I have been using the Kipon EF Mount Adapter on my Olympus EM1 using both the Canon 100-400mkII L f/4.5 and the Canon 70-200mkII L f/2.8 with interesting results.
I have found that of the lenses that Kipon didn’t test, the 100-400 performs better on speed of focus, and locking focus than the 70-200mkII. This includes when using 1.4xMkIII extender.
Lighting: HUGE thing they don’t mention is that you have to have nice bright ambient light to use autofocus, but need to switch to manual focus in dimmer light. I suggest using a flash if you have to do this.
Also, with or without the 1.4x extender, when using the 100-400, when zooming about 50 to 60% (200mm to 300mm) the image in the viewfinder goes dark, f stop misreads to 0.09. It does return normal when I get to the full 400mm zoom.
Focus Speed: Today, it was cloudy, rather dim ambient light. The 100-400mkII focusing speed is fractionally slower than using the Olympus Lens with the MFT adapter. The 70-200mkII is extremely slower, if it would focus at all.
I think part of the problem with this is that it’s the “Mark One” adapter, mostly tested with older lenses. You have to really dig to find out which lenses were tested with this adapter before it was put out on the market. The older Canon L glass were tested as were the ‘kit’ lenses. Who knows, maybe Mark Two will be address these issues.
Do not take this to mean it’s of no use. For the price, it’s a great way to extend what lenses you can use with your MFTs kit. Just remember, if you’re using the MkII lenses, make sure it’s nice and bright outside and expect it to be a bit slower to focus.