Sigma AF 18-200mm f/3.5-6.3 DC OS (Canon) - Review / Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

Typical for extreme range zooms the Sigma exhibits a fairly heavy amount of barrel distortion at 18mm (~3.2%). However, beyond the wide end the problem is actually very well controlled with only slight to moderate pincushion distortion.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
18mm 35mm 50mm 100mm 200mm

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

Vignetting

The Sigma is a dedicated APS-C lens but despite the ambitious design vignetting is generally well controlled except at 18mm @ f/3.5 (~1EV) and 200mm @ f/6.3 (~0.8EV).

MTF (resolution)

In terms of resolution the Sigma shows a bit of a roller-coaster ride throughout the zoom range. At 18mm the results are surprisingly good - the center resolution leaves nothing to be desired and the border quality is very good even at large aperture settings. At 35mm the situation changes dramatically. The center performance is still exceptionally high but the border quality is absolutely dismal at f/4 and f/5.6. The field curvature is quite hefty at this setting. It is advisable to avoid anything faster than f/8 at this setting. (FWIW, the Sigma 18-200mm non-OS showed also a performance breakdown at 35mm). Interestingly the weakness is gone at 50mm where the lens is capable to produce a pretty good and even center to border quality. At 100mm the lens shows a downward trend again - the center quality is very good but the borders and specifically the extreme corners are softer. The very good peak performance is reached at f/11 here. Surprisingly the border quality can recover again at 200mm.

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)

For most of the zoom range CAs (color shadows at harsh contrast transitions) are well controlled with an average pixel width between 0.5px to 1.1px at the image borders. This is very decent for such a zoom lens (even for a zoom lens in general).

Bokeh

The quality of the bokeh (out-of-focus blur) is not exactly a strength of the Sigma 18-200mm OS - it is fairly "nervous" and, to be honest, distracting. Here's a 100% sample portion:

You may also check out the "knight" sample image below where the background quality isn't all that hot.




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