Take your flash and put it on your camera:
Aim at a subject while looking through the viewfinder. Take a picture.
Did you see the flash? Through the viewfinder? Yes you did.
How is this possible? When the picture is taken on an SLR, the mirror is raised. When the mirror is raised, the viewfinder is black. So it is impossible that you see the flash through the viewfinder. You cannot have seen what you just saw!
Those of you who do not know, click on.
Your flash and camera use a system called “Through the lens flash metering”, or “TTL”. Canon calls it E-TTL II, Nikon calls it i-TTL (and when using more than one flash, CLS, “Creative Lighting System”).
TTL does a LOT of work behind the scenes. You think the camera just takes a picture? Nah. Here’s what actually happens when you press down all the way on the shutter button. The camera rapidly goes through this sequence, every time:
- It tells the flash to a fire low-power test flash.
- Uses its light meter to see how much light returns.
- Based on that, calculates actual flash power needed.
- Raises the mirror.
- Opens the shutter.
- Tells the flash to fire at the level it calculated in step 3.
- Closes the shutter.
That’s what happens. And what you are seeing through your viewfinder is step 1: the low-power test flash.
Now you know.