Sigma 56mm f/1.4 DC DN Contemporary (Sony E-mount) - Review / Test Report - Analysis
Lens Reviews - Sony Alpha/NEX (APS-C)

Distortion

Unfortunately, the technical analysis has to start with a weakness of the lens - image distortion. The original characteristic is hardly impressive with a pincushion distortion in excess of ~2.8%. This is rather extreme for such a prime lens.
Of course, in real life, this may be a lesser issue if you keep image auto-correction activated. Surprisingly it just eases the issue but doesn't eliminate it completely. However, at just 0.6% it's not really something to worry about.

Vignetting

A similar pattern is present with respect to vignetting. RAW images show a heavy light-falloff (for an APS-C lens) at f/1.4. A vignetting of ~1.7EV (f-stops) is easily visible. Stopping down to f/2 helps but the issue is only mostly dissolved from f/4 onward.

Once again, image auto-correction comes to the rescue. It reduces the vignetting by about 1EV at f/1.4. While you may still spot the resulting ~0.7EV drop in critical scenes, it's usually not a dealbreaker. The light falloff is basically gone from f/2.8 in this case.

MTF (resolution)

The Sigma 56mm f/1.4 DC DN Contemporary has a superb resolution characteristic. It is already very sharp at f/1.4 in the image center and just a tad less so in the outer image field. The center quality is superb between f/2 and f/5.6 and the borders/corners are also highly impressive. Diffraction effects are only visible beyond f/8.

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

Please note that the MTF results are not directly comparable across the different systems!

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 CAs (color shadows at contrast transitions) are very low peaking at an average CA pixel width of less that 0.4px at f/1.4. Beyond the CAs are virtually non-existent.

Bokeh

"Bokeh" is a Japanese word referring to the way, out of focus blur is rendered. There are two main aspects - out-of-focus (background) highlights and the rendering of contrasty objects in the focus transition zones. If you buy a lens such as the 56mm f/1.4 DC DN, you will usually target shallow depth-of-field applications thus the bokeh isn't a secondary criterion here.

Let's have a look at the "highlights" first - and they are almost perfect in the image center at least. The inner zone of the discs is very clean and there is no outlining to speak of. The discs are circular at f/1.4 and (mostly) f/2. The more edgy aperture shape starts to emerge at f/2.8.

When looking at the whole image frame, the situation is a little different though. As you can see below, a fully circular highlight shape is only maintained near the image center. The shape deteriorates towards "cat eyes" from the midfield into the corners. This is a mechanical vignetting effect from the lens barrel but it sets in a little "early" here. Stopping down broadens the "good" zone in the center but cuts the corner highlights. A circular shape in the corners is mostly restored at f/4 (not shown).

The general rendition of out-of-focus areas is silky smooth in the critical image background (shown to the left below). The foreground blur (to the right) is a tad more edgy but still very good.

Bokeh Fringing (LoCA)

Bokeh fringing - also referred to as LoCAs - is a color fringing effect on the Z-axis. It shows up as purplish halos in front of the in-focus zone and greenish beyond. The effect is visible at f/1.4 and f/2 and starts fading at f/2.8 - this is a typical behavior for a non-APO lens.