Sigma AF 17-70mm f/2.8-4.5 DC (Nikon) - Review / Lab Test Report - Analysis
Lens Reviews - Nikon / Nikkor (APS-C)
Article Index
Introduction
Analysis

Distortion

Typical for many standard zoom the Sigma lens produces a pronounced degree of barrel distortion at 17mm (~3%) changing to slight pincushion distortion when zooming towards 70mm (~0.45%). At about 35mm both forces even out so the lens shows basically no distortion here.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
17mm 35mm 70mm

The chart above has a real-world size of about 120x80cm.

Vignetting

Standard-zoom lenses with a reduced image circle tend to vignette quite a bit specifically towards the wide end of the zoom range. This is also true for the Sigma AF 17-70mm f/2.8-4.5 DC. At 17mm the level of vignetting is pronounced with ~1.2EV at f/2.8 but stopping down to f/4 reduces the problem to a manageable degree. At 35mm the issue is very well controlled whereas the vignetting increases again at 70mm (0.83EV @ f/4.5).

MTF (resolution)

The Sigma AF 17-70mm f/2.8-4.5 DC produced quite a mixture of quality levels in the MTF lab. At 17mm the center performance is excellent straight from the max. aperture. The borders are very good but the extreme corners are soft at f/2.8 and to a lesser degree also at f/4. Stopping down to f/5.6 resolves some of the corner problems but the peak performance is not reached prior of f/8 (very good corners). The sweet spot of the lens is at 35mm - the resolution is already evenly high at f/3.8 and outstanding from f/5.6 onwards. This is a surprising level for a standard zoom lens. There's a slight drop in resolution at 70mm but the quality remains easily very good at f/4.5 and excellent from f/5.6 (and that's despite a slight centering defect here).

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)

The Sigma manages to keep lateral chromatic aberrations (color shadows at harsh contrast transitions) very well controlled for a zoom lens. The problem is a bit more pronounced at 17mm @ f/2.8-4 with an average width of ~1.1px at the image borders. However, stopping down to f/8 reduces the CAs to a very good level. Typical for most standard zooms the problem decreases when zooming towards longer focal lengths.

Verdict

The Sigma AF 17-70mm f/2.8-4.5 DC is a very good standard zoom and a viable alternative to the Nikon offerings in this range. Generally it is capable to produce very sharp results although the corner performance leaves a bit to be desired at 17mm at large aperture settings. The vignetting level could also be a bit better here but most competitors don't offer a f/2.8 settings anyway and from f/4 it follows the mainstream. The level of distortions is about typical for a standard zoom whereas chromatic aberrations (color shadows) are comparatively low. The build quality is very fine for a lens in this price league.

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