Carl Zeiss Touit 12 mm f/2.8
6. Distortion
If you look at RAW files, developed by independent software (like e.g. dcraw) the results are quite different. In that case you can notice distinct barrel distortion which value we measured as – 2.54%. Such a level is not especially bothersome but it is also nothing to boast of, especially for a fixed-focus lens. Let’s remind here that 12-24 mm zoom lenses such as the Nikkor or the Tokina, at 12 mm could show the distortion ranging from –2.5 to-2.0% so slightly lower. I suppose we don’t have to add that the correction of that aberration in a zoom lens is more difficult than in a ‘prime’.
The approach of Zeiss when it comes to the fields of view is interesting, though. Usually, when you deal with a system that gives up on distortion correction, the image before the correction features a wider field of view than declared so, after the cropping, the resulting field of view remains in accordance with the focal length of the lens, stated on the casing. In the case of the Zeiss the situation is different. The field of view of the uncorrected RAW file amounts to 99.23 degrees (with the measurement error on a level of 0.05 of a degree) so it is even a tad narrower than 99.5 degrees the 12 mm focal length should provide on the 1.5x APS-C/DX sensor. The field of view of the distortion-corrected JPEG file is just 97.03 degrees (with the error on a level of 0.05 of a degree) so over 2 degrees smaller than declared. It is a flaw to some extend but, on the other hand, an optics producer such as Zeiss shouldn’t be demanded to state some focal lengths equivalents instead of a real focal length, which is, after all, closely connected with the optical construction and doesn’t depend on the size of the detector.
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