Sigma A 14 mm f/1.8 DG HSM review - Coma, astigmatism and bokeh

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Sigma A 14 mm f/1.8 DG HSM

14 July 2017

Arkadiusz Olech

7. Coma, astigmatism and bokeh

If you stick to the APS-C/DX sensor you shouldn’t have any problems connected wirh coma or other aberrations causing deformations of point-like sources of light. The situation changes after passing to the corner of full frame where the deformations are already quite distinct even on stopping down the aperture by 1 EV. You can also mention the fact that another aberration of higher degree (a component of sagittal oblique spherical aberration) makes itself felt here more than coma, which deformations are radial.

The result of the Sigma is similar to the performance of the Irix and a bit weaker than that of the Samyang 2.8/14 at the same time. The Canon 2.8/14 II fared the weakest in this category.

Center, f/1.8	Corner APS-C, f/1.8	Corner FF, f/1.8

Center, f/2.5	Corner APS-C, f/2.5	Corner FF, f/2.5

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Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to 3.8%. That level is low so the tested lens should be praised here. The Irix had a very similar result and the Samyang 2.8/14 fared even better. In this company the Canon 2.8/14 II with its 17% looks like a beggar even though it is the most expensive lens in this group.

When it comes to out-of-focus images we have almost no reservations whatsoever. The light spread in the circles is very even, without any rims or local extremes. Even the ring on the very edge is not especially pronounced. Still if you take a closer look at the interior of the circles you’ll be able to notice traces of onion ring bokeh.

Center, f/1.8	Corner APS-C, f/1.8	Corner FF, f/1.8

Center, f/2.5	Corner APS-C, f/2.5	Corner FF, f/2.5

Center, f/3.5	Corner APS-C, f/3.5	Corner FF, f/3.5