Does the fancy automatic “TTL” flash mode work when you bounce your light off the wall behind you?
Yes, and that is exactly the point of TTL (“eTTL in Canon terms; iTTL for Nikon).
You press the shutter button: Click.
But it is not one click! In the milliseconds after you press the shutter, your camera does all the following:
- Fires a low power test flash
- Measures light returned
- Calculates power needed
- Raises mirror
- Open shutter
- Flashes with power setting calculated in step 3
- Closes shutter
- Drops mirror.
Steps 2 and 3 are crucial: that’s why it works wherever you are pointing the flash.
And that is also why you see the flash through the viewfinder: you are seeing the pre-flash. Try it: look through your viewfinder and shoot. If you see a flash, that cannot the be real flash – after all, the mirror is up when that goes. It is the preflash that you (and your camera’s light meter, near the pentaprism) are seeing!