Sigma 100-400mm f/5-6.3 HSM DG OS Contemporary (Canon EOS) - Review - Analysis
Lens Reviews - Canon EOS (Full Format)

Distortion

The Sigma 100-400mm f/5-6.3 HSM DG OS Contemporary produces a varying degree of distortions throughout the zoom range. At 100mm it is basically distortion-free. Pincushion-type distortions increase steadily from 100mm till reaching their peak at 300mm. At 2% they are fairly obvious here. Interestingly the distortions decrease slightly towards the far long end (~1.5%) at 400mm.

Vignetting

The vignetting characteristic is about typical for a lens in this class. At fully-open aperture, the light falloff varies between 1EV (f-stop) and 1.5EV (100mm). While this is visible at times, it is usually not overly disturbing. Stopping down to f/8 resolves most of the issue.

MTF (resolution) at 50 megapixels

Considering its price tag and versatility, the Sigma does a good job in terms of resolution albeit it has, of course, some issues. The broader center quality is high at 100mm but the borders/corners are fairly soft and they don't really recover when stopping down. The sweet spot (or range) is clearly in the middle section where the center remains very good and the outer image region improves substantially. There is a slight drop in quality at 400mm which is primarily affecting the borders/corners again but the quality is pretty good at f/8 actually.

The centering quality of the tested sample was good. There's some field curvature at 100mm.

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

Coming later ...

Chromatic Aberrations (CAs)

Lateral chromatic aberrations (color shadows at the image borders) are very low in the middle to upper range. At 100mm they can get more obvious when stopping down. However, even so we aren't talking about massive CAs here.

Bokeh

The Sigma 100-400mm f/5-6.3 HSM DG OS Contemporary is certainly not aligned to shallow depth-of-field scenes - it is just too slow for that. However, if you stick to moderate focus distances you can still separate your main subject from the surroundings. Thus let's have a quick look at the bokeh characteristic now.

Near-center highlights are nicely circular till f/11. The inner highlight zone is fairly smooth and there's some outlining at the highlight edge - see the lower portion of the sample crops below. The circular highlights deteriorate to the usual "cat eyes" the more you move to the image corners (not shown).

When looking at the general quality of the blur in the focus transition zone, it's very smooth in the image foreground (to the right below). The image background (left) shows some roughly rendered edges at high contrast transitions though.