Sigma C 12 mm f/1.4 DC
6. Distortion and field of view
Field of view
A rectilinear 12 mm lens on the APS-C/DX sensor should provide an angle of view amounting to 99.5 deg. In the official specification the producer states a slightly higher value, that of 99.6 degWe 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 as JPEG files with distortion correction switched on and off. 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 distortion corrected JPEG files our transformation was based on positions of 125 stars spread evenly across the frame. An average mesh-fitting error amounted to 59 seconds of arc. In this case we reached an angle of 100.39 degrees with a measurement error on a level of 0.03 of a degree. In other words our result is a tad higher than the value stated in the official specifications.
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When it comes to uncorrected JPEG files our transformation was based on positions of 113 stars and an average mesh-fitting error amounted to 3 minutes and 6 seconds of arc. The field of view proved to be noticeably higher, with a result of 104.01 degrees with a measurement error below 0.07 of a degree. Such a value of angle of view is an equivalent of a rectilinear 11 mm lens.
As you can notice Sigma leaves here over 4 deg of spare field of view. In a moment you are going to find out why; let's just mention here that it is necessary to correct significant distortion.
Distortion
When it comes to distortion, the significant field of view, small physical dimensions and the fact that the lens belongs to the C line of products suggested at once that the producers might try to economize a bit in this category, giving up on correcting this aberration. Indeed it is the case.A photo below shows an image you can get if you save it as an uncorrected JPEG file. You see a distinct barrel distortion of -6.27%. What's more, when you look closely at the edge of the field of view you can notice a slight trace of moustache distortion. Our measurements confirm it – if you limit them to an area limited by 1:1 markings the result increases to -6.90%.
| Sony A7R IIIa, 12 mm, UNCORRECTED JPEG | |||
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In case of an ultra wide angle lens you are in a very short distance from your aim, even if you take a photo of a very big testing chart. Still it's worth checkin how distortion changes when you set the focus at infinity. With such parameters as these of the tested Sigma you can, without changing its position, set the foccus at infinity and the aperture set by f/16 and still get a more or less sharp image of the tested chart. Our result we present below.
| Sony A7R IIIa, 12 mm, UNCORRECTED JPEG, INFINITY | |||
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In this case distortion decreases to -5.29%, a level noticeably lower than previously but still significantly high, easily noticeable in real life photos.
At the end we would like to check the distortion level for corrected JPEG files. In this case you still get a field a tad wider than in official specifications; at the same time this wide field is completely devoid of deformations, with a result of -0.17%. It is, within the measuring error, in accordance with zero.
When you take photos and save them as corrected JPEG files you get the declared field of view completely devoid of distortion. Still there is a price to be paid – 'pumped up' pixels. After all the corrected images are first cropped and then scaled up so they are in accordance with the official resolution of the detector used.
| Sony A7R IIIa, 12 mm, CORRECTED JPEG | |||
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