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Canon EF-M 11-22mm f/4-5.6 STM IS - Review / Test Report - Analysis |
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Lens Reviews -
Canon EOS (APS-C)
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Page 2 of 3
Distortion
Many mirrorless lenses rely on digital auto-correction but fortunately Canon follows a more conservative approach.
The Canon EF-M 11-22mm f/4-5.6 STM IS produces a strong degree of barrel distortion at 11mm but at ~3.1% this is way
better than e.g. the 7% that we have recently seen from the (pricey) Olympus M.Zuiko 7-14mm f/4 Pro. The barrel distortion
is substantially reduced at 15mm (1%) and absent at 22mm (funny figure there ...).
Vignetting
Unfortunately there's a skeleton in the closet ... vignetting. It's shockingly high and in fact it's the
worst that we have ever seen in an APS-C format review. A light falloff of 2.9EV (f-stops) at 11mm f/4 is
immediately obvious in most scenes. Stopping down to f/8 is advisable to reduce the issue to a more
manageable degree of ~1EV. The falloff is still pretty bad 15mm @ f/4.5 but, again, quite Ok from f/8 onward.
The situation has eased a bit at the 22mm setting.
Given these findings and similar tendencies from the EF-M 22mm f/2, we are beginning to wonder whether
we are really facing lens issues here. The 24mp sensor of the camera may have a substantial share in all
this. It is likely that Canon simply took the unmodified sensor used in some of their DSLRs and this
may not have been the best idea after all. Mirrorless lenses tend to be a more picky due to their difficult
light transmission characteristics (=non-perpendicular light rays unless you are using a so-called tele-centric
design). We'll see whether this behavior is confirmed once more in our next EF-M review.
Here are two samples taken at 11mm f/4 and f/8. You may notice that the falloff is quite smooth which could
also confirm the sensor as the prime suspect. The vignetting is normally more emphasized in the corners.
The smooth characteristic is less disturbing than a rather abrupt falloff.
 
MTF (resolution)
The resolution figures of the Canon EF-M 11-22mm f/4-5.6 STM IS are more impressive again.
The center quality is excellent throughout the relevant aperture- and zoom-range.
The borders are also very good. The corners are still decent at 11mm although there's a
drop just outside of the zone that we define as corners (somewhat emphasized by a bit
of field curvature). The sweet spot is clearly in the middle range. The extreme image corners
are really sharp here as well. The quality is slightly reduced at 22mm again but the
quality remains fine. Diffraction has a limiting effect from f/11 onward.
The centering quality of the tested sample was good.
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 (CAs)
The amount of lateral CAs is very moderate with an average CA pixel width of around 1px at the
image borders. The issue increases quite a bit beyond that in the corners at 11mm though.
That all being said - CAs are usually taken care of by the camera's auto-correction or
in various RAW converters (Canon DPP, Adobe Photoshop, etc).
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