Sony FE 16 mm f/1.8 G
7. Coma, astigmatism and bokeh
Images of a diode gets slightly bigger in the corners of the APS-C sensor so the colour of the diode returns to its original shade without any distinct comatic deformations.
The situation changes slightly in the corner of full frame. Here you can notice not only coma (that turns the image of a diode into a comma), but also aberrations of higher degree that contribute to these characteristic wings. The wings decrease significantly on stopping down the aperture by 1 EV but the elongation, connected to the coma, remains visible. Still these effects aren't especially strong and the level of this aberration can be assessed as on the borderline between low and medium values.
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| Center, f/1.8 | Corner APS-C, f/1.8 | Corner FF, f/1.8 |
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| Center, f/2.5 | Corner APS-C, f/2.5 | Corner FF, f/2.5 |
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In order to show all aspects of the performance in this category we decided to show also crops taken from photos of the sky, and more precisely from the left-hand upper corner of full frame. This time these are shots by f/1.8, f/2.5, and f/3.5. In this case coma is similar to the results of our diode test. The fact that these characteristic wings disappear only on stopping down the aperture to f/3.5 is clearly visible here. Even then the star still looks like a short line.
| Corner FF, f/1.8 | Corner FF, f/2.5 | Corner FF, f/3.5 |
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Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to just 2.0%. It's a negligible value so we don't have any reasons to complain. It's worth noticing that the low level of astigmatism concerns also the maximum relative aperture. Once again the Sony optics specialists did a good job.
For an ultra wide angle lens defocused circles of light aren't very bad but of course the situation could have been better. The light spread is quite even inside but if you take a closer look you can notice onion ring bokeh stemming from aspherical elements inside the construction. Fortunately mechanical vignetting is not especially pronounced here.
| Center, f/1.8 | Corner APS-C, f/1.8 | Corner FF, f/1.8 |
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| Center, f/2.5 | Corner APS-C, f/2.5 | Corner FF, f/2.5 |
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| Center, f/3.5 | Corner APS-C, f/3.5 | Corner FF, f/3.5 |
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