Distortion

Move the mouse cursor over the focal length text marks below to observe the respective distortion 24mm 40mm 70mm

The chart above has a real-world size of about 120x80cm.

Vignetting

Field Curvature

Let's assume a perfect lens - its focus field would be perfectly flat so the famous "brick wall" test scene would be in focus from the very center all the way up to the extreme corners of the image field. Depending on the chosen aperture you'd still have a sharp zone in front and beyond the focus plane - this zone is called the depth-of-field. However, in the real life such perfect lenses do not exist. The focus "plane" is never really a plane but more or less (radially) distorted. Normally the amount of focus distortions or "focus field curvature" isn't overly significant but there're some lenses where the field curvature is much more pronounced and, unfortunately, the Canon EF 24-70mm f/2.8 USM L is among them.

At the wide-end of the zoom range the outer focus field is distorted to the background resulting in the unfortunate effect that the foreground corners are pushed out of focus compared to the center (or vice versa). The following illustration may help to visualize this kind of field curvature:

/ We've experienced the problem in the lab but it's also very obvious in field conditions. The scene below has been taken at 24mm f/10. Now at f/10 you'd normally just rely on the AF which suggests a near-infinity focus distance here. You'd waste some depth-of-field by this but normally everything would still be sharp at this setting (based on this kind of scene). The result can be seen BELOW the sample scene. The extreme corners are blurry - despite f/10 - whereas the lower center is perfectly sharp.

Remember that we mentioned that the outer focus field is distorted towards background ? Well, the solution is to "pull" the focus field towards you. The sample crops ABOVE the sample scenes show the effect of this action - the corners are pulled into focus and the center remains sharp (within the depth-of-field). This "workaround" is viable at small to medium aperture settings but it does naturally fail at large apertures where the depth-of-field is not sufficient for such an approach anymore.

MTF (resolution)

The Canon lens has a mixed but still decent resolution characteristic across the range. The center performance is very good at 24mm f/2.8 and it reaches excellent levels from f/4 till f/11. The border quality is also very good at max. aperture which is quite an achievement for a full format lens and it continuous to increase slightly towards medium apertures. The extreme corners are relatively soft at f/2.8 but they improve gradually the more you stop down. At f/8-11 they're also on a very good level (just). The sweet spot of the lens is located in the middle part of the zoom range. The lens is already very sharp across the image field at 40mm @ f/2.8 and the local peak performance is reached between f/5.6 and f/8 here. There's a quite pronounced drop in performance at 70mm. The borders and corners are both quite soft here and the center isn't all that stellar either at f/2.8. However, the results get pretty snappy at medium aperture settings.

It should be mentioned that the Canon EF 24-70mm f/2.8 USM L is a bit of a personal nemesis. A couple of years ago I tested the lens on the old EOS 350D (APS-C) and it took me 4 (f-o-u-r) samples of the lens to get a good one - please note: "good", not a "great" sample. The first three variants showed rather hefty centering defects which spoiled the results quite a bit. Actually I didn't really want to test this lens anymore but we were able to get our hands on the "good" sample again so ...

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)

Bokeh

The background blur is very smooth and buttery resulting in a very pleasing bokeh in many "mainstream" scenes. However, the foreground blur isn't quite as harmonious. Out-of-focus highlights show a near circular shape at large aperture settings but the inner zone of these "discs" is quite busy/nervous (see also the "meadow" in the sample image section). All-in-all the bokeh may not be flawless but it's still decent for a standard zoom lens (such lenses are rarely perfect here).

Bokeh Fringing / Longitudinal Chromatic Aberrations (LoCA)

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs f/2.8 f/4 f/5.6

These sample portions were taken at ~70mm.