Does anyone know how TTL calculates the amount of power to put in after the preflash is done? I understand that the value is somewhat brand specific, but what is the general logic? Because if you think about it, before flash the camera calculates the exposure to the 18% rule. But this makes no sense for flash.
For example, if I'm already at the perfect exposure (18%) before flash, the flash will still fire. If it followed that logic, it should not fire at all.
Also, if the logic is to bring the whole scene to 18%, doesn't that mean the flash power is inversely proportional to my continuous light exposure? i.e., the more I underexpose the background, the more power it will flash?
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