Sigma A 28-105 mm f/2.8 DG DN
6. Distortion and field of view
Field of view
A rectilinear 28-105 mm lens on a full frame sensor should provide you angles of view changing from 75.4 to 23.3 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 180 stars spread evenly across the frame and average mesh-fitting error amounted to 2 minutes and 44 seconds of arc. Our result amounts to 79.29 deg with a measuring error not exceeding 0.07 deg. As you see the field measured by us is almost 4 degrees wider than official values stated in specifications. It's obvious the producer left some margin for distortion correction. At the same time you get a bonus – when you work with uncorrected files you get a field of view almost like in a 26 mm rectilinear lens.
In case of the 105 mm focal length we used the positions of 91 stars and average mesh-fitting error amounted to just 5 seconds of arc. Our result amounts to 23.19 deg, also with a measuring error not exceeding 0.05 deg. Here the field of view is a bit narrower than stated in official specifications but the difference is really very slight.
Distortion
At the smaller APS-C sensor distortion can be noticed without any problems at 28 mm where you deal with its barrel variant amounting to -2.80%. After passing to the 35 mm focal length that value decreases to -1.14%, then it becomes zero very fast, changes its sign, and at 50 mm you already deal with low pincushion of +0.47%. Then it increases pretty fast - at 70 mm it is +1.09%, and at 105 mm it reaches +1.32%.
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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, 50 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 70 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 105 mm | |||
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As some distortion problems were already visible on the smaller sensor we were very curious how they present themselves on full frame. Let's glance at photos below.
| 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, 50 mm | |||
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| Sony A7R IIIa, FF, JPEG, 70 mm | |||
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| Sony A7R IIIa, FF, JPEG, 105 mm | |||
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Here the level is noticeably higher; additionally optics constructors decided to get some support from moustache distortion that is detectable not only at 28 mm but even at the 35 mm focal length.
Now about concrete values: at the widest angle of view you deal with barrel distortion of -4.63%. If you limit the field of measurement to an area inside 1:1 markings the result increases to as much as -5.15%. Small wonder the Sigma offers here a far wider field of view than declared because correction of such distortion demands a significant cropping of images.
At the 35 mm the aberration, described here, decreases to a not especially bothersome level of -1.54%; still if you once again assess the area inside 1:1 markings that value increases to -2.08%.
Slight increase of the focal length makes the deformations reduce to zero and then they pass to the pincushion side so at 50 mm you already deal with a level of +1.51%. Problems increase quite swiftly because at 70 mm you already deal with a quite impressive value of +2.72% that additionally goes up to +3.13% when you increase the focal length to the maximum.
There are no doubts – the Sigma's 28-105 mm distortion is quite bothersome and for both sides of the focal range.
