Sigma A 28-105 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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| Center, 105 mm, f/2.8 | Corner APS-C, 105 mm, f/2.8 | Corner FF, 105 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. Stars proved to be a bit more demanding because deformations, observed in these photos, seem to be a tad more pronounced than in the case of the diode. The influence of the coma itself is clearly visible here as it turns images of stars into commas, along with an influence of aberrations of higher degree that are responsible for the length and width of 'wings'.
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| A7R IIIa, 28 mm, f/2.8,top right corner | A7R IIIa, 28 mm, f/4.0, top right corner |
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| A7R IIIa, 105 mm, f/2.8, top left corner | A7R IIIa, 105 mm, f/4.0, top left corner |
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Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to just 5.3%. It is a value on the borderline between very low and low levels so there is no reason to worry. It's also worth noticing the fact that we didn't notice any meaningful differences between particular focal lengths.
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.
Also mechanical vignetting is nothing you can have serious reservations about – it disappears completely on stopping down the aperture by 2 EV.
| Center, 105 mm, f/2.8 | Corner APS-C, 105 mm, f/2.8 | Corner FF, 105 mm, f/2.8 |
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| Center, 105 mm, f/4.0 | Corner APS-C, 105 mm, f/4.0 | Corner FF, 105 mm, f/4.0 |
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Center, 105 mm, f/5.6 | Corner APS-C, 105 mm, f/5.6 | Corner FF, 105 mm, f/5.6 |
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