I am rather new to modern digital cameras. I have not been able to find detailed info on how exposure is actually controlled during flash photos. I am interested in both built-in and external slave flash(es). Can someone give some detailed explanation of how it works? One reason for my question is that I am old enough to remember when the OM-2 came out and, as I recall it, was the only camera able to close the shutter when enough light had hit the film. Other cameras at the time, as I understood it, had to determine flash/exposure time prior to opening the shutter. I assume with modern electronics, this is a lot more sophisticated, but exactly how? For example, if I have two slave flashes and the built-in flash, can they all be controlled by TTL? What happens if, say, during the exposure, there is another light source, not controlled by the camera, adding to the light in the image - will the shutter close, and I have a correctly exposed image? Sorry if my question reveals my stupidity, but I may as well admit it.