I have a 600ex-RT and when it is turned on and not connected to one of my Cameras, 6D and 70D, and it is in ETTL mode. When I attach it directly to the Cameras and press the shutter release button it goes into TTL mode and the flash is fired at full power. I go into the C.Fn settings , #5 is already set at 0: (For ETTL) but I select it again and then the flash reads ETTL but as soon as I fire it again it goes back to TTL . What is a possible cause for this problem? Thank you Kindly
This sounds like a communication problem. On the foot of the flash, there are a number of contacts ... there are springy (each pin should be able to be pushed and should bounce back down.) Make sure nothing is jamming any pin.
Also... make sure the foot is snug on the flash. I've never known a 600 series flash to have this issue, but the 580 EX II had an issue where some internal screws would come loose and cause the foot to not make good contact with the hot shot. If you attached that flash to the camera you would notice that it had quite a bit of play (you would wobble the flash more than should have been able to wobble it once it was snugged down.) The "fix" was to simply re-snug the screws.
One other thing that occasionally happens is the flash might not be fully seated on the camera. Make sure the foot slides ALL THE WAY forward into the hot-shoe and that you flip the locking lever until it actually does "lock". Once locked the flash should refuse to slide in the shoe (there is actually a locking pin which pushes into a tiny hole so it really is physically "locked" and not merely snugged tight.) If it's not slid into and fully locked then the pins on the foot wont align with the contacts on the shoe.
If you are convinced you are fully seating the flash into the shoe and locking it then it may have a defective contact (or perhaps some other defect) and require service. Contact Canon service.
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