Sigma AF 18-50mm f/3.5-5.6 DC - Review / Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

The level of distortion varies across the zoom range. At 18mm there is quite extreme barrel distortion - nothing for architecture photography where you prefer straight lines to remain straight. Towards 28mm the situation eases a bit but there is still some pronounced barrel distortion here. At the 50mm setting the lens is virtually free of distortion.

18mm:

28mm:

50mm:

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

Vignetting

Typical for most true APS-C lens there's some vignetting at wide-open aperture but the issue remains comparatively well controlled. As to be expected the problem is most pronounced at 18mm where should stop down to about f/5.6 - at wide-open aperture vignetting can be quite disturbing. At 28mm and 50mm vignetting is already quite a bit lower being quite acceptable even at large aperture settings.

MTF (resolution)

The lens exhibited a surprisingly good performance regarding resolution. At the wide-angle settings you may prefer to stop down a little to avoid edge softness but from f/5.6 and up the performance is quite impressive for such an affordable lens. At 18mm and 28mm the center can even reach excellent levels at medium aperture settings and the border can follow not all that far behind with good to very good figures. At the 50mm setting the MTF figures are slightly lower but still on a very high level. Quite baffling actually.

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

The lens produced a rather extreme degree of chromatic aberrations (color shadows at harsh contrast transitions) at the image borders at 18mm. Stopping down helps to ease to problem a little here. At 28mm and more so at 50mm the problem is pretty well controlled though.




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