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Sigma AF 17-70mm f/2.8-4.5 DC macro - Review / Test Report - Analysis |
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Lens Reviews -
Canon EOS (APS-C)
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Page 2 of 3
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.
Please note that the MTF results are not directly comparable across the different systems!
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.
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