Sigma AF 17-70mm f/2.8-4.5 DC (Pentax K) - Review / Test Report - Analysis
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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 (~0.5%).

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 vignetting is pronounced with ~1.3EV at f/2.8 but stopping down to f/5.6 reduces the problem to a manageable degree. At 35mm the issue is pretty well controlled whereas vignetting increases again at 70mm (0.84EV @ 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 very good straight from the max. aperture. The borders are good to very good but the extreme corners are soft. 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/4 and outstanding at f/8. This is a surprising level for a standard zoom lens. There's a slight drop in resolution at 70mm but the quality remains easily in very good territories here.

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)

The Sigma manages to keep lateral chromatic aberrations (color shadows at harsh contrast transitions) comparatively well controlled for a zoom lens. That said the problem is still fairly pronounced at 17mm @ f/2.8-4 with an average width of ~1.5px at the image borders. However, stopping down to f/8 reduces the CAs to a good level. Typical for most standard zooms the problem decreases when zooming towards longer focal lengths.



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