I have had two or three of the Yongnuo 568 speedlites and use them with a Canon 24–70mm L lens on a Canon 5D MkIII and a 6D.
The problem I have on both is when I zoom in the power is not correct. If I shoot wide open the flash is perfect in E-TTL but if I zoom to say 50mm it gets darker, and if I zoom to 70mm it gets darker still. Is this a zoom issue? It happens on all three of my Yongnuo flashes and on both cameras. I will be delirious with glee if you can help... in the hope I don't have to revert back to Canon flashes.
All my Yongnuos act the same. Surely this cannot be a fault in all three. I am sure I am doing something wrong.