Will this allow F8 focusing on 5DMIII

[RGF]

Just wondering if I placed a very thin 3rd party extension tube on the camera and then attached the lens/converters, would I be able to focus at F8 or higher?

[neuroanatomist]

Depends on the lens, I expect.  I've tried stacking two TCs with a 12mm tube between on a 600/4 on my 1D X - only one is reported, phase AF tries and fails, contrast AF works with the 2x in back, fails with the 1.4x in back.

[Mt Spokane Photography]

Yes, it seems to work, depending on the lens.  My 100mmL struggles doing this, but others focus fine.  Of course, you can only focus on things that are close.

[jon_charron]

The firmware that comes out in a few days will allow f8 focusing through the cross point sensors.

[neuroanatomist]

The firmware that comes out in a few days will allow f8 focusing through the cross point sensors.

Not all of them, presumably just the center AF point with 4 surrounding points available as expansion (but not user selectable).

[RGF]

Thanks.