Sigma AF 18-125mm f/3.8-5.6 DC HSM OS (Canon) - Review / Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

Unlike most of its class mates the Sigma has a comparatively moderate distortion characteristic. Nonetheless it exhibits a fairly strong amount of barrel distortion at 18mm (~2.6%). However, beyond the 18mm end the problem is actually well controlled. There's no distortion at 24mm and only moderate pincushion distortion at 50mm and 125mm.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
18mm 24mm 50mm 125mm

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

Vignetting

The Sigma shows the quite typical characteristic of a dedicated APS-C lens with a strong degree of vignetting at wide-open aperture at the extreme ends of the zoom range. However, stopping down to apertures smaller than f/5.6 helps to reduce the problem to unobjectionable levels.

MTF (resolution)

In terms of resolution the Sigma is one of the better representatives of its species although it's not a stellar performer in absolute terms. The resolution is very decent at 18mm and 24mm with a very high center performance and good (max. aperture) to very good borders. Even the extreme corners are able to keep a good level here. The sweet spot is reached around 50mm with a very good and even quality across the frame. The border quality deteriorates to only fair levels at 125mm whereas the center can keep its very good quality. While not formally tested the global contrast seems to be a little lower than average. Field curvature is quite well controlled.

FWIW, the lens exhibited a slight centering defect at the wide end of the range.

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)

CAs (color shadows at harsh contrast transitions) are generally well controlled especially considering the extreme nature of this lens. Unsurprisingly they're more pronounced at 18mm with a peak around 2px on the average at the image borders. They can also be visible at 125mm, less so in between.




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