the tubes report aperture and retain AFsorry i forgot to mention i have those extension tubes too that report aperture, i get the same error with the extension tubesKenko extension tube 36 and 400-2,8 IS and Canon EF 2,0x III Extender, no problems for me on my 5Dmk3
Af works fine for me
thanks mate that confirms that something is really flakey with my unit
if i put the tube and a lens on it displays 00 for aperture
then take it all off and put the lens only on turn the camera back on it still wont register the lens
have to take the battery out and put it back for it to work.
I think an "extension tube" is something different then a TC. So you cannot say: "Because an extension tube (of someone else) works on the 5DmkIII, the TC (of my own) should also work on the 5DmkIII".
That suggest that it is a problem with the connections. Most extension tubes (at least the ones I own/owned) supplied a direct connection between the contacts on the lens and contacts the camera. No electronics involved (but I don't know about your tubes). I think you might want to check if the connections are good.
I once owned a Sigma TC which gave the same problem. If I attached the lens (a Sigma 70-210/2.8 in this case) with the TC and locked it in place it gave a lens failure (aperture 00, etc). But when I twisted the lens and TC a little back and forth (there is always some slack) it sometimes started working. I tried to clean the contacts of the TC which gave some results. Not enough though so I stoped using the TC (I didn't want to short ciruit my camera). For me it was a easier decision because it was a 2x TC which had quite a impact on IQ.
Of course getting support is quite impossible when using a third party part: Canon 'blames' Sigma and Sigma 'blames' Canon. Same with Kenko ...
if i put the TC or tubes on the 5dmk2 or 1D they work fine
on the 5dmk3 from day 1 just displays aperture 00 so it doesn't seem to be a problem with the kenko parts
kenko suggested cleaning the contacts too but no joy there