Sigma AF 30mm f/1.4 EX DC (Pentax K) Review / Test Report - Analysis
Lens Reviews - Pentax

Distortion

The Sigma exhibits a moderate amount of barrel distortion (1.7%) which isn't not overly impressive for a fix-focal length but it's not an unusual level either.

The chart above has a real-world size of about 120x80cm.

Vignetting

The Sigma shows a fairly unusual vignetting characteristic (which also showed up in the test of the Canon variant of the lens) - normally the vignetting curve is logarithmic (steep drop in the beginning followed by moderate fading) whereas the curve of the Sigma shows a quite linear behavior. This isn't really favorable because it takes a couple of f-stops till the vignetting reaches really low values. At wide-open aperture the issue is quite pronounced (1EV) and even at f/2.8 it is not much better. The vignetting is no longer field-relevant from about f/5.6. Vignetting is not disturbing in all situations but other lenses perform better here.

MTF (resolution)

The Sigma shows a rather unusual resolution characteristic - it has a very pronounced priority on center performance but it doesn't really care about the border and especially the extreme corner quality. The center resolution is already on a very good level at f/1.4 and f/2 and reaches excellent and even absolutely outstanding levels beyond. The borders are only fair at wide-open aperture but in the extreme corners there isn't much resolution left. The borders improve to good levels by f/2 whereas the corner quality remains poor and it doesn't get much better at f/2.8. At f/4 the border/corner quality starts to lift a little but it takes f/8 till the extreme corners reach good levels. This is again in line to what we've seen in the corresponding test of the Canon variant.

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)

The degree of lateral chromatic aberrations (color shadows at harsh contrast transitions) is acceptable for a 30mm lens but nothing to rave about either. CAs with an average width around ~1.2px at the image borders can be visible at times.

You may be able to spot some Longitudinal Chromatic Aberrations (LoCA) at large apertures. These are greenish to reddish out-of-focus halos near the in and out-of-focus transitions. This is normal for an ultra-large aperture lens. See the sample image of the wooden barrels below.


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