The F3 doesn't meter at the film plane once the mirror is up. It holds the reading taken before the mirror is up and thus the shutter speed displayed in the viewfinder is the one it's going to use. It sends out a signal thru the hot shoe proportional to the intensity of the flash it reads off the film plane during exposure. For the F3 OTF is only for flash as almost all film slrs with TTL flash do.
Originally Posted by Les Sarile