Sigma AF 18-200mm f/3.5-6.3 DC OS (Canon) - Review / Test Report - Analysis
Lens Reviews -
Canon EOS (APS-C)
Page 2 of 3
Typical for extreme range zooms the Sigma exhibits a fairly heavy amount of barrel
distortions at 18mm (~3.2%). However, beyond the wide end the problem is actually
very well controlled with only slight to moderate pincushion distortions.
Move the mouse cursor over the focal length text marks below to observe the respective distortions
The chart above has a real-world size of about 120x80cm.
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).
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 in line widths per picture height (LW/PH)
which can be taken as a measure for sharpness. If you want to know more about it you may check out the corresponding
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).
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.