Strobist Question

I tried this topic before but got no takers.

I am a fan of Sly Arena and have read/studied his book "Speedlighter's Handbook"
and seen some of his classes at Kelby Training.

One question I can not find an answer to is:

When using canon off camera flash (580EXII) with an A and B channel, and using ETTL.
The preflash fires from the master and this calculates the correct/predicted best flash exposure.

When I use A and B. Assuming the ratio is set to 1:1.
Is 1/2 of the calculated flash exposure sent to A and 1/2 to B
or does each channel get a full dose?

What if I have multiple flashes set to B?

What if A is set to flash 'disable' and only fires the preflash?

I know very little about ETTL matters, so I am of no help. :-)

Sean
Are you doing all your flash in manual flash exposure mode?

I keep my camera in manual mode 99% of the time, and my strobes the same. Every now and then, I'll be in a "run & gun" situation where ETTL is best. However, most of the time, it's not what I use.

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When I use A and B. Assuming the ratio is set to 1:1. Is 1/2 of the calculated flash exposure sent to A and 1/2 to B
or does each channel get a full dose?

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When using ratios set at 1:1 Both flashes have an equal output. If set 1:2 then one will flash at half the power of the other.

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What if I have multiple flashes set to B?

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Then they will both have equal output if ettl or selected output if manual.

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What if A is set to flash 'disable' and only fires the preflash?

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The A will only fire a pre-flash but will not be used in the actual exposure or when the shutter is open. All of the flash calculations/adjustments will be sent to the B channel if in Ettl

Hope this helps. BTW, Sly Arena does a very good job of explaining this in on of his B&H Videos here: http://speedliting.com/how-to/canon-...+(Speedliting)

Maybe I am not able to explain my question clearly.

Lets say there is only one speedlight and say we could put a value on the expected 'power' needed for the determined flash exposure.
Lets say that has a value of 10.
One speedlight, one preflash, determines a 'power' of 10 is needed for correct flash exposure.
So the single flash fires at 10.

Then take the same situation but have two flashes, one on and one off camera, set ratio to 1:1
Ok preflash, determine a power of 10 is needed for correct flash exposure,
Does each flash get a 10? or is it divided 5 and 5.
If it is not divided how do we keep from getting over exposed.
If there were 10 flashes would each get 10 then we have 10x10 and we would be way overexposed.

Discussion:
My guess is each flash get a power of 10.
Also the master does not have any idea of how may other flashes are out there, there is no feed back from the slave.
Also the master has no idea of how far the slaves are from the subject.

10 and 10, or 5 and 5. The thing is that you don't really know the output, except that they're equal... The camera is going to behave as if you are using a single flash.. It won't blow it out, but it won't under expose, so distance of your various flashes will still have an impact on the photo, BUT it is going to adjust both flashes equally until it "sees" the correct exposure.

For example, you like the amount of light on group A, but group B is darker than you'd like, you can simply move group B closer to the subject. But of course, you have more flexibility by just using a different ratio.

Short answer: They're the same power, at whatever power level makes your camera think it's properly exposed.. without giving any indication of actual output. And THAT is the reason why so many strobists shoot with manual control.

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Originally Posted by Rocco

10 and 10, or 5 and 5. The thing is that you don't really know the output, except that they're equal... The camera is going to behave as if you are using a single flash.. It won't blow it out, but it won't under expose, so distance of your various flashes will still have an impact on the photo, BUT it is going to adjust both flashes equally until it "sees" the correct exposure.

For example, you like the amount of light on group A, but group B is darker than you'd like, you can simply move group B closer to the subject. But of course, you have more flexibility by just using a different ratio.

Short answer: They're the same power, at whatever power level makes your camera think it's properly exposed.. without giving any indication of actual output. And THAT is the reason why so many strobists shoot with manual control.

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Rocco, Thanks for answering this----You have some killer teaching skills.