Viltrox AF 27 mm f/1.2
6. Distortion and field of view
Field of view
A rectilinear 27 mm lens on the APS-C/DX should provide an angle of view of 55.4 deg. The Fujifilm X system sensor is slightly smaller than the classic APS-C/DX so the official specifications of this lens feature also a slightly lower value of the field, amounting to 55.3 degrees.We decided to check the real size of the field In order to do that we took photos of the starry sky, saving them as uncorrected JPEG files and 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 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.
In case of JPEG files our transformation was based on positions of 116 stars spread evenly across the frame. An average mesh-fitting error of the coordinates amounted to 33 seconds of arc. Our result of 54.94 degrees, with the measuring error not exceeding 0.03 of a degree, is a bit lower than official values stated by the producers.
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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 110 stars and the mesh-fitting error reached 34 seconds of arc. Te field of view also proved to be a bit wider – our result amounts to 55.35 degrees with an error below 0.04 of a degree. In this case the result is in perfect accordance with official declarations of the producer.
Distortion
When it comes to distortion we didn't expect any serious problems. In case of older standard lenses, based on the double gauss system, distortion could reach even -2% but current constructions deal with it much better. It's worth to remember, though, that you deal with a wider standard here, with a bar set a bit higher.In case of JPEG files we noted a result reaching -1.16% and RAW files proved to be more demanding, with a value of -1.21%. Both these results are in accordance with each other within the margin of error and they don't provide any more serious reasons to complain. Of course deformations aren't corrected in a perfect way and you can notice a slight 'barrel' effect in the photos.
| Fujifilm X-T2, 27 mm, JPEG | |||
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| Fujifilm X-T2, 27 mm, RAW | |||
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