A rectilinear 28-70 mm lens on a full frame sensor should provide you angles of view changing from 75.4 to 34.3 deg and such values exactly are given by the producers in official specifications. Of course we decided to check how big this field really is. In order to do so we took photos of starry sky saving 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 were able to measure the field very precisely as it should be done, for rays of light coming from infinity.
At the 28 mm focal length the transformation was based on positions of 161 stars spread evenly across the frame and average mesh-fitting error amounted to 3 minutes and 43 seconds of arc. Our result amounts to 76.9 deg with a measuring error not exceeding 0.1 of a deg. As you see the field measured by us is by 1.5 degrees wider than official values stated in specifications. It's obvious the producer left some margin for distortion correction and it was a very wise decision because in the next section we are going to need it.
Distortion
On the smaller APS-C sensor we noticed distortion without any special problems at 28 mm where you deal with its barrel variation of -2.82%. After passing to the 35 mm focal length that value decreases to -1.28%, then it becomes zero, changes the sign and at 50 mm you already deal with small pincushion distortion of +0.50%. It increases quite fast, though, and at 70 mm it amounts to +1.13%.
Sony A7R IIIa, APS-C, JPEG, 28 mm
Sony A7R IIIa, APS-C, JPEG, 35 mm
Sony A7R IIIa, APS-C, JPEG, 50 mm
Sony A7R IIIa, APS-C, JPEG, 70 mm
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On full frame the level of deformations is much higher; additionally the optics constructors decided to introduce moustache distortion, visible not only at 28 mm but even at 35 mm.
Talking about tangible values, at the widest angle of view you deal with barrel distortion of -4.32%. If you limit the field of your measurement to an area within 1:1 markings the result increases up to -5.18%. Small wonder the Sigma offers here a wider field of view than the declared one because the correction of such distortion needs a significant cropping of images. In fact it's possible that this spare field should have been even wider.
At the 35 mm focal length this aberration decreases to a not especially bothersome level of -1.07% but, once again, if you limit your measurements to just the 1:1 area distortion increases to -2.88%.
A slight lengthening of the focal length zeroes all deformations and then they pass to the pincushion part. At 50 mm you already deal with a level of +1.93%. The problems increase very quickly because at 70 mm you have to deal with a quite significant value, amounting to +2.72%.
There are no doubts that distortion bothers the Sigma 28-70 mm so much that it influences both ends of its focal range. However, it's one of these compromises that all the potential users of any of the Contemporary series instruments have to take into account because the producers inform about them openly.
