Canon EF 24-105mm f/4 USM L IS II - Review / Test - Analysis
Lens Reviews - Canon EOS (Full Format)

Distortion

The Canon lens produces a strong barrel distortion of 3.2% at 24mm. In the middle range you can observe medium pincushion distortions whereas at 105mm the pincushion distortions are on the verge of getting annoying (1.98%) again. While this characteristic isn't unusual in this class, it is nothing to write home about but at least a tiny bit improved over the mk I (at 24mm).

Vignetting

The Canon EF 24-105mm f/4 USM L IS II has a fairly typical vignetting characteristic for a full format standard zoom lens. At 24mm @ f/4, you can notice a heavy light-falloff of almost 2EV (f-stops). However, the issue is greatly reduced at f/5.6 and not a major issue beyond. The vignetting isn't as pronounced at the other tested focal lengths although stopping down to f/5.6 remains advisable.

MTF (resolution) at 50 megapixels

Unfortunately the lens failed to impress us in the 50 megapixel scope. At 24mm, the center quality is very good at f/4 and it manages to reach excellent levels at f/5.6. Beyond diffraction is limiting the quality already. The border quality is just "Ok" throughout the relevant aperture range. The corners are downright soft at f/4 but at least they are catching up with the mediocre borders when stopping down. The sweet spot of the lens is around the 40mm mark. The outer image region could be better at f/4 but images are nicely sharp across the frame from f/5.6 onward. From here on it's all downhill again ... At 70mm the dead center is still sharp (albeit not bitingly so) at fully open aperture but there's a rapid loss of quality toward the very soft corners. However, if you stop down to f/8 the quality is at least decent. The center quality remains alright at 105mm but the outer image field is soft again and stopping down doesn't really help. All in all this is rather disappointing ...

Honestly, we were a little shocked by these results. The first tested sample also showed a higher than usual optical decentering. Thus we repeated the exercise with a 2nd sample. This one was well centered (albeit still back-focusing like hell) but not really better. The results from this 2nd sample are shown below.

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

MTF (resolution) at 21 megapixels

Why are the MTFs sometimes "better" on 21 megapixels compared to 50 megapixels ? There are two reasons for this. Lateral CAs are lower in terms of pixel widths at 21mp simply because the pixel density is also lower. Extreme CAs that may exist at 50mp are therefore less affecting the measurements at 21mp. Generally we are also using a certain degree of sharpening during the image conversion (just like in real life images) and because the 21mp results are "sharper" on pixel level they are relatively more receptive to (mild base-) sharpening.

The majority of users are still using cameras with a more moderate pixel count so let's have a look how the lens performs with the eased requirements at 21 megapixels - also in order to be able to compare the results to older reviews.

Unsurprisingly the results are "better" across the board. The center quality is generally very good to excellent albeit still not tack sharp (on pixel level). The borders are "improved" with good to very good results. The sweet spot around 40mm remains, of course. The corners are usually quite good as well although stopping down to f/5.6 remains a good idea especially at 24mm and 70mm.

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) are visible at the extreme ends of the zoom range but they manage to stay below an average pixel with of 2px at the borders. The issue isn't quite as pronounced in the middle range.

PS: The CA figures were taken at 50 megapixels.

Bokeh

Standard zoom lenses are rarely perfect when it comes to the quality of the bokeh (out-of-focus blur). However, while the Canon EF 24-105mm f/4 USM L IS II has its rough edges, it isn't all that bad here.

Thanks to the 10 circular aperture blades, out-of-focus highlights have a nice disc shape - that is except at the image borders where you have to live with the usual "cat eyes". The inner highlight disc isn't perfectly smooth but still quite decent. A slight outlining effect is visible at f/4.

The quality of the "general" blur is actually quite good in the critical image background whereas the foreground is more on the rough side here.

Note: the samples were taken at 105mm here.