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Unlike on my canon 5Dc, when I use the Canon-M42 adaptor on my 5D3 the focus confirmation is pretty much spot on accurate at f1.4 using the center focusing point. The other ones are not so good and I found out that as the lens is not recognised by the camera, it's switched off all the cross sensors apart from the middle ones. There is a way to program the lens to change the focal length (by default it shows up in exif and to the camera as a 1-65553mm zoom) and although it works when the lens is on my 5Dc, shows up as a 50mm, on the 5D3 is still shows up as that wacky zoom.
I know this is probably a somewhat niche problem but does anyone have any suggestions for activating more of the cross sensor focus points?