Viltrox AF 25 mm f/1.7 Air
6. Distortion and field of view
Field of view
A rectilinear 25 mm lens on the APS-C/DX sensor should provide an angle of view amounting to 59.1 deg. In the official specification the producer states a slightly higher value, that of 60 deg exactly.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 developped 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 107 stars spread evenly across the frame. An average mesh-fitting error amounted to 58 seconds of arc. In this case we reached an angle of 58.53 degrees with a measurement error on a level of 0.05 of a degree. In other words our field was slightly lower than official declarations and also the value resulting directly from the focal length.
In the Fujifilm system RAW files always cover a bit wider field of view than JPEG files. Here our transformation was based on positions of 121 stars and the mesh-fitting error reached 58 seconds of arc. The field of view indeed proved to be a tad wider because we got here a result of 58.67 deg with an error below 0.07 deg. Still it's an angle lower than the declared value. The differences are slight here because when we converted this value into rectilinear focal length the result, 25.2 mm, was just slightly higher than the declared 25 mm.
Distortion
When it comes to distortion a significant angle of view, small dimensions and an affordable price suggested that the producers tried to look for savings in this area and give up on correcting this aberration. Still Viltrox managed to surprise us nicely once again not leaving everything to the software of the camera – it's obvious they decided to fight off distortion with the optics of the lens.
In case of the JPEG file we got a result of -1.82%, in case of RAW file the level is slightly higher, amounting to -1.87%. Distortion is just moderate; additionally it looks like almost uniquely barrel distortion, with a very slight influence of moustache deformations.
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| Fujifilm X-T2, 25 mm, JPEG | |||
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| Fujifilm X-T2, 25 mm, RAW | |||
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