Today’s flash units use E-TTL, which stands for ‘Evaluative through the lens’. Canon refers to it as ETTL, while Nikon uses iTTL (intelligent TTL) or to simplify: just TTL. The pre-flash only fires when you have depressed the shutter enough to start taking the actual picture. Most importantly, it only measures flash output, and does not fire when the camera is also metering the ambient light. The pre-flash usually evaluates the exposure through the lens just like the camera metering ambient light. This system makes it harder to fool the camera into an incorrect measurement of light. The content you are trying to access is only available to members. Sorry.

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