Master flash not turning off

[bjyoder]

I was out shooting the other night, and came up with this problem: When I am using OCF, the master unit situated on camera will fire to light the exposure even when the display on the flash shows that the main unit's flash should be off.

I think this is more of a camera problem, as I tried both my 580EX II and 580EX, and both suffered the same problem, but I figured I'd post here to start since the problem seems to fit here a bit better.

The only thing I noticed different was that, when using the 580EX as my master, the icon next to the IR Transmitter (on the flash display) would blink. I have not used this flash as master before - relying solely on the 580EX II for that duty - so this may be normal.

Here is a sample shot. No editing other than some noise reduction. I had my 430EX II as a slave above the grave stone with a dark blue gel on it (seen in the background grass). The master unit was set for master, and for the flash not to fire; obviously it didn't listen:
http://i396.photobucket.com/albums/pp44/bjyoder913/POTN/flashflashflash.jpg

I have been through the custom function menus and cannot think of what this could be. Hopefully it's something simple that I'm just overlooking. TIA

[Curtis N]

The master fires, while the shutter is open, at a very low power, to "command" the slave. Even when it is set to "master off."

This usually does not affect your exposure significantly, but if it does, aim the flash directly at the slave and put some sort of snoot on it to prevent its light from reaching the subject.

[bjyoder]

I appreciate the response, but this seems to be more than the command flash. In my past experiences, this problem has not shown itself to this extent. The light was very rapidly fading, and the resulting exposure should have rendered the stone blue.

[Wilt]

EXIF said 'Flash fired, compulsory flash'

[Curtis N]

What was the approximate camera-subject distance, aperture and ISO for this shot?

When you're close and you crank up the ISO, the amount of flash power required to affect the exposure is very small.

[bjyoder]

Make - Canon
Model - Canon EOS 40D
ExposureTime - 1/30 seconds
FNumber - 2.80
ExposureProgram - Manual control
ISOSpeedRatings - 800
ShutterSpeedValue - 1/30 seconds
ApertureValue - F 2.80
ExposureBiasValue - 0
MeteringMode - Multi-segment
Flash - Flash fired, Compulsory flash mode
FocalLength - 35 mm

I was about 3.5-4 feet away from the stone.

I was thinking my shutter speed was faster, so seeing that, you could be right. I just thought the control flash wouldn't effect the exposure that drastically.

[Curtis N]

Some calculations, just for yuks...
GN of the 580EX II zoomed to 50mm is 42 meters or 139 feet at ISO 100.
That's 392 feet at ISO 800.
Divide by the aperture...
So at full power the 580EX II will illuminate to 139 feet at f/2.8, ISO 800.
70 feet at 1/4 power
35 feet at 1/16 power
17.5 feet at 1/64 power
8.75 feet at 1/256 power
4.38 feet at 1/1024 power

So even if the command flash is less than 0.1% of full power, it throws enough light to illuminate your gravestone at that distance.

[PacAce]

Make - Canon
Model - Canon EOS 40D
ExposureTime - 1/30 seconds
FNumber - 2.80
ExposureProgram - Manual control
ISOSpeedRatings - 800
ShutterSpeedValue - 1/30 seconds
ApertureValue - F 2.80
ExposureBiasValue - 0
MeteringMode - Multi-segment
Flash - Flash fired, Compulsory flash mode
FocalLength - 35 mm

I was about 3.5-4 feet away from the stone.

I was thinking my shutter speed was faster, so seeing that, you could be right. I just thought the control flash wouldn't effect the exposure that drastically.
Listen to Curtis. He is right. ;)

Re the shutter speed, as long as it's at or below max sync speed, it will not affect the flash exposure one bit. It's the aperture and the shutter speed that will and you have yours set so that the command flash will affect your image.

[Curtis N]

Listen to Curtis. He is right. ;)

Re the shutter speed, as long as it's at or below max sync speed, it will not affect the flash exposure one bit. It's the aperture and the shutter speed that will and you have yours set so that the command flash will affect your image.I'm guessing you meant to write "aperture and ISO?"

[PacAce]

I'm guessing you meant to write "aperture and ISO?"

Oops! Yes, that's what I meant but my fingers never type what I'm thinking. Thanks for pointing that out. :o

[bjyoder]

Got it. Thanks so much guys! :)

The more I read this and thought about it, the more I can understand it. Just never thought the command flash would have that impact, but I've never used OCF quite that way (in such dark surroundings).