Sigma AF 18-125mm f/3.5-5.6 DC - Review / Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

A major weakness of most long range zoom lenses is the level of distortion and the Sigma is no exception. At 18mm the lens produces a rather extreme degree of barrel distortion changing to very pronounced pincushion distortion from 35mm and up. If you´re looking for a lens for architecture photography this one is clearly not for you.

18mm:

35mm:

70mm:

125mm:

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

Vignetting

Typical for most reduced image circle lenses the Sigma produces quite strong vignetting. At 18mm the vignetting exceeds 1.2EV so stopping down to ~f/5.6 is generally a good idea here. The problem isn't quite as pronounced at the other tested focal lengths but you have to be careful towards the long end of the zoom range where the vignetting increases again.

MTF (resolution)

The lens exhibited a baffling performance regarding resolution and more so relative to its extreme zoom ratio. At 18mm the lens is capable to produce an excellent center quality and very good borders and there isn´t even any noticeable performance drop towards the extreme corners which is otherwise quite typical for reduced image circle lenses. The situation remains as positive at the 35mm setting. Beyond there´s a decrease in performance but the center quality continues to stay on a very good level but the borders suffer from a more pronounced drop specifically at max. aperture. At 70mm it´s possible to lift the quality to very good figures by stopping down a little whereas at 125mm the borders stay "just" on a good level.

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 relatively high degree of chromatic aberrations (color shadows at harsh contrast transitions) at the image borders at the extreme ends of the zoom range. However, the situation isn't quite as extreme as with some other (even higher priced) lenses and actually pretty acceptable for such an extreme zoom lens.




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