Sigma AF 17-70mm f/2.8-4.5 DC macro - Review / Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

The distortion characteristic of the lens is about average (in the true sense) for a zoom lens in this class. At 17mm there is quite pronounced barrel distortion changing to relatively moderate pincushion distortion at 70mm. The neutral setting is somewhere between 24mm and 40mm.

17mm:

24mm:

50mm:

85mm:

The chart above has a real-world size of about 120x80cm or 40x the focal length. Expect more distortion towards closer focus distances.

Vignetting

Typical for lenses with a reduced image circle vignetting is pronounced compared to classic full frame lenses. As to be expected the problem is most significant at 17mm @ f/2.8 where the vignetting exceeds 1.1EV - this will be visible in many scenes. However, at all other focal lengths and aperture settings the issue is actually pretty well under control. If needed stopping down by 1-1.5 f-stops helps to reduce the vignetting down to a marginal degree.

MTF (resolution)

In the lab the lens produced pretty impressive resolution figures but it's not a lens without flaws. At 17mm the center performance is very good at f/2.8 and excellent beyond. The borders follow a little behind with good to very good figures. Unfortunately this isn't true for the extreme corners (roughly 85% off the center) which are soft at f/2.8. Stopping down helps to lift the performance here as well but you should use at least f/4, better f/5.6 for a decent quality level. At f/8 even the extreme corners are very good though. At 24mm the extreme border problem eases quite a bit and the setting is already quite usable at wide-open aperture. At f/5.6 the quality is absolutely exceptional. At 40mm and 70mm the resolution figures are pretty flawless.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

Chromatic aberrations (color shadows at harsh contrast transitions) are slightly better than average for a zoom lens in this class. CAs are somewhat pronounced at 17mm peaking in an average width of 1.3 pixels at the image borders. The problem eases significantly when moving towards the long end of the zoom range.




© by photozone.de