How much light does CNTL flash "spill" into scene?

Started Jan 10, 2013 | Discussions thread
cplunk
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Re: How much light does CNTL flash "spill" into scene?
In reply to William Porter, Jan 10, 2013

William Porter wrote:

So, using the Sony wireless flash system, with a flash on the camera acting as control or triggering flash, and a remote flash "out there" as the unit being triggered, the question arises, how much flash does the control contribute to the scene?

It's my understanding that the answer is supposed to be None. The on-camera control flash is supposed to act as a trigger only, and is not supposed to add its light to the scene.

But, basic understanding of physics should explain to you that's impossible.

Now I've heard a number of people in this forum and elsewhere say that this is basically how it works, but that there may be a little spillage from the scene if you're really close to what you're shooting. In other words, some posters admit that it can be a problem but insist that the amount of light added is small and that you have to be really in the subject's face for it to matter.

The further from the flash, the less effect it has, regardless of power. Wireless flash or otherwise, it's still light, inverse square law applies. So if you move from three feet to six feet, you'll loose 75% of that light. Twelve feet, 75% of that 25%.

So reading through your tests, I'm curious about two things that I don't really understand and how they might effect your tests.

  1. How does a wireless remote work with ADI? ADI takes the distance, measured from focusing, and figures out the amount of flash based based on that? With a certain aperture, ISO, a quick calculation should tell you the amount power the flash needs to output based on the distance, using that inverse square law. But, the camera has no idea how close, or far that wireless slave is so how could it ever determine the power required for that?
  2. With TTL, the camera fires the flash, figures out how much power it needs metering through the lens, then opens the shutter and fires at that power. Wirelessly, I'm assuming, the controller must tell the slave to fire, the camera meters that, decides how much the power it needs the slave to actually fire, tells the slave that, shutter opens, command to slave to fire. Does that sound about right? But if there is no slave for the preflash, what's it do? It has nothing to meter from the slave, so what does it do?
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