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

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

cplunk wrote:

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?

My guess (emphasize: ﻿guess﻿) is, when using wireless flash, it doesn't make a bit of difference whether the in-camera setting for flash is ADI or P-TTL: Just as when you use a single flash in the hot shoe and you turn the flash head off straight forward, you get P-TTL even if the camera setting is ADI, so here, I think it's got to be P-TTL. The camera can't measure the distance of the remote flash from the subject. I mentioned ADI only because I was trying to be aware of everything that could possibly be a variable, whether I understand why it's a variable or not.

Now I don't know exactly how the metering is done in wireless flash mode. I wasn't trying to take a "good" exposure here in the normal sense. I was only trying to see how much light was coming from the on-board, control flash unit, as compared to the amount coming from the remote. That's why I turned the FEC down to negative 3: so that the remote flash could do anything it needs to communicate with the camera but so that it's final output will be minimal.

Will

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