Sigma A 20 mm f/1.4 DG HSM review - Coma, astigmatism and bokeh

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Sigma A 20 mm f/1.4 DG HSM

3 November 2015

Arkadiusz Olech

7. Coma, astigmatism and bokeh

In the case of fast, ultra wide angle lenses the coma correction and the correction of other aberrations distorting point-like sources of light in the frame corners remains invariably difficult. It pains us to say the Sigma didn’t manage to fare well here. The slight influence of the coma is distinct even on the small APS-C/DX sensor. In the corners of full frame the situation deteriorates and becomes very serious with that aberration being very bothersome both by f/1.4 and by f/2.0. In the case of the Nikkor AF-S 20 mm f/1.8G ED we dealt with a similar performance – it had a noticeable coma even by f/2.5.

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

Immediately after publishing sample shots I got a lot of requests, both form the Polish Optyczne.pl Readers and from the English-speaking Readers of LensTip.com, to take photos of the starry night with the tested Sigma. As I am an astrophysicist by education and by avocation I don’t have to be told twice, especially if I get at my disposal a lens which would be perfect to commemorate the vast chunks of the Milky Way, to hunt for meteors or the Northern Lights. Still do take into account the fact that I live almost in the centre of Warsaw and the place from which a sensible photos of dark sky can be taken is in a distance of about 30 kilometers. Sometimes showing ordinary photos is a priority and an astrophoto trip must wait for good weather, more free time or both.

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Still the end of October and the beginning of November this year was rich with cloudless nights and a great number of interesting astronomical phenomenons (e.g. the flyby of the 2015TB145 asteroid, the maximum of the Taurids) which inclined me to position the EOS 5D MkIII with the Sigma 1.4/20 attached to it on an equatorial mount with a motorized tracking and take several shots, three of which are in the sample shots section of this test. Below there are 1:1 crops taken from a frame corners with the Lyra constellation. The influence of the coma is clear by f/1.4 and by f/2.0; it is less distinct by f/2.8. The correction of that off-axis aberration by a lens with such a wide angle of view proved to be a really difficult task.

5D MkIII, f/1.4, corner 1:1

Sigma A 20 mm f/1.4 DG HSM - Coma, astigmatism and bokeh

5D Mk III, f/2.0, corner 1:1

5D Mk III, f/2.8, corner 1:1

Apart from the coma also the astigmatism might contribute toward the worsening of the image quality on the edge of the frame. That aberration’s level is not very high but the average difference between vertical and horizontal MTF50 function values amounted to 10% which allows us to describe it as medium.

It would be difficult to have any reservations when it comes to the appearance of circles created by defocusing light points. Their light spread is quite even, without any local extremes. Still the deformations connected to the mechanical vignetting stick out quite a bit.

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