Sigma 45mm f/2.8 DG DN Contemporary - Review / Test Report - Analysis
Lens Reviews - Sony Alpha (Full Format)

Distortion

Standard prime lenses tend to show minimal distortions if any. However, in its native design, the Sigma 45mm f/2.8 DG DN Contemporary produces a medium pincushion distortion (~1.7%) thus more than usual in this class. With activated image auto-correction this is, of course, a lesser concern so most users don't need to bother with this aspect.

Vignetting

Image auto-correction also comes handy with respect to vignetting. The RAW images reveal a light fall-off of ~2.4EV (f-stops) at f/2.8 which rather high for such a lens. You have to stop down to f/5.6, better f/8 to reduce the issue to a reasonable degree. Image auto-correction reduces the light falloff substantially by almost 1 1/2 f-stops at fully-open aperture. This is still visible albeit usually not disturbing anymore. Stopping down improves this further, of course.

MTF (resolution)

The Sigma lens is already pretty sharp across the frame at f/2.8 albeit it doesn't bite yet. The center does bite - hard - at f/4 and f/5.6 and the outer image region is very good indeed at these settings. The center quality suffers from diffraction effects from f/8 onward. However, the resolution is still very high across the image frame at f/11. Image softening is more pronounced at f/16, of course.

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

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)

Once again - on Sony cameras, you don't really have to worry about chromatic aberrations due to image auto-correction. However, even if you prefer to disable this for whatever reason, the CAs are low at around 0.5px on the average at the image borders.

Bokeh

The most controversial aspect of the Sigma 45mm f/2.8 DG DN Contemporary is the comparatively small max. aperture. This is limiting its shallow depth-of-field potential. However, at closer focus distances, you can still produce a decent amount of out-fo-focus blur.

The quality of the bokeh (=the blur in the focus transition zone) is good in the background (shown to the left below). It's not perfectly symmetrical but pretty smooth. The foreground blur (to the right) is somewhat "harder" without being nervous though.

Out-of-focus highlights are rendered nicely. The inner zone of the discs isn't perfectly even (probably due to aspherical elements) but the typical "onion rings" aren't too pronounced. The circular aperture shape remains intact and edgy traces do only show up from f/5.6.

The circular shape of the highlights deteriorates the more you move to the corners but it's not that pronounced on the Sigma lens. Namely, there are no "cat eyes". Stopping down improves the highlights in the broader center zone but they are not ideal in the corners at f/4. At f/5.6 the highlight discs are pretty good across the image frame.