Lens review

Viltrox PFU RBMH 85 mm f/1.8 STM

14 October 2019
Arkadiusz Olech

4. Image resolution

The resolution test (resoltuion meaning the MTF50 function value) of the Viltrox PFU RBMH 85 mm f/1.8 STM was based on RAW files from the Sony A7R II camera. In the case of that mirrorless device the decency level we set about 39-41 lpmm and the best fixed-focal lenses could exceed 70 lpmm. So far, the Samyang AF 85 mm f/1.4 FE (76.5 lpmm) and the Voigtlander Apo-Lanthar 65 mm f/2 Aspherical 1:2 Macro (78.5 lpmm) have been the record holders.

Let’s check how the tested Viltrox PFU RBMH 85 mm f/1.8 STM compares – its performance in the frame centre, on the edge of the APS-C sensor and on the edge of full frame presents a graph below.

Viltrox PFU RBMH 85 mm f/1.8 STM - Image resolution

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Near the maximum relative aperture the Viltrox fares very well, with MTFs reaching 53-54 lpmm, noticeably higher than, for instance, 46-48 lpmm achieved by the Sony 1.8/85 FE. Still the Sony reacted better on stopping down so by f/2.8 it was able to outdistance the Viltrox by 2.5 lpmm and by f/4.0 - by less than 1 lpmm. The differences in maximum values both lenses are able to reach amounted to just 1-2 lpmm so, by and large, they can be positioned on the margin of error's borderline.

A significant physical dimension of Viltrox and its complex optics suggested that its performance on the edge of the frame might be quite impressive. On the edges of the APS-C sensor everything looks very sensible indeed - images are useful already up from the maximum relative aperture and the resolution amounts to over 55 lpmm at the most. It is higher than 51 lpmm the Sony 1.8/85 FE could reach at that place.

What's interesting, the situation changes diametrically on the edge of full frame. In the large area next to the maximum relative aperture images provided by the Viltrox are not especially useful; they become so only from f/4.0. This result is distinctly worse than that of the Sony.

To sum up, we can observe two different approaches here. The Viltrox optics specialists, contrary to their Sony counterparts, focused more on better correction of wide areas of the frame centre and a bit less attention they gave to the edges. As a result you deal with two lenses with similar optics quality but with a different approach – the selection is wide.

At the end of this chapter, traditionally, we present crops taken from photos of our resolution testing chart saved as JPEG files alongside RAW files we used in our analysis above.

A7R II, JPEG, f/1.8
Viltrox PFU RBMH 85 mm f/1.8 STM - Image resolution
A7R II, JPEG, f/4.0
Viltrox PFU RBMH 85 mm f/1.8 STM - Image resolution