Sigma 24-70mm f/2.8 DG HSM ART OS - Review / Test Report - Analysis
Lens Reviews - Canon EOS (Full Format)

Distortion

The Sigma 24-70mm f/2.8 HSM DG OS ART produces comparatively moderate image distortions. At 24mm it shows a barrel distortion of just over 2.1% which is usually not overly disturbing. The 35mm is essentially free of distortions. They are increasing again towards the long end of the zoom range with a peak of 1.7% (pincushion style) at 70mm. Pincushion distortions are usually less acceptable but these are still within reasonable limits.

Vignetting

The vignetting analysis didn't reveal any special surprises. The light falloff is worst at 2.5 EV (f-stops) at the 24mm setting - this is clearly visible. Stopping down to f/4 helps but it's more acceptable from f/5.6 onward. Even so, you will still be able to spot traces of vignetting in critical scenes. Vignetting is a lesser issue in the middle range. The light falloff increases again at the far end with a max. of 2 EV (f-stops) at 70mm f/2.8. The vignetting is substantially reduced at f/4 and pretty much gone from f/5.6 here.

MTF (resolution) at 50 megapixels

Well, you can't state that the Sigma 24-70mm f/2.8 HSM DG OS ART didn't entertain us regarding its resolution characteristic. At 50mp, it gave us a wild ride from the deepest depth of our quality scale up to the near maximum. Note: 50mp are still a killer for most lenses.

Let start with the positives - the broader center zone is very good at f/2.8 and excellent at medium aperture settings. That's throughout the zoom range and quite an achievement in this lens class.

That being said ... this trend didn't hold in the outer image region. At 24mm f/2.8, the worst spot of the lens, there isn't much in it at the borders/corners. The quality is very soft to be polite. On the positive side, the quality is pretty good between f/5.6 and f/8 here. The border/corner issue is slightly improved at 35mm and, more so, at 50mm which is the sweet spot in the zoom range. The corners are still not great here at large apertures but the results are very sharp from f/5.6 onward till diffraction impacts the quality beyond f/11. Looking at the far end of the range ... well ... the borders/corners are showing their misery again and they don't really get much better when stopping down - which is disappointing.

The centering quality of the tested sample was good. The field curvature is minimal.

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 MTF numbers 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 cameras with a more moderate pixel count so let's have a look how the lens performs with the eased requirements at 21 megapixels - also in order to be able to compare the results to older reviews.

As usual, the results are "better" on pixel level because of the lower pixel density that just can't grasp that much detail. This doesn't mean that all is good though. The extreme corner weakness at 24mm f/2.8 remains albeit the borders are at least acceptable here. Stopping down to f/4 helps, again, and the quality is really nice from f/5.6 to f/11. The 35mm and 50mm settings show no obvious weaknesses with just minor drops at f/2.8. However, once again things aren't quite as good at 70mm. The border quality is good here whereas the corners continue to struggle on rather low levels.

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) ... Borders

Lateral chromatic aberrations (color shadows at the image borders) are fairly high for such a high-priced lens. The average CA pixel width vary around 2px at 24mm and 70mm. This will be visible unless you deal with CAs during post-processing. They aren't quite as pronounced in the middle zoom range.

Chromatic Aberrations (CAs) ... Extreme Corners

Normally we don't really bother you with the CAs in the image corners because even more charts don't produce more clarity about what's really important. However, as an exception to the rule, we'd like to show you the corner CAs this time ... because they are extreme. As you can see the CA pixel width exceeds 4px at 24mm and it's not much better at 70mm. This is way too much for a modern lens.

Bokeh

The bokeh is the aesthetic quality of the blur produced in the out-of-focus areas of an image. Just like other lenses in this class, the Sigma 24-70mm f/2.8 DG HSM OS ART is certainly not as capable in producing a very shallow depth of field as fast prime lenses in its range but it is certainly usable for portraits etc.

One of the typical short-comings of standard zoom lenses is a somewhat busy rendering of out-of-focus highlights. That also applies to the Sigma although relative to its direct competitors it's actually quite decent. There's also a bit of outlining as you can see below but it is, again, better than average here. The circular highlight shape is well maintained till f/5.6. As usual, the shape deteriorates towards the borders/corners (cat eyes).

When looking at the general quality of the blur in the focus transition zone, it is actually quite smooth both in the more critical image background whereas the foreground blur is rough ("Nisen-Bokeh").