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Sigma AF 18-200mm f/3.5-6.3 DC - Review / Lab Test Report - Analysis |
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Lens Reviews -
Canon EOS (APS-C)
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Page 2 of 2
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 pronounced pincushion distortion from 35mm
and up.
18mm:
35mm:
80mm:
200mm:
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 a little careful towards
the long end of the zoom range where the vignetting increases again.
MTF (resolution)
The lens exhibited a very mixed performance regarding resolution. At 18mm
the lens is capable to produce a very good to even excellent center quality and
good to very good borders - quite a positive surprise here! At 35mm the
center quality remains on a very high level but the borders suffer dramatically
at "large" aperture settings. However, the situation eases quite a bit from
f/8 and up where the border quality reaches very respectable levels.
At 80mm the quality is on a very good level throughout the tested aperture
range whereas there's a performance penalty towards the long end of
the zoom range where the quality falls to generally good levels.
All-in-all pretty good for a 11x zoom lens. Nonetheless it is worth to mention
that there was a fairly strong centering defect at 18mm (blurry right side till f/8).
Please note that the MTF results are not directly comparable across the different systems!
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 quite high degree of chromatic aberrations (color shadows at harsh
contrast transitions) at the image borders throughout 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.
Verdict
There's no such thing as a free lunch in lens land and the Sigma AF 18-200mm f/3.5-6.3 DC
is no exception here. The resolution figures are generally quite decent and regarding the
extreme zoom range you may accept the very pronounced distortions present throughout the zoom
range as well as a few other shortcomings. Unfortunately the AF accuracy leaves something to
be desired with this lens especially towards the long end of the zoom range.
Typically such a type of lens is used by beginners on entry level DSLRs where a rather dark
viewfinder combined and an average AF system don't really help here. So if you're interested
in this lens it's a good idea to take your DSLR to your trusted photo dealer around the
corner to check whether the camera's AF system is really up to the task. If so it is a good
compromise between quality and zoom range.
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