Canon EF 24-105mm f/3.5-5.6 STM IS - Review / Test Report - Analysis
Lens Reviews - Canon EOS (Full Format)

Distortion

The Canon lens produces a strong barrel distortion of 3.4% at 24mm. In the middle range there's a slight degree of pincushion distortion whereas at 105mm the pincushion distortions are on the verge of getting annoying (1.9%). While this characteristic isn't unusual, it is nothing to write home about either.

Vignetting

The Canon EF 24-105mm f/3.5-5.6 STM IS has a fairly typical vignetting characteristic for a full format standard zoom lens. At 24mm @ f/3.5, you can notice a heavy light-falloff of almost 2EV (f-stops). However, the issue is greatly reduced at f/5.6 and not much of a deal beyond. The vignetting isn't quite as pronounced at the other tested focal lengths although stopping down will improve the results here, of course.

MTF (resolution & chromatic aberrations)

The resolution characteristic of the Canon lens is above average in its class. The center performance is generally excellent till f/8. The border quality is good to very good. Interestingly the extreme corners are slightly better than the borders at 24mm although this advantage can't be kept at longer focal lengths. That being said, it is worth to note that while the field curvature may be negligible at close to medium focus distances, it increases significantly and noticeably at (near-)infinity in the 24-28mm range. In the real life the corners are often pushed out of focus. Beyond f/8 diffraction is a limiting factor but f/11 remains easily usable while the effect is more obvious at f/16 and beyond.

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)

Lateral chromatic aberrations (color shadows at the image borders) reach an average pixel width of around 1-1.5px at the image borders. Again, this is nothing out of the ordinary. The CAs in the extreme corners are still somewhat worse than that at the 24mm setting though.

Bokeh

Due to the small max. aperture the out-of-focus blur is somewhat underdeveloped and the lens is certainly not meant for serious portraits photography. Nevertheless you can achieve a shallow depth-of-field when choosing short object distances - after all the lens can focus down to a max. object magnification of 1:3.3.

In this context, the bokeh quality is about average for a standard zoom lens. Out-of-focus highlight discs show a slightly "onion-like" substructure and an outlining effect. As so often the disc shape deteriorates towards the borders/corners at "large" aperture settings. Stopping down helps to improve these "cat's eyes" but the more edgy aperture shape is getting more obvious at f/8 already.

As far as the general out-of-focus blur is concerned - while it isn't silky smooth, it is actually quite good for a lens in this class.

Note: the samples were taken at 105mm here.