Viltrox AF 15 mm f/1.7 Air
6. Distortion and field of view
Field of view
A rectilinear 15 mm lens on full frame should provide an angle of view amounting to 86.8 deg. In the official specification the producer states a value of 84.9 deg and it might suggest that the real focal length of the tested Viltrox might be somewhere between 15 and 16 mm.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 159 stars spread evenly across the frame. An average mesh-fitting error amounted to 4 minutes and 15 seconds of arc. In this case we reached an angle of 84.87 degrees with a measurement error on a level of 0.05 of a degree. In other words our result was in perfect accordance with the official specification.
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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 191 stars and the mesh-fitting error reached 3 minutes and 57 seconds of arc. The field of view indeed proved to be a tad wider because we got here a result of 85.08 deg with an error below 0.04 deg.
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 a measurement averaged out across the frame provided a result of -1.12%, a very moderate value for the offered angle of view. Still it's worth noticing the fact that it was reached with a help of noticeable moustache distortion. If you limit your measurements to 1:1 markings distortion will increase to a level of -2.59%. It's a tad more bothering value but, after all, we can quote here a whole list of lenses that are more expensive, and provide narrower angles of view and yet their performance in this category has been noticeably weaker.
It's worth adding that in case of RAW files with a tad wider field of view distortion averaged out across the frame is a bit lower, reaching -1.06%. It stems from the fact that the result is based on an area where bending of curves the other way round, an effect connected to moustache distortion, is more distinct. It confirms the fact that the measurement inside 1:1 markings provides a result of -2.56% so, within the limits of measurement error, the same as for JPEG files.
| Fujifilm X-T2, 15 mm, JPEG | |||
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| Fujifilm X-T2, 15 mm, RAW | |||
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