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Sigma AF 17-70mm f/2.8-4.5 DC macro - Test Report / Review

Conclusão: evitar 17mm wide open.

Summary
is a DC ("Digital Camera") equivalent to 27-112mm on classic full frame SLRs. The min. focus distance of 0.20m resulting in a max. magnification of 1:2.3. don't expect wonders at extreme close focus settings. 79x83mm and 455g, filter diameter 72mm. The front element does not rotate
Distortions: At 17mm there're quite pronounced barrel distortions. The neutral setting is somewhere between 24mm and 40mm. Expect more distortions towards closer focus distances.
Vignetting: the problem is most significant at 17mm @ f/2.8 – visible in many scenes. However, at all other focal lengths and aperture settings the issue is actually pretty well under control. stopping down by 1-1.5 f-stops helps to reduce the vignetting down to a marginal degree.
MTF (resolution): At 17mm the center performance is very good at f/2.8 and excellent beyond. extreme corners (roughly 85% off the center) 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.
Chromatic Aberrations: CAs are somewhat pronounced at 17mm. The problem eases significantly when moving towards the long end of the zoom range.
Verdict: the lens has only one really weak spot - 17mm at wide-open aperture where the extreme border performance is soft and vignetting is quite pronounced. At medium aperture settings the delivered quality is very impressive. decent build quality (only marginally worse than the AF 18-50mm EX) and fast AF



Introduction
The new Sigma AF 17-70mm f/2.8-4.5 DC macro is a highly anticipated lens especially among Canon users seeking for a higher quality, affordable alternative to the original Canon EF-S 18-55mm f/3.5-5.6 or EF-S 17-85mm f/4-5.6 USM IS. In fact the interest in the Photozone forum was big enough to purchase the lens for testing rather than loaning it from users like you which is the usual approach for the local lens tests. The Sigma is a DC ("Digital Camera") lens specifically designed for the reduced image circle of today's APS-C DSLRs where its field-of-view is equivalent to 27-112mm on classic full frame SLRs.
The optical construction is made of 15 elements in 12 groups with 2 aspherical and one SLD (Special Low Dispersion) element. The aperture mechanism features 7 aperture blades. The min. focus distance of 0.20m resulting in a max. magnification of 1:2.3. The Sigma has no floating elements for close focus correction so don't expect wonders at extreme close focus settings. The lens extends significantly during zooming (see below) reaching its max. length at 70mm. Despite the quite large max. aperture the lens is still pretty compact and light-weight at only 79x83mm and 455g respectively. It's interesting that the AF 17-70mm DC is marginally bigger than the Sigma AF 18-50mm f/2.8 EX both regarding physical size as well as filter diameter (72mm vs 67mm) . A petal-shaped hood is part of the package.
Unlike its cousin the AF 17-70mm DC is not a designated EX lens (Sigma's pro grade lineup) but the build quality is still pretty good (tight controls, no wobbling) and only marginally worse than the AF 18-50mm EX. The lens has a slight crinkle finish (with a tendency to collect dust) and broad rubberized control rings. The focusing action feels smooth but a little lifeless (not damped) whereas the zoom mechanism has a slightly varying friction across the range. The lens doesn't show any tendency for zoom creeping but Sigma has provided a transport lock (17mm only). The front element does not rotate so using a polarizer remains easily possible. Unfortunately Sigma has still not managed to implement a focus ring that does not rotate in AF mode. The 17-70mm DC features a conventional AF micro motor so the AF operation is a little on the noisy side but thanks to an extremely short focus path (~45 degrees) the AF speed is very fast. The accuracy seems on par with the Canon kit zoom.
Distortions
The distortion characteristic of the lens is about average (in the true sense) for a zoom lens in this class. At 17mm there're quite pronounced barrel distortions changing to relatively moderate pincushion distortions at 70mm. The neutral setting is somewhere between 24mm and 40mm. Expect more distortions 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.
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.
Verdict
Sigma has positioned the AF 17-70mm f/2.8-4.5 DC macro in one of the sweet spots of the market. It is priced beyond the (usually pretty crappy) kit zooms yet it still remains within the reach of budget shoppers. Even better than that it delivers a performance which matches and sometimes even exceeds more expensive lenses. This includes the in-house competition (AF 18-50mm f/2.8 EX) as well as genuine brand lenses (such as the Canon EF-S 17-85mm f/4-5.6 USM IS). All in all the lens has only one really weak spot - 17mm at wide-open aperture where the extreme border performance is soft and vignetting is quite pronounced (but still not worse than the direct competition). At medium aperture settings the delivered quality is very impressive. Combine that with the quite decent build quality and fast AF and it doesn't need a genius to foresee that this lens will be a quite hot seller.

Optical Quality: 3 stars

Mechanical Quality: 3 stars

Price/Performance: 5 stars


http://www.photozone.de/Reviews/Canon%20EOS%20Lens%20Tests/312-sigma-af-17-70mm-f28-45-dc-macro-test-report--review