Canon EF-S 18-200mm f/3.5-5.6 IS - Review / Test Report - Analysis
Lens Reviews -
Canon EOS (APS-C)
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Typical for many extreme range zoom lenses the Canon exhibits an extreme amount of barrel distortion at 18mm (~4.5%). However, beyond the wide end the problem is relatively very well controlled with only moderate barrel distortion at 24mm and moderate pincushion distortion from 50mm onwards.
Move the mouse cursor over the focal length text marks below to observe the respective distortion
The chart above has a real-world size of about 120x80cm.
It's fairly common for dedicated APS-C lenses to show a more pronounced vignetting at max. aperture settings. The Canon lens is no exception here especially at 18mm with a rather extreme peak vignetting of 1.5 stops at 18mm @ f/3.5. At 200mm @ f/5.6 it's also somewhat hefty at around 0.9EV. The situation isn't as bad at other settings but usually you should generally stop down a bit to avoid the problem.
The most interesting aspect is naturally the resolution of the lens but, unfortunately, this is nothing to rave about. At 18mm the lens struggles to come up with an acceptable performance even at medium aperture settings. The center performance is great but the border quality is soft based on a 15mp sensor - the potential is roughly sufficient for 6mp here. The situation improves significantly (good borders) at 24mm. At 50mm the resolution is on a very good level even at max. aperture. At 100mm @ f/5 and 200mm @ f/5.6 the border quality worsens significantly although it helps to stop down to medium apertures. Overall the lens shows a roller-coaster ride in terms of resolution but that's admittedly that's not unusual in the extreme zoom range lens league. While not formally tested the same seems to apply for the contrast behavior.
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
Chromatic Aberrations (CAs)
The level of lateral CAs (color shadows at harsh contrast transitions) is just acceptable for such a lens although the problem is very significant in absolute terms. At 18mm and 100mm the LCAs can exceed 2px on the average at the image borders and it's not much better elsewhere. The problem is clearly field relevant (although correctable to some degree via image post-processing).