Sigma A 40 mm f/1.4 DG HSM review - Coma, astigmatism and bokeh

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Sigma A 40 mm f/1.4 DG HSM

10 December 2018

Arkadiusz Olech

7. Coma, astigmatism and bokeh

The Sigma A 1.4/40 doesn’t have almost any coma-related problems. Deformations of point-like diode images in the very corner of the frame are slight. In this category the new Sigma model prevails over its 1.4/35 and 1.4/50 cousins which had more issues concerning comatic aberration and deformations of point-like images in the frame corners. The fans of night sky photography will be enthused about the images provided by the Sigma A 40 mm f/1.4 DG HSM – not only the edge of the frame is almost perfectly sharp already by f/1.4 but also stars captured in that part of the frame are going to be as point-like as you like.

Center, f/1.4	Corner APS-C, f/1.4	Corner FF, f/1.4

Center, f/2.0	Corner APS-C, f/2.0	Corner FF, f/2.0

When it comes to astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, we have no reservations whatsoever. The value measured by us amounted to 4.5%; it means you might as well forget about this aberration at all.

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A bright, fast lens with the focal length close to journalistic 35 mm or standard 50 mm should provide out-of-focus area pleasing to the eye and here the Sigma’s performance is beyond reproach. Circles of light are very nice and smooth, without any local extreme points. Even the rim which appears on stopping down the lens is not especially intensive. The influence of mechanical vignetting is discernible in the f/1.4-2.0 aperture range but its influence disappears practically completely by f/2.8.

Center, f/1.4	Corner APS-C, f/1.4	Corner FF, f/1.4

Center, f/2.0	Corner APS-C, f/2.0	Corner FF, f/2.0

Center, f/2.8	Corner APS-C, f/2.8	Corner FF, f/2.8