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Sigma AF 15-30mm f/3.5-4.5 EX DG - Review / Lab Test Report - Analysis |
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Lens Reviews -
Canon EOS (APS-C)
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Page 2 of 2
Distortion
Typical for many ultra wide-angle lenses the Sigma AF 15-30mm EX suffers from very heavy
barrel distortion at the wide end of the zoom range. The situation eases continuously
towards the 30mm where the lens is almost free of distortion.
15mm:
20mm:
30mm:
The chart above has a real-world size of about 120x80cm.
Vignetting
Many ultra-wide lenses suffer from vignetting at large aperture settings
and the Sigma AF 15-30mm f/3.5-4.5 EX is no exception despite enjoying
the sweet spot advantage on APS-DSLRs.
At 15mm f/3.5 the vignetting exceeds 1EV so it's a good idea to stop
down a little. The issue is already quite negligible from f/5.6 and
up. This is also true for 20mm and 30mm where the issue is much less
pronounced even at the max. aperture setting.
MTF (resolution)
The lens produced a very even performance accross the focal length and
aperture range with very-good to excellent results.
The difference between center and border performance is most pronounced
at 15mm (no surprise) but in absolute terms the borders are already
very good here. The center performance is best at 15mm but still
impressive at 20mm and 30mm.
Thanks to the APS-C crop there's only a marginal gain in technical
quality when stopping down.
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 very pronounced
with this lens but not much worse than the rest of the gang (in this lens class).
As usual for an ultra wide lens the problem is most pronounced towards the wide end
of the zoom range where the average CA pixel width varies around 1.5 pixels on the average
at the image borders.
Verdict
The Sigma AF 15-30mm f/3.5-4.5 EX DG proved to be a worthy representative within the ultra-wide
arena. It very sharp throughout the range without any more weakness in terms of resolution.
On the downside there're moderately strong barrel distortions at 15mm and visible lateral CAs
(color shadows at the image borders).
The build quality is very nice but it is a little unfortunate that Sigma chose to implement
a noisy micro motor with an imperfect AF/MF switch rather than a HSM drive. The huge protruding
front element may provide some design advantages but leaving the vulnerable front element
unprotected feels a little uneasy. So at the end of the day it isn't a
perfect lens but still a highly interesting option specifically regarding sharpness.
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