NB!! It is the Eg-S (6D version) that works on the 5DIII, not Ec-S!
Ahhh!! that explains why I couldn't force my Ec-s into the 5DMK III cavity.
Will order the Eg-s now & try that.
Sorry I was a bit unclear on that to begin with. I hope you didn't break anything ...
5DIII, 1DX, 8-15/4L, 16-35 f4L IS, 24-70/2.8L II, 70-200/2.8L IS II, 70-300/4-5.6L IS, 200-400/4L IS 1.4x, Zeiss 15/2.8, 17/4L TS-E, Zeiss 21/2.8, 24/3.5L TS-E II, Zeiss 35/1.4, Zeiss Otus 55/1.4, Zeiss Otus 85/1.4, 100/2.8L IS Macro, Zeiss 135/2, 600/4L IS II