questions about features of 600EX-RT

I've had my 600EX-RT for about a week now and am currently using it as a studio light for gelling backgrounds. I have a couple of questions I'm hoping someone knows the answer to.

1) When mounted on my flash zebra hot shoe (for use with my cybersync receiver) it occasionally turns off. Is this the 90 second auto off feature? Its a pain because in order for it to fire again, I have to trigger it via the test button on my receiver, which means getting on the floor where my light is positioned. Does Custom Function 1 disable this?

2) What specifically are the External Flash Metering A/M modes for? I've read those sections a dozen times and it still doesn't make sense to me. When would you want to use them?

3) Does the 600EX have a feature to prevent damage from overheating? I've read the section on limiting number of flashes in a short period of time, but when you're shooting in a studio sometimes its hard not to get trigger happy.

Yes, the auto off feature can be disabled with a custom function.

External metering uses the light sensor on the flash itself to determine the proper flash power. This is different from the E-TTL II system, where the power is determined prior to exposure, by having the camera meter the pre-flash. When using external (to the camera) metering, the flash itself is instead metering the flash on the fly, while it's burning to illuminate the subject.
The auto/manual versions of external metering are for using this mode when the flash is on the camera versus elsewhere. The auto mode gets information from the camera about aperture and ISO. But when the flash isn't in the hot shoe, that's not possible, so in the manual mode, you have to set thes values yourself, on the flash. External manual can be used to provide automatic flash power regulation even when triggering through a "dumb" sync cable, for example.

The 600 EX-RT should, just like the 580 EX II, refuse to fire when there's risk for overheating.

apersson850 wrote in post #15475815
External metering uses the light sensor on the flash itself to determine the proper flash power. This is different from the E-TTL II system, where the power is determined prior to exposure, by having the camera meter the pre-flash. When using external (to the camera) metering, the flash itself is instead metering the flash on the fly, while it's burning to illuminate the subject.
The auto/manual versions of external metering are for using this mode when the flash is on the camera versus elsewhere. The auto mode gets information from the camera about aperture and ISO. But when the flash isn't in the hot shoe, that's not possible, so in the manual mode, you have to set thes values yourself, on the flash. External manual can be used to provide automatic flash power regulation even when triggering through a "dumb" sync cable, for example.

Okay, I follow. But its still not clear when you would prefer this mode (assuming automatic) vs. ETTL II.

Disable auto off in the flash menu - Custom Function 01

I highly recommend watching this if you have a 600EX-RT - http://www.youtube.com​/watch?v=KpxTjmlEsm8

coeng wrote in post #15475910
But its still not clear when you would prefer this mode (assuming automatic) vs. ETTL II.

A few years ago, so many photogs were complaining about E-TTL and saying their old-fashioned, thyristor-controlled flash units were more reliable and consistent than E-TTL. I think Canon added that functionality to the 580EX II to answer those people.

Then there were complaints that the external metering on the 580EX II was consistently underexposing and the user had to dial in +2 FEC to get it to expose properly. I experienced this myself.

If you're going to use that option, you need to remember that it has a fixed field of view for flash metering. It's likely to overexpose if you're using it for outdoor fill since the background is too distant to bounce light back into the flash. Maybe try it indoors if you want to experiment.

I think, over the years, Canon has improved their E-TTL system and event shooters have learned to use it better. I never use the external metering option because I've finally figured out how to get the results I want with E-TTL.