Sigma 14-24mm f/2.8 DG HSM ART - Review / Test Report - Analysis
Lens Reviews - Canon EOS (Full Format)

Distortion

Ultra-wide zoom lenses tend to have a rather extreme distortion characteristic BUT ... surprisingly ... the Sigma 14-24mm f/2.8 HSM DG ART does a very good job here. At 14mm, the barrel distortion is rather slight at just ~1.6%. There is a tiny bit of barrel distortion left at 16mm but beyond it's negligible.

Vignetting

The vignetting is about in line to what you can expect from such a lens. Thus the light falloff is very heavy (2.7 EV/f-stops) at 14mm f/2.8. Stopping down to f/4 helps a lot but there's still some vignetting at f/11. The issue isn't QUITE as pronounced at 16mm with a "fairly" moderate vignetting of 1.8 EV at f/2.8. The issue is mostly gone from f/5.6. It's similar beyond 16mm but f/4 is sufficient enough for taming the vignetting.

MTF (resolution) at 50 megapixels

The resolution characteristic of the Sigma lens is somewhat mixed. On the positive side, the quality is very impressive at 14mm. The center is already great at f/2.8 and superb at f/4. The near center is at least very good. The outer image region is still good at fully-open aperture which is surprising for a lens in this class at 50 megapixels. The peak performance is reached between f/5.6 and f/8 with a very good quality across the image field. The wide-open performance is a bit reduce at 16mm with some soft corners. However, once again, the results are pretty good at medium apertures. The performance is maintained at 20mm. The 24mm setting isn't quite on that level though. The center quality remains great but the borders/corners are soft - not only at f/2.8 but also at f/5.6. However, not all is lost - the quality is decent at f/8. Diffraction effects are getting more obvious from f/11 onward (as usual).

The centering quality of the tested sample could have been better (again).

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

MTF (resolution) at 21 megapixels

Why are the MTFs sometimes "better" on 21 megapixels compared to 50 megapixels ? There are two reasons for this. Lateral CAs are lower in terms of pixel widths at 21mp simply because the pixel density is also lower. Extreme CAs that may exist at 50mp are therefore less affecting the measurements at 21mp. Generally we are also using a certain degree of sharpening during the image conversion (just like in real life images) and because the 21mp results are "sharper" on pixel level they are relatively more receptive to (mild base-) sharpening.

Many users are still using 21mp DSLRs so let's have another look how it performs in this scope then. Of course, the lower resolution is boosting the charts; visually at least. The quality is very high at 14mm - once again straight from f/2.8. The corner softness isn't quite as obvious at f/2.8 in the middle ranage and there isn't really anything to complain beyond. At 24mm the quality isn't that great at large aperture settings but not terrible either.

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)

Lateral chromatic aberrations (color shadows at the image borders) are very low with an average CA pixel with of 1px or less. That's nothing to be concerned about really.

PS: The CA figures were taken at 50 megapixels.