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Lens review

Viltrox PFU RBMH 20 mm f/1.8 ASPH

20 May 2019
Maciej Latałło

6. Distortion and field of view

Field of view

When a new player appears on the market it's always worth checking whether parameters of their lenses, stated officially in the specifications, are in accordance with real life measurements. In an ultra wide angle lens the field of view is very important – one degree less than declared and optics specialists have it a bit easier.

We decided to check the situation in practice on JPEG files (uncorrected, so reflecting well the real parameters of the device). We took a photo of the starry sky and we transformed the pixel layout (X,Y) into the equatorial coordinate system (right ascension and declination) which locates a star on a celestial sphere. That way we could determine the field of view of the lens very precisely. In the case of JPEG files it amounted to 94.57 degrees with the measurement error never exceeding a value of 0.05 of a degree. The field proved to be exactly in accordance with official declaration of the producer – a round of applause.

Distortion

As long as you work with the smaller APS-C sensor you are able to notice moderate barrel distortion which value we assessed as −1.40%. What's interesting, after passing to full frame the distortion actually decreases to −1.06%. If you look at photos below the reason becomes obvious: on full frame there is distinct moustache distortion which bends lines the other way so the results averaged out across the frame decrease when compared to the performance on the APS-C sensor.

Traditionally we might add that after removing the areas where the lines are bent the other way and limiting your measurements to the part of the frame which is situated within 1:1 markings the distortion increases to −2.15%

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Still it is very important to remember that in this category the Viltrox fares noticeably better than the Tokina which also had moustache distortion; its distortion level amounted to as much as −3.1% in areas devoid of moustache deformations.

Sony A7R II, APS-C
Viltrox PFU RBMH 20 mm f/1.8 ASPH - Distortion and field of view
Sony A7R II, FF
Viltrox PFU RBMH 20 mm f/1.8 ASPH - Distortion and field of view