Sigma C 17-70 mm f/2.8-4.0 DC Macro OS HSM
5. Chromatic and spherical aberration
Chromatic aberration
The longitudinal chromatic aberration is not a problem at all. Consulting the photos below, taken at 70 mm, it would be difficult to notice any visible influence of that aberration.
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The lateral chromatic aberration is invisible only at longer focal lengths where it doesn’t exceed the level of 0.06%. At 28 mm and near the maximum relative aperture it reaches a medium level but fortunately it decreases swiftly with the stopping down. The most serious problems you can encounter at 17 mm and by f/2.8 where that aberration gets near high values. It becomes lower when you stop down but nowhere it drops below medium level.
The maximum values, reached by the new Sigma lens, are higher than those of its predecessor which, at 17 mm, got to about 0.13% at most. There is a difference, though: the old lens kept the level of 0.13% in practically the whole range of apertures and the new one allows, on stopping down, to decrease the aberration to less than 0.11%.
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Spherical aberration
The aspherical aberration of the tested lens is corrected in a good but not perfect way. You have no problems with the focus shift but the images of defocused light points in front of and behind the focus aren’t identical. It means that the aberration can make itself felt but, fortunately, its degree is not high.
