Sony FE 14 mm f/1.8 GM
7. Coma, astigmatism and bokeh
I don't know what lens was used to take the photos presented during that Sony press conference but I am sure I got a completely different specimen for testing. First symptoms of classic coma are visible already on the edge of the APS-C sensor - by f/1.8 and also by f/2.5, point-like images of the diode change into small commas. In the corners of full frame apart from coma you see wings and it means a higher degree aberration makes itself felt (it can be e.g. a component of sagittal plane diagonal spherical aberration).
As you see, something that was announced as a strong point of the Sony in reality proved to be its slip-up.
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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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Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to as much as 25.7%. It is, unfortunately, a high value. Combination of that aberration and significant depth of field, provided by this lens, produces very interesting effects which we are going to mention also in the autofocus chapter. Here we would like to add only that astigmatism, as every off-axis aberration, increases as you go further from the frame centre and really interesting things might happen on its very edge. Even if you set the focus in the centre in such a way so you can minimize differences between horizontal and vertical MTF50 function values, these differences might reach as much as 100% on the very edge. You might observe more interesting performance when you set the focus in such a way that image quality in the centre (as a median of horizontal and vertical values) is at its highest; in such a situation huge differences stemming from the presence of astigmatism can be observed. Even if the overall image quality and sharpness in the frame centre is very good, its horizontal and vertical components on the very edge (depending on the setting of the autofocus) can feature significant blur.
Perhaps in the case of a 14 mm lens out-of-focus areas are not exactly a feature you pay the most attention to but still, they are worth a closer look because, once again, you can notice several interesting things. First, circles of light in the corners are deformed mainly because of mapping of such a wide angle of view on the flat surface of the detector but not because of mechanical vignetting. Second, aspherical elements, used in the optical construction of the lens, make themselves felt as onion-ring bokeh. Third, you can notice a brighter rim in circles we got in the frame centre without any problems. All these effects were present also in the test of the Sigma A 1.8/14 but to a lower degree; in this category the rival lens undoubtedly prevails.
| 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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