Nikon Nikkor Z 28-75 mm f/2.8 review - Coma, astigmatism and bokeh

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Nikon Nikkor Z 28-75 mm f/2.8

22 November 2023

Maciej Latałło

7. Coma, astigmatism and bokeh

An analysis of coma correction provides you another tip why the Nikkor 28-75 mm fares so weakly when it comes to resolution on the edge of the frame. This aberration is slightly bothersome already on the edge of the APS-C/DX sensor, where the point-like images of a diode start showing some deformations. It is a huge surprise and not a positive one, especially if you take into account the physical dimensions of the tested Nikkor. In the corners of full frame, no matter what focal length you employ, coma remains significant. Fortunately you can't call it very high or even huge but we are sure it seriously contributes towards deterioration of image quality on the edge of the frame.

Center, 28 mm, f/2.8	Corner APS-C, 28 mm, f/2.8	Corner FF, 28 mm, f/2.8

Center, 45 mm, f/2.8	Corner APS-C, 45 mm, f/2.8	Corner FF, 45 mm, f/2.8

Center, 75 mm, f/2.8	Corner APS-C, 75 mm, f/2.8	Corner FF, 75 mm, f/2.8

Astigmatism, understood as an average difference between vertical and horizontal MTF50 function values, amounted to 7.6% and it is a moderate value. The maximum focal length makes biggest contribution to that value as in that place the astigmatism level reaches a medium level of 11.9%. At shorter focal lengths the results slightly exceeded 5%.

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The optical construction of the Nikkor Z 28-75 mm f/2.8 features as many as three aspherical elements so we expected problems with onion-ring bokeh. As it happens, it is perfectly visible. Fortunately, it is also the only flaw of defocused circles of light; generally out-of-focus areas, provided by the tested lens, look quite sensible. It is interesting, though, that, despite big physical dimensions of this instrument, mechanical vignetting by f/2.8 is quite significant. If you put a circle in the very corner of the frame it becomes really significantly truncated. Fortunately, this vignetting decreases very quickly on stopping down the aperture; by f/5.6 you don't notice it practically at all.

Center, 75 mm, f/2.8	Corner APS-C, 75 mm, f/2.8	Corner FF, 75 mm, f/2.8

Center, 75 mm, f/4.0	Corner APS-C, 75 mm, f/4.0	Corner FF, 75 mm, f/4.0

Center, 75 mm, f/5.6	Corner APS-C, 75 mm, f/5.6	Corner FF, 75 mm, f/5.6