Viltrox AF 56 mm f/1.7 Air
6. Distortion and field of view
Field of view
A rectilinear 56 mm lens on the APS-C/DX sensor should provide an angle of view amounting to 28.4 deg. In the official specification the producer states a bit different value, that of 29.8 deg. It's a noticeable discrepancy that suggests that a real focal length of this lens is closer to 53 mm than to 56 mm.Of course we decided to check the dimensions of this field in real life. 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 analysed files our transformation was based on positions of 92 stars spread evenly across the frame. An average mesh-fitting error amounted to just 5 seconds of arc. In this case we reached an angle of 28.95 degrees with a measurement error not exceeding a value of 0.03 of a degree. This result is between a value stemming from the focal length and the one stated in the specification. At the same time it is an equivalent of an angle of view provided by a rectlinear 55 mm lens.
Distortion
In case of a lens providing an angle of view on a level of almost 30 deg geometric deformations shouldn't be a big 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 clever way and I admit I am truly impressed.
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In case of the JPEG files we got a result of +0.64% and in case of RAW file this value was slightly higher, +0.67%. Pincushion distortion is here rather slight and it certainly won't disturb you while shooting.
| Fujifilm X-T2, 56 mm, JPEG | |||
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| Fujifilm X-T2, 56 mm, RAW | |||
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