Let me start with a absolutely geek post by R. Cicala and lensrentals team. They teared down three lenses, among them the Canon EF 24-70mm f/2.8 Mark II (price & specs) (the other two are Nikon 24-70 f/2.8 AF-S and the Tamron AF 24-70mm f/2.8 VC (price & specs)). Tear down in the sense they opened the lenses and had a look deep inside. A pretty technical post that will appeal to the engineers among us. Lots of images of the various steps. Check it out!
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.
Leaningcameras reviewed the Canon EF 24-70mm f/4 IS. In the conclusion they write:
The end result is that this is a great f/4 lens. It has stabilization which works extremely well, it is sharp, produces notably good results overall, and has the macro bonus.
They also made a video comparing the Canon 24-70mm f/4 IS to the Tamron 24-70mm f/2.8 VC:
Another comparison between the EF 24-70mm f/4L IS and the EF 24-105mm f/4L IS comes from Ketan Gajria’s blog. Samples and crops provided.