Sigma C 28-70 mm f/2.8 DG DN
7. Coma, astigmatism and bokeh
| Center, 28 mm, f/2.8 | Corner APS-C, 28 mm, f/2.8 | Corner FF, 28 mm, f/2.8 |
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| Center, 50 mm, f/2.8 | Corner APS-C, 50 mm, f/2.8 | Corner FF, 50 mm, f/2.8 |
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| Center, 70 mm, f/2.8 | Corner APS-C, 70 mm, f/2.8 | Corner FF, 70 mm, f/2.8 |
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In order to make this test more interesting we would like to show an image of a bright star put in the corner of the frame. The photos in this case only confirm all conclusions, drawn above.
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| A7R IIIa, 28 mm, f/2.8 top left Corner | A7R IIIa, 28 mm, f/4.0, top left Corner |
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| A7R IIIa, 70 mm, f/2.8, top right Corner | A7R IIIa, 70 mm, f/4.0, top right Corner |
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Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to 7.7%. It is a value we consider to be low so there is no reason to worry. It's also worth adding that the lowest astigmatism level we measured at the shortest focal length.
The look of out-of-focus images, for a zoom, is very sensible indeed. There are no features that might make you worry but if you take a closer look you can manage to notice traces of onion-ring bokeh stemming from the presence of aspherical elements. Mechanical vignetting is also nicely reduced and it disappears completely on stopping down the aperture by 2 EV.
| Center, 70 mm, f/2.8 | Corner APS-C, 70 mm, f/2.8 | Corner FF, 70 mm, f/2.8 |
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| Center, 70 mm, f/4.0 | Corner APS-C, 70 mm, f/4.0 | Corner FF, 70 mm, f/4.0 |
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| Center, 70 mm, f/5.6 | Corner APS-C, 70 mm, f/5.6 | Corner FF, 70 mm, f/5.6 |
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