LensTip.com

Lens review

Viltrox PFU RBMH 20 mm f/1.8 ASPH

20 May 2019
Arkadiusz Olech

8. Vignetting

First let's check the vignetting performance on the smaller APS-C sensor – appropriate thumbnails are shown below.

A7R II, APS-C, f/1.8 A7R II, APS-C, f/2.0
Viltrox PFU RBMH 20 mm f/1.8 ASPH - Vignetting Viltrox PFU RBMH 20 mm f/1.8 ASPH - Vignetting


No reasons to worry – at the maximum relative aperture the vignetting reaches a moderate level of 20% (−0.64 EV) and it decreases to 18% (−0.58 EV) on stopping down to f/2.0. By f/2.8 and higher apertures the brightness loss stays around 17% (−0.54 EV).


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Now let's see how the situation changes after passing to full frame.

A7R II, FF, f/1.8 A7R II, FF, f/2.0
Viltrox PFU RBMH 20 mm f/1.8 ASPH - Vignetting Viltrox PFU RBMH 20 mm f/1.8 ASPH - Vignetting
A7R II, FF, f/2.8 A7R II, FF, f/4.0
Viltrox PFU RBMH 20 mm f/1.8 ASPH - Vignetting Viltrox PFU RBMH 20 mm f/1.8 ASPH - Vignetting


At the maximum relative aperture the light fall-off in the frame corners amounts to as much as 56% (−2.38 EV) and it decreases to 53% (−2.21 EV) on stopping down the aperture to f/2.0. It is a better result than that of the Tokina Firin which had as much as 61% of brightness loss at its maximum relative aperture.

What's interesting, vignetting reacts very slowly to stopping down. By f/2.8 we got 45% (−1.74 EV), and by f/4.0 its level decreased slightly to 43% (−1.61 EV). Further stopping down didn't have any measureable influence on vignetting; it means a high result of over 40% is maintained even by f/8 and f/16.

It is a very curious situation. In the previous chapter we noticed lack of truncation of light circles; here we deal with very sluggish vignetting decrease even after significant stopping down of the aperture. The conclusion can be only one: glass and coating of this lens influence vignetting the most. Perhaps the producer used such anit-reflective coatings which provide weak transmission for light falling at obtuse angles. You cannot also exclude a pronounced influence connected to the lack of telecentricity and very big angles at which rays of light fall on microelements of the sensor.

Sony A7R II, JPEG, f/1.8
Viltrox PFU RBMH 20 mm f/1.8 ASPH - Vignetting