Viltrox AF 13 mm f/1.4 XF
6. Distortion and field of view
Field of view
A 13 mm rectilinear lens on the APS-C/DX should provide an angle of view of 95 degrees. The sensor of the Fujifilm X system is a tad smaller than the classic APS-C/DX so official specifications of the lens feature a slightly lower value of 94.06 degrees.We decided to take a reality check. In order to do so we took photos of starry sky and saved them as uncorrected JPEG files. 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. Our transformation was based on over 220 stars positioned evenly across the frame and average mesh-fitting error amounted to 1 minute and 58 seconds of arc.
Finally we got a result of 94.1 degrees with measuring error not exceeding 0.2 of a degree. As you see our result is by and lare in perfect accordance with the official specifications – a round of applause!
Distortion
Optics constructors often give up on distortion correction and let the software of a camera to deal with this task. In case of a lens with such a wide angle of view and fast aperture as the Viltrox, tested here, I imagine it was a very tempting option. It would allow more room of maneuver when it comes to correcting other off-axis aberration. That's why we were very curious what option was chosen by Viltrox engineers.
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As it happened they decided to throw themselves into the deep end and correct distortion. By the way their approach was quite successful. The results for uncorrected JPEG and RAW files are in accordance with themselves within the margin of measuring error and amount, respectively, to -1.05% and -0.99%. As you see, we deal with slight barrel distortion. This result is definitely worth praising because with such a wide angle of view it's not easy to correct distortion so well. Of course we should mention the fact that slight moustache distortion helped to achieve it – it's noticeable that lines near their end bend the other way a bit. Our measurements taken right in the middle of the image, so limited by 1:1 markings, confirm that. For that area barrel distortion increases to -1.55%.
We don't approve of moustache distortion but its presence cannot change our positive assessment of the Viltrox performance in this category. We think its constructors deserve our respect for a quite successful attempt at limiting geometric deformations with such a wide angle of view.
| Fujifilm X-T2, 13 mm, JPEG | |||
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| Fujifilm X-T2, 13 mm, RAW | |||
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