Question about flash metering/exposure/output

Hello,

I am getting more and more into discovering the amazing possibilities of using flash but am having a difficult time understanding a key technical element of it. I use TTL flash exclusively (at least for now) and my question has to do with the metering and output of the flash.

I understand that you use your camera's shutter, aperture and iso pretty much normally to expose for the ambient light, and then the output of the flash determines your subject's exposure. So how does the flash know what your subject is? Is it measuring the spot where you locked focus? Is it measuring the wherever the center focus point happens to be at the time you snap the picture? What if I lock focus then recompose and move the camera - where is the flash now metering from? And is the brain behind TTL trying to reproduce middle greys just as your camera's meter is on a non-flash exposure? Which would lead me to believe that in that case I would increase the FEC for the "white dog in the snow" and decrease the FEC for the "black dog in the coal mine."

I want to start to understand what the flash is actually doing (or trying to do) so that I can more acutely focus on and adjust for my primary goal. Right now it's a lot of trial and error.

Thanks!

Your flash settings in the camera have two settings. Averaging and evaluative. So you can select how the flash is metered in ETTLl to some degree, nothing as precise as spot metering would be for the ambient. I'm no expert on all of the elements that are taken into account in evaluative but it does give weight to focus area, distance (with a reporting lens), and is weighted. Averaging does just that tries to average across the frame. But to your point about white dog in snow. The flash is still subject to the same vagaries as the reflective metering in your camera. So it assumes average reflectivity and you will have to use flash exposure compensation to accommodate those difficult subjects.

You may find this of interest:

http://www.tom-morrow-land.com/tests/fillred​/chuckw.htm

Here is another absolutely critical first place to start (hint - always look at the sticky posts):
https://photography-on-the.net/forum/showthre​ad.php?t=138907

Thank you for the replies. I have read a few of those articles in the past, and just re-read again last night. There's something that is still not clicking for me, however.

I think I can best describe my confusion as this:

-Let's say I am shooting with bounce flash off a ceiling or wall (as I often do).
-I'm in manual mode on my camera and ETTL on my flash.
-Assume I adjust my camera's exposure using whatever combination of aperture, iso and shutter speed results in the light meter being centered (what the camera determines to be a "correct exposure).
-Now I press the shutter and the flash does its pre-flash and calculations to fire what it thinks is the "correct exposure."
-If the camera already measured a "correct exposure" with its light meter, then why would the flash fire at all?

From what I understand, the TTL technology is being used to balance the overall exposure; but if the overall exposure is already balanced then what is the flash trying to do?

RandMan wrote in post #16413472
Thank you for the replies. I have read a few of those articles in the past, and just re-read again last night. There's something that is still not clicking for me, however.

I think I can best describe my confusion as this:

-Let's say I am shooting with bounce flash off a ceiling or wall (as I often do).
-I'm in manual mode on my camera and ETTL on my flash.
-Assume I adjust my camera's exposure using whatever combination of aperture, iso and shutter speed results in the light meter being centered (what the camera determines to be a "correct exposure).
-Now I press the shutter and the flash does its pre-flash and calculations to fire what it thinks is the "correct exposure."
-If the camera already measured a "correct exposure" with its light meter, then why would the flash fire at all?

From what I understand, the TTL technology is being used to balance the overall exposure; but if the overall exposure is already balanced then what is the flash trying to do?

The flash will fire because you told the camera, by attaching it and turning it on, that you wanted it to. The camera doesnt question your motives, but assumes you want light coming from the direction you have the flash pointed. For instance fill flash to prevent raccoon eyes from overhead fixtures.