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Hello,
Just brought the 7D and have a question about the wireless flash. I use it as a master and the 580EXII as a slave. When I fire it the popup flash fires and then the slave. I am just wondering if this effects the exposure of the pictures taken at all. I'm thinking that the popup flash that fires from teh 7D does not effect the exposure but I am jsut wondering why it fires..
any thoughts?
Thanks!
I'm not an ETTL expert, but I think the answer to your question is this--the pop-up may be contributing to the exposure, and maybe not. It depends on how you have it set up. However, I will say that the initial burst of flash you see coming from the pop-up isn't contributing to the exposure. The initial burst is used to communicate information from the master flash (the pop-up) to the slave (580EX, in this case). However, once the inital burst is finished, the pop-up may fireat the same time as the 580EX thereby contributing to the exposure ifthe pop-up isdesignated to do so through the menu system. For testing purposes this weekend, I had the pop-up set to contribute to exposure in a ratio (designated by me) while using the 580EX as a slave flash.
From the 7D manual (p. 121) , "The slave unit is controlled by the built-in flash's light pulse signal." Translation - the popup flash must fire to trigger the slave(s). It's normal, and the amount of light doesn't contribute meaningfully to the total exposure if you're set up to fire the remote flash only. I suppose it could cause an issue with a reflection in the image, perhaps as a small additional catchlight in an eye, etc. Unfortunately, there's no way around it short of additional hardware (ST-E2 which sends out an IR-only optical pulse to control the slaves, or a radio triggering system like Pocketwizards, etc.).
Interesting. Thank you for the comments.
I am just worried that the popup flash does contribute to the exposure and make it harsh and direct which defeats the point of offcamera flash. I
A couple of things could be happening:
1) The pop-up is set to only trigger the slave flash and not contribute to exposure.
2) The pop-up is set to trigger the slave flash and contribute to exposure, but the ratio is such that the pop-up merely fills in the shadows a bit and therefore the shot still looks quite good.
In your On-Camera Flash menu, there should be an option for it to be enabled or disabled; note that even if it is disabled, it will still flash. The flash is a pre-flash that sends out the ETTL settings to the slave unit, and this occurs in the moment before the shutter is released. If your on-camera flash is enabled it will fire twice in rapid succession, once to send out the ETTL signal and again to contribute to the exposure. It happens fast enough to look like a single flash.