Unlike digital cameras, there are no pre-flashes in film cameras that do off the film metering. In absence of pre-flashes I want to know how camera can determine accurate flash power when flash is bounced off a surface.
You triggered a distant memory. It appears to have been a genuine one :-) -
TTL (through the lens) metering was invented by Olympus in 1975. My memory says that they measured light reflected from the film surface to determine the light level and that they obtained a large number of film samples from many countries to arrive at a typical reflectance value to assign to the film. Olympus's history page here says that I recall correctly :-). They say -
- OM-2 This camera went on sale in 1975. It features an automatic exposure (AE) system with an aperture-preferred shutter. The OM-2 also had the world's first TTL direct metering system, which measures light reflected off the surface of the film. This allowed exposure control during shooting, and automatic TTL strobe adjustment using a specially designed strobe. To develop the TTL direct metering technology, Olympus collected 35mm film from throughout the world and measured the reflection ratios for each roll. The results were used to determine the density of printing on the shutter curtain.
That's a little concentrated: they used the derived value of film reflectance to set a value to use when the shutter was open but they also printed the shutter with a pattern with the same reflectance level so that when the shutter was closed they could use "preflash" to reflect light onto the sensor via the shutter surface. Very clever.
It is common with small battery power flashguns to regulate flash power using "tail trimming" i.e. shutting the flash circuit off before the capacitor is fully discharged.
TTL flash metering systems without pre-flashes work by monitoring the light passing through the lens in realtime during the exposure. The flash is fired as soon as the shutter opens, and a sensor records the light entering the camera. As soon as a certain threshold (as determined by the film speed) is reached, a signal is sent by the camera to electronically turn off the flash.
This prevents overexposure, but it can still result in underexposure as there is a maximum duration of the flash pulse, and this scheme does not allow the camera to change aperture as is the case when flash power is estimated using a pre-flash (though it can still delay the shutter closing if not enough light is detected).
Incoming flash light bounces off the film surface in a predictable way, if you place a sensor at the front of the mirror box it can measure this reflection and cut the flash off when the desired amount has been reached.
Edit: Ah, Russel McMahon beat me to it by seconds :)