Sigma C 23 mm f/1.4 DC DN
6. Distortion and field of view
Field of view
A 23 mm rectilinear lens on the APS-C/DX sensor should provide an angle of view amounting to 63.3 deg. What's insteresting, in the official specifications of the lens published on the official Sigma website they give a bit higher value, that of 63.4 deg.We decided to check the real field and measure it as it should be done, for rays of light coming from infinity. In order to do so we took photos of starry sky and saved them both as automatically corrected JPEG files and RAW files which weren't corrected and 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.
The transformation for the JPEG file photo was based on 137 stars evenly spread across the frame. Average mesh-fitting error of coordinates amounted to just 13 seconds of arc. We got a result of 65.6 deg with measuring error not exceeding 0.05 of a degree. Our result is a bit higher than the official specifications but the difference is not significant. Still, it indicates that even after the correction of geometric deformations and cropping you deal with a lens with an effective focal length of 22 mm.
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If you can tolerate a slight distortion level you can even get a wider field of view taking and saving photos as uncorrected RAW files. In their case our transformation was based on 205 stars and its error reached 49 seconds of arc. The field of view we got this way amounted to 67.9 deg with a measuring error of 0.1 of a degree. In this case you brush against an effective focal length reaching 21 mm.
Distortion
When you join the Sigma C 23 mm f/1.4 DC DN with the Fujifilm body JPEG files are automatically distortion corrected. As a result in this format you see a very slight pincushion distortion of +0.49%.How the optics really performs you can find out only when you analyze RAW files developped by neutral software. In this case you get distinct barrel distortion which level we determined as -2.75%.
It's interesting that, while passing from RAW to JPEG files, the software of the camera changes the type of distortion. I find it a bit strange. Perhaps better results you would get by reducing the level of -2.75% to near -0.5% and the absolute value of corrected distortion would remain the same.
What about the rivals? In this category the Viltrox fares the best, the Sigma gets the second place and the most expensive Fujinon ends as the third.
| Fujifilm X-T2, 23 mm, JPEG | |||
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| Fujifilm X-T2, 23 mm, RAW | |||
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