Samyang AF 50 mm f/1.4 FE II
6. Distortion and field of view
Field of view
A classic rectilinear 50 mm lens on full frame should provide an angle of view of 46.8 deg. The Samyang's official specifications state a slightly lower value, that of 45.7 deg. It might suggest that the real focal length is a bit higher.We decided to check how much it really is. In order to do that we took some photos of starry sky and save them 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 could determine the field of view of the lens with utter precision and in the right way, so for rays of light coming from infinity. Our transformation was based on over 100 stars positioned evenly across the frame and average mesh-fitting error amounted to 22 seconds of arc. Our result was even lower than the official one, amounting to 45.1 deg with the measuring error not exceeding 0.1 of a degree.
Such a result means that, for rays falling from infinity, the real focal length of the Samyang AF 50 mm f/1.4 FE II amounts to almost exactly 52 mm.
Distortion
The Sigma A 50 mm f/1.4 DG DN, tested by us not so long ago, didn't bowl us over with its distortion correction because on full frame it showed a quite significant 'pincushion', reaching +2.58%.
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Fortunately Samyang engineers didn't follow that path and took the right decision of correcting distortion with optics of the lens. The result is a success – on the smaller APS-C sensor you deal with negligible pincushion distortion of +0.39%, that, after passing to full frame, increases to still low level of +0.55% that we find not especially inconvenient.
| Sony A7R III, 50 mm, APS-C | |||
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| Sony A7R III, 50 mm, FF | |||
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