Sigma A 50 mm f/1.4 DG HSM review - Coma, astigmatism and bokeh

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Sigma A 50 mm f/1.4 DG HSM

17 April 2014

Szymon Starczewski

7. Coma, astigmatism and bokeh

Many 50 mm f/1.4 devices experience a lot of problems with coma correction as it is not an easy task. The Sigma is doing great but only on the APS-C/DX sensor. In the corner of full frame, especially by f/1.4, that aberration is distinct, a bit higher than in the case of the Zeiss Otus. Fortunately on stopping down the lens to f/2.0 you see that problem disappear.

Sigma A 50 mm f/1.4 DG HSM - Coma, astigmatism and bokeh

When it comes to the astigmatism the average difference between horizontal and vertical MTF50 function values for three biggest relative apertures amounted to 5.4% which is a good result. To be honest the f/1.4 aperture contributed the most to that value. On stopping down to f/2.0 or to f/2.8 makes the astigmatism level decrease to 1-3% so it becomes imperceptible. Here the Sigma sticks out in a positive way even compared to the Zeiss Otus as that lens had a measurable level of astigmatism up to f/2.8 inclusive.

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I don’t have any reservations concerning defocused light points. First of all the light spread is very even, without even a trace of onion rings bokeh, so pronounced in the case of the Sigma 1.4/35. The comparison with the Zeiss Otus shows how good the situation really is. The light spread is nice for both lenses. Circles of light generated by the Otus in the frame corner were distinctly truncated not only by f/1.4 but also by f/2/0; even by f/2.8 the truncation remained noticeable. In the case of the Sigma the circle you get in the corner of full frame is more or less round already by f/2.0 which means the optical path is connected properly to the casing and there are no serious vignetting issues. A round of applause!