Sigma A 28-45 mm f/1.8 DG DN
6. Distortion and field of view
Field of view
A rectilinear 28-45 mm lens on a full frame sensor should provide you angles of view changing from 75.4 to 51.3 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 176 stars spread evenly across the frame and average mesh-fitting error amounted to 29 seconds of arc. Our result amounts to 75.34 deg with a measuring error not exceeding 0.05 deg. As you see the field is in almost perfect accordance with official declarations.
In case of the 45 mm focal length we used positions of 117 stars and average mesh-fitting error amounted to just 15 seconds of arc. Our result amounts to 50.59 deg, also with a measuring error not exceeding 0.05 deg. It's a piece of good news – in reality the lens is able to provide a bit wider range of fields than we thought.
Distortion
At the smaller APS-C sensor distortion is not a serious problem and you can notice it mainly on both ends of the focal range. At 28 mm you deal with low barrel distortion of -1.18% that becomes zero near 35 mm (with an official result of +0.25%) and then it changes the sign and increases quite quickly so at 40 mm its level is +0.85% and at 45 mm it jumps to +1.18%.
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| Sony A7R IIIa, APS-C, JPEG, 28 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 35 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 40 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 45 mm | |||
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On full frame there are more problems but, fortunately, you never deal with high levels. At the widest angle of view barrel distortion reaches -2.49%, it becomes zero right before the 35 mm focal length and at that value it turns into pincushion of +0.78%. A distinct 'explosion' of that pincushion variant is visible at 40 mm where we measured a level of +2.30% and then distortion increased more to +2.92% at the maximum focal length.
It is interesting that the Sigma 28-45 mm shows the biggest deformations (and, of course, they differ when it comes to the sign) not at the widest angle of view but at the narrowest one.
| Sony A7R IIIa, FF, JPEG, 28 mm | |||
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| Sony A7R IIIa, FF, JPEG, 35 mm | |||
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| Sony A7R IIIa, FF, JPEG, 40 mm | |||
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| Sony A7R IIIa, FF, JPEG, 45 mm | |||
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