Viltrox AF 35 mm f/1.7 Air
6. Distortion and field of view
Field of view
A rectilinear 35 mm lens on the APS-C/DX sensor should provide an angle of view amounting to 44.1 deg. In the official specification the producer states a slightly higher value, that of 45 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 distortion uncorrected JPEG files and then as RAW files developed by neutral software. 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.
In case of JPEG files our transformation was based on positions of 82 stars spread evenly across the frame. An average mesh-fitting error amounted to 18 seconds of arc. In this case we reached an angle of 42.64 degrees with a measurement error not exceeding a value of 0.03 of a degree. This result is a tad lower than official declarations and also than a value stemming directly from the focal length. In other words the real focal length of the tested lens amounts to a tad over 36 mm.
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As you see, the Viltrox AF 35 mm f/1.7 Air is an equivalent of a full frame 55 mm standard rather than a 50 mm class model.
Distortion
In case of a standard lens geometric deformations shouldn't be a serious problem. In times of mirrorless devices, when producers can be very lax with optics-induced distortion correction, we've seen many strange results. The Viltrox deals with this problem in a brilliant way and I admit I am truly impressed. In case of JPEG files we got a result of +0.26% and in case of RAW files this value is slightly higher, +0.32%. Pincushion distortion is simply negligible, brushing against zero within the margin of measuring error – a round of applause!
| Fujifilm X-T2, 35 mm, JPEG | |||
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| Fujifilm X-T2, 35 mm, RAW | |||
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