Sigma A 28-45 mm f/1.8 DG DN
7. Coma, astigmatism and bokeh
Still the problems are visible only in the corner of full frame and they decrease noticeably on stopping down the aperture by 1 EV. The performance in this category can be described as medium but rather closer to the upper ranges of this level.
| Center, 28 mm, f/1.8 | Corner APS-C, 28 mm, f/1.8 | Corner FF, 28 mm, f/1.8 |
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| Center, 35 mm, f/1.8 | Corner APS-C, 35 mm, f/1.8 | Corner FF, 35 mm, f/1.8 |
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| Center, 45 mm, f/1.8 | Corner APS-C, 45 mm, f/1.8 | Corner FF, 45 mm, f/1.8 |
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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 upper left-hand corner of the frame. The result is in accordance with the photos of the diode – the coma decreases on stopping down the aperture by 1 EV and its level becomes distinctly lower (but not zero).
| A7R IIIa, 28 mm, f/1.8,lewy-górny Corner | A7R IIIa, 28 mm, f/2.5, lewy-górny Corner |
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| A7R IIIa, 45 mm, f/1.8, lewy-górny Corner | A7R IIIa, 45 mm, f/2.5,lewy-górny Corner |
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Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to just 7.8%. It is a low level that shouldn't worry you at all. Still we noticed an upward trend of astigmatism as the focal length increases.
The appearance of out-of-focus areas is very sensible for a zoom lens. You can't notice any worrying symptoms but, if you take a closer look, you manage to perceive traces of onion-ring bokeh caused by aspherical elements inside the lens. You can also have some reservations when it comes to the level of mechanical vignetting as it remains bothersome even on stopping down the aperture by 2 EV. It influences the overall vignetting level in a distinct way, especially at the maximum focal length, and we are going to deal with it in the next chapter.
| Center, 45 mm, f/1.8 | Corner APS-C, 45 mm, f/1.8 | Corner FF, 45 mm, f/1.8 |
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| Center, 45 mm, f/2.5 | Corner APS-C, 45 mm, f/2.5 | Corner FF, 45 mm, f/2.5 |
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Center, 45 mm, f/3.5 | Corner APS-C, 45 mm, f/3.5 | Corner FF, 45 mm, f/3.5 |
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