Canon EF-M 18-55mm f/3.5-5.6 STM IS - Review / Test - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

Unlike other mirrorless manufacturers, Canon tries to keep its sanity by not fully relying on digital auto-correction but on a decent base design. The Canon EF-M 18-55mm f/3.5-5.6 STM IS does produce strong barrel distortions at 18mm (~3.8%) but these are far from embarrassing levels. At 24mm a mild amount of barrel distortion remains. At 35mm there are barely any distortions and there are just marginal pincushion distortions at 55mm.

Note: Imatest has sometimes a different opinion on this (35mm, 55mm) as you may conclude from the numbers below the images.

Vignetting

The vignetting is somewhat higher than average for a lens in this class. At 18mm it exhibits very strong vignetting at fully open aperture. Stopping down helps but the light falloff never disappears even at f/8 and beyond. The issue is reduced at 24mm but it's still a good idea to stop down to f/5.6, better f/8 - or activate image auto-correction. At 35mm you may still wish to stop down a little but otherwise the issue is well controlled here as well as a 55mm.

MTF (resolution)

The resolution figures of the Canon EF-M 18-55mm f/3.5-5.6 STM IS are fairly decent. The center quality is very high at all focal lengths throughout the relevant aperture range (till f/11). Conversely, the outer image region is very weak at 18mm f/3.5 but improves drastically to good to very good quality when stopping down to f/5.6 already. The corners are still a bit softish at 24mm f/4 but, again, that's no issue anymore at f/5.6. The image quality is evenly high at 35mm and 55mm without being tack sharp though.

The centering quality of the tested sample was good.

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 quite moderate with an average CA pixel width at the image borders around the 1px mark. It peaks at ~1.6px at 18mm f/3.5 - which is visble albeit not extreme. CAs are usually taken care of by the camera's auto-correction or in various RAW converters (Canon DPP, Adobe Photoshop, etc) so there's little to worry about here anyway. CA auto-correction is a lossless procedure.