Sigma A 20 mm f/1.4 DG DN
7. Coma, astigmatism and bokeh
In their premiere release concerning the A 20 mm f/1.4 DG DN model, Sigma informed that they did their best to correct coma properly – after all the new lens was supposed to be designed for, among other uses, landscape astrophotography. In case of such images it is very important that the stars are as point-like as only possible across the frame.
Photos below prove that Sigma specialists gave to this task their full attention and priority. Despite the fact that the bar was raised a bit higher, here coma is corrected much better than in case of its predecessor. In the corners of the APS-C sensor you don't see it practically at all; it appears in the corners of full frame but it would be difficult to call its level even medium.
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| Center, f/1.4 | Corner APS-C, f/1.4 | Corner FF, f/1.4 |
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| Center, f/2.0 | Corner APS-C, f/2.0 | Corner FF, f/2.0 |
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In order to check the performance thoroughly we also went outside and tested the Sigma at night. We are pleased to say we were able to get very similar images to these took in our testing studio, with an artificial diode instead of stars.
| A7R III, top left corner f/1.4 |
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| A7R III, top left corner f/2.0 |
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Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to just 6.5%. It's a small value and not a reason to worry. In this category the new model fared a bit better than its reflex camera predecessor and, within limits of measuring error, exactly the same as the slower Sony.
Out-of-focus areas are not bad for an ultra-wide angle lens. Light spread in the circles is quite even; still, if you take a closer look, you can notice traces of onion-ring bokeh. Such bokeh is characteristic for constructions with aspherical elements inside. We don't like a significant influence of mechanical vignetting that remains visible even on stopping down the aperture by 2 EV. It doesn't bode well the results in the next chapter.
| Center, f/1.4 | Corner APS-C, f/1.4 | Corner FF, f/1.4 |
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| Center, f/2.0 | Corner APS-C, f/2.0 | Corner FF, f/2.0 |
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Center, f/2.8 | Corner APS-C, f/2.8 | Corner FF, f/2.8 |
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