Lens review

Viltrox PFU RBMH 20 mm f/1.8 ASPH

20 May 2019
Arkadiusz Olech

4. Image resolution

The resolution test (resolution meaning MTF50 function values) of the Viltrox PFU RBMH 20 mm f/1.8 ASPH was based on RAW files from the Sony A7RII. In the case of that mirrorless device the decency level we set about 39-41 lpmm and the best fixed-focal lenses have been able to exceed 70 lpmm. So far, the Tokina Fírin 20 mm f/2 FE MF (72.6 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 20 mm f/1.8 ASPH 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 20 mm f/1.8 ASPH - Image resolution

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The performance in the frame centre should be assessed very positively indeed. Already at the maximum relative aperture MTFs exceed a level of 40 lpmm so images are fully useful. On stopping down the aperture the resolution improves very quickly and its maximum, amounting to 68.3 lpmm, the lens reaches by f/4.0. It is a truly excellent result even if the direct rival of this lens, the Tokina Firin 2/20, fared significantly better – both at the maximum relative aperture and on stopping down.

The performance on the edge of the frame, however, is a noticeable problem for the tested lens. On the edge of the APS-C sensor and in close proximity of the maximum relative aperture the images are not especially useful and in order to get that much you have to close the lens down to near f/2.8. The edge of full frame is even weaker, as the lens has to be stopped down to near f/3.5. The peak of its possibilities the tested lens reaches by f/5.6 where its MTFs are on a level of about 50 lpmm.

While comparing these results to the performance of the Tokina 2/20 we can say that the rival lens prevailed on the edge of the APS-C detector but on the edge of full frame the Viltrox had higher resolution results.

To sum up the Viltrox lost its duel with the rival Tokina but its position is hardly hopeless; it still offers an excellent image quality in the frame centre remaining faster and noticeably cheaper than the Tokina (the Vlitrox now is priced at about $486). At the end of this chapter, traditionally we present crops taken from photos of our resolution testing chart; these were saved in JPEG format along with RAW files used by us for the analysis presented above.

A7R II, JPEG, f/1.8
Viltrox PFU RBMH 20 mm f/1.8 ASPH - Image resolution
A7R II, JPEG, f/4.0
Viltrox PFU RBMH 20 mm f/1.8 ASPH - Image resolution