Sony FE 20 mm f/1.8 G
7. Coma, astigmatism and bokeh
The image of a diode gets slightly enlarged in the corners of the APS-C sensor so the colour is right in that place but still you don't notice any serious comatic deformations.
The situation changes in the corner of full frame – you notice not only coma (turning the diode into a comma) but also aberrations of higher degree that are responsible for characteristic 'wings'. These 'wings' disappear almost completely on stopping down the aperture by 1 EV but elongation connected to coma remains visible.
Please Support UsIf you enjoy our reviews and articles, and you want us to continue our work please, support our website by donating through PayPal. The funds are going to be used for paying our editorial team, renting servers, and equipping our testing studio; only that way we will be able to continue providing you interesting content for free. |
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Center, f/1.8 | Corner APS-C, f/1.8 | Corner FF, f/1.8 |
|
|
|
| Center, f/2.5 | Corner APS-C, f/2.5 | Corner FF, f/2.5 |
|
|
|
With photos of starry sky you can also check the situation with rays of light falling from infinity. There are no special differences here – once again in the corner of full frame by f/1.8 coma and aberrations of higher degrees make themselves felt but on stopping down the aperture to f/2.5 only coma remains.
| A7R III, bottom left corner f/1.8 |
 |
| A7R III, bottom left corner f/2.5 |
 |
Are the results of the Sony weak? Not at all – the bar is set very high here. The Nikkor and the Viltrox dealt much weaker in this category; the Tokina could compete with the Sony on equal terms but its task was easier, as it is a bit slower lens.
Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to 5.3%, being on the borderline between very low and low levels. We shouldn't complain here at all, especially that both the Viltrox and the Nikkor fared here notably weaker. Once again only the slower Tokina can compete with the Sony in this area.
Defocused circles of ligth look very well for an ultra wide-angle lens, with even, smooth spread of light and barely visible onion-ring bokeh stemming from aspherical elements. What's more, mechanical vignetting is not especially pronounced either. A round of applause!
| Center, f/1.8 | Corner APS-C, f/1.8 | Corner FF, f/1.8 |
|
|
|
| Center, f/2.5 | Corner APS-C, f/2.5 | Corner FF, f/2.5 |
|
|
|
Center, f/3.5 | Corner APS-C, f/3.5 | Corner FF, f/3.5 |
|
|
|
