ISO 12233 resolution chart, distortion and vignetting test results, along with specs and measurements, have been added to the Canon EF 24-70mm f/4 L IS USM Lens Review
Here are some of my initial comparative thoughts based on multiple copies tested:
You will see very low distortion from the 24-70 f/4 L IS relative to similar zoom lenses. Noticeably less than from the 24-105 f/4 L IS - significantly less at 24mm. Landscape photographers shooting over a flat horizon (such as water) will especially appreciate the difference at 24mm, though the 24-70 f/4 L IS still has some barrel distortion.
The 24-70 f/4 L IS is nearly void of CA (Chromatic Aberration) until the longer end of the focal length range where mild CA becomes apparent at 70mm. The 24-70 f/4 L IS has noticeably less CA than the 24-105 f/4 L IS at the wide end, but more at 70mm.
The 24-70 f/4 L IS is similar to the 24-70 f/2.8 L II in regards to both CA and distortion.
The 24-70 f/4 L IS has less vignetting than the 24-105 f/4 L IS at the wide end at f/4, but more at the long end. By f/5.6, the two lenses are close in this regard. The 24-70 f/2.8 L II, with its 1-stop wider aperture, has the comparable-aperture vignetting advantage until about f/8 where the two are nearly equal.
If you are shooting at f/11, the 24-70 f/4 L IS, 24-105 f/4 L IS and 24-70 f/2.8 L II lenses produce essentially equal sharpness. The 24-105 has the longest focal length range to its advantage in this comparison. The 24-70 f/2.8 L II lacks IS.
At 24mm with a wide open aperture, the 24-70 f/4 L IS bests the 24-105 L IS in sharpness by a modest amount. The 24-70 f/4 L IS gets softer (especially in the mid and peripheral image circle) by 50mm f/4 where the lens performs its worst.
That this lens performs its worst at a mid-focal length vs. an extremity focal length is unusual. Since most people shoot their highest percentage of images at the focal length range extremes of a lens, the 24-70 L IS weakness is perhaps well placed. Roger at LensRentals.com has confirmed the 50mm weakness in his vast stock of this lens.
The 24-105 L delivers a sharper image at 50mm f/4. Sharpness improvement at 70mm brings the 24-70 f/4 L IS back up to near equality with the 24-105 L. These two lenses perform more similarly at f/5.6 and at f/8, results from these two lenses are nearly comparable. Again, the results are similar at f/11.
The Canon EF 24-70mm f/2.8 L II USM Lens
is the sharper lens of this group - even at f/2.8 vs. f/4.
Overall, the Canon EF 24-70mm f/4 L IS USM Lens appears to be a great lens, but ... perhaps overpriced with the bargain-priced Canon EF 24-105 f/4 L IS Lens