Samyang AF 85 mm f/1.8 P FE
6. Distortion and field of view
Field of view
A rectilinear 85 mm lens on the full frame sensor should provide an angle of view amounting to 28.56 deg. In the official specification the producer states almost exactly the same value, that of 28.6 deg. We decided to measure the field of view on our own. In order to do so we took photos of a starry sky and saved them first as uncorrected JPEG files. Then we transformed the pixel layout (X,Y) from the photo into the equatorial coordinate system (right ascension and declination), which locates a star on a celestial sphere. That way we were able to measure the field very precisely, and exactly as it should be done, for rays of light coming from infinity. The transformation was based on the positions of 116 stars spread evenly across the frame. An average mesh-fitting error amounted to 18 seconds of arc.Finally our result amounted to 28.40 degrees with a measuring error of 0.03 of a degree, a tad more than in official specifications. Still the difference is merely cosmetic, reaching just 0.5% so it fits tolerance limits.
Distortion
In case of a telephoto 85 mm lens we don't expect any problems with geometric deformations. When it comes to the APS-C/DX sensor indeed there are none, with a result of +0.97%, not providing you any reasons to complain.
Still the situation changes after passing to full frame where pincushion distortion increases to +1.19% so to a level that you can notice in photos. It's worth reminding here that the expensive Nikkor Z 85 mm f/1.8 S took proper care to correct this aberration, reducing it to +0.47%. An even slightly lower result achieved the Sony FE 85 mm f/1.8, just +0.38%. The Panasonic S 85 mm f/1.8 fared a tad weaker, with a value of +1.14%. The worst situation was observed in case of the Viltrox PFU RBMH 85 mm f/1.8 STM because its pincushion reached +1.89%. Now this disgraceful record belongs to the Samyang – as you see, miniaturization doesn't come without a price.
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Still it's worth remembering that the results lower than 2% in absolute terms we still consider as moderate so they shouldn't worry you too much.
| Sony A7R IIIa, 85 mm, APS-C | |||
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| Sony A7R IIIa, 85 mm, FF | |||
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