance (compared to the real life distance) in order to achieve acceptable corner results (displayed in the graphs below). This effect is usually not overly field relevant because it is hidden by the depth-of-field which is rather pronounced with such an ultra-wide lens anyway. At 18mm and 24mm the Sigma proved to be a very solid performer with a very high center performance and just slightly worse corners.

The chart has been revised due to an incorrect reading at 12mm.

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)

Chromatic aberrations (color shadows at harsh contrast transitions) can be a problem with this lens. In fact the Imatest tool refused to analyze the CAs at the worst image corner (12mm, lower right) where the problem probably exceeded 3 (!) pixels on the average - I had to obtain the CA figures from a slightly different place here in order to keep Imatest from stalling. The CA characteristic was also slightly uneven in this specific lens sample because at the upper right the average CA pixel width was just ~1 pixel (at 12mm). The chart below shows the averaged value of these image portions. At the 18mm setting the problem remains very pronounced, less so at 24mm.
Usually CAs can be corrected via imaging tools but this is only possible if the problem is symmetrical which was not the case here.




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