AF Assist confusion?

Could someone possibly explain to me (relative newcomer to flash photography) what exactly AF assist looks like? I have 2 flashguns: 600EX-RT and 270EX. They both have a habit of emitting a very white burst of light for around half a second or so prior to a shot being taken. I assumed this was AF Assist however the 600EX-RT emits a very brief red beam which I also believe is a focus aid. Please excuse my ignorance.

The better flashes, like the 600 EX-RT, use a couple of high-intensity red LEDs to project an AF assist pattern on the subject. Due to optics in front of the LEDs, the pattern consists of perpendicular lines, and actually allows focusing on a white wall, something the AF system otherwise couldn't do.

Simpler flashes, including those built-in into the APS-C format EOS cameras, don't have dedicated AF assist lamps/LEDs, but instead uses the main flash bulb for that purpose. Much more obvious, and doesn't create any pattern for focusing. Thus a white wall is still an impossible target, just brighter.

If you see a burst of white light from the 600 EX-RT, you've probably triggered the modelling light, something which usually is done by touching the depth of field preview button on the camera.

the white light you are seeing is the preflash that is used for the ettl exposure

Thanks Anders. Quick reply!

tsilva, thanks also...the burst is happening without my finger on the shutter button. If I'm in AL servo mode on my 5Dmkii it happens at will if I move the camera around.

I was just about to make a statement about the pre flash. 1/2 a second before the exposure seems pretty slow to me. Typically we can't see the pre flash because it is too fast for us. I know it is recommended to look through you viewfinder to see if the pre flash is working. If you see a flash that was the pre flash because the mirror is still down. If you don't see a a flash before the exposure the pre flash is not working.

I'm probably over thinking it and it is the pre flash.

Are you pressing the DOF preview button on the camera? This will trigger Canons version of a modeling light. You should be able to turn this feature off in your camera.

2 things - 1st the half-second I mentioned is the duration of the beam, not its time gap from the main flash nb there is no main flash as the shutter button has not been pressed. 2nd - it's def not the modelling light as I have deactivated that in the menu. Weird or what?

oingyboingybob wrote in post #16253238
the burst is happening without my finger on the shutter button. If I'm in AL servo mode on my 5Dmkii it happens at will if I move the camera around.

Classic poor contact with hotshoes/hotfoots.

When contact is lost, there can be a flash dump. This can be seen sometimes if (contrary to procedures) the flash and 622C have not been switched off before unmounting. In the situation you describe, the digital data system resets itself.

Clean all shoe and foot contacts including inside the clamping rails, and make sure that the hotfoot on each device is fully pressed forward before locking down.

Thanks to everyone who took the time to respond. I shall persevere.