Speedlite 600 ex-rt flashes rapidly & then camera won't focus

[simplyouphoto]

What causes a speedlite to fire rapidly, almost like a strobe light, while on top of the camera angled at the ceiling? Afterwards, it take a bit for my camera to do anything (Mark ii 5D)...won't focus or anything, like it's thinking or something. It really is annoying & I'd like to avoid it. 

[amfoto1]

I don't have 600EX, but do have 5DII that I use with several 580EX II and 550EX flashes.

I suspect one of two things is happening...

First possiblity, many times a Canon flash will fire a very brief "pre-flash" that it uses to determine correct exposure. It's usually very quick and low powered (about 1/64 of full power, if memory serves). This might be what you are seeing.

However, since you also are stating that the camera won't focus, it's possible that the flash is trying to provide Focus Assist, and because you have the flash oriented for bounce, it's failing to achieve focus. I'm not sure this is what's happening, though... The Focus Assist from all my flashes is done using a rather faint, near IR grid that's projected onto the subject, to help the camera achieve focus. But, on the other hand, the uilt-in flashes on many Canon cameras (which 5DII doens't have, of course)  use a short, fast series of white light flashes to provide Focus Assist. I don't know which method 600EX use, so this may or may not be what you're seeing.

I'd encourage you to stop trying to do bounce. It introduces a lot of uncontrollable variables such as the distance and color of the bounce surface, forces the flash to fire much more fully to cover the far greater distance as well as compensate for light absorbed by the bounce surface, which in turn makes the flash much slower recycling.... And (I hadn't though of this before) might cause exposure or Focus Assist problems.

Use direct flash. Put a diffuser on it, if the flash is too harsh. The only way I'll bounce a flash is using a bounce card mounted on the flash. Putting the flash on a bracket with an off-camera shoe cord also improves the look of flash a lot, hleping to reduce redeye and shadow issues in particular.

Also it should be possible to disable the Focus Assist feature... I think it's in the menu of most camera models.

Hope this helps!

***********

Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER 

[simplyouphoto]

I think you're probably right. The quick burst of flashes has to be to help focusing because it happens mostly in low light situations. 

I use a bounce card & it's awesome for fill & in daylight, that spot of light in the eyes. For direct flash in dark rooms such a a reception, where you can't really run around with off camera flash, what is your favorite diffuser? I have a wescott & I'm not really impressed but it's pretty old. 

[TCampbell]

You are probably triggering this by hitting the "modeling flash" button.  If you hit the "depth of field preview" button (on the front of the camera, just below the lens release button -- near the "Mark II" badge) when a flash is on the camera it will cause the speedlight to flash very rapidly for about 1 second.  This is the "modeling flash".  

The 600EX-RT has a better focus-assist beam built into the base of the flash (behind the red lens.)  Make sure you've enabled the focus-assist beam on the flash.  It's a much better way to perform auto-focus when there isn't enough light.

Check TWO different settings...

With flash attached and powered on, hit "Menu" and navigate to Gold Wrench #3 across the top.  

Scroll down to and select "External Speedlite Control"

Scroll down and select "Flash C.Fn settings"

Using the rear dial, scroll to setting "2" (Flash C.Fn setting #2) which is named "Modeling Flash". 

-- This is what causes the flash to pulse very rapidly to create a near-constant light that lasts about 1 second.  The default is setting "0: Enabled (DOF preview button.)"  You can see the other options listed (press the "Set" button in the middle of the rear-dial then use the rear-dial to change the setting to a different value if you wan to change it.)  If you are inadvertently triggering this then you can either disable it or move it to a different button.

Using the rear dial, scroll to setting "8" (Flash C.Fn setting #8) which is named "AF-assist beam firing".

-- Make sure this option is set to "0:Enabled".  This allows the AF-assist beam located in the lower half of the flash to fre the red focusing pattern.  If this is set to "1:Disabled" then the beam will not fire and camera will not be able to focus in the dark without adding more light.

Assuming you've set Flash C.Fn 8 to enable the AF-Assist beam, give this a quick test.  In a dark environment, point the camera at a subject and auto-focus the camera.  You should see the beam fire a red pattern of light on your subject.  The beam is behind that red lens on the lower half of the flash and, since it's on the lower half of the flash, it will fire the beam forward even if the flash head is pointed in a different direction.