Almost there, but not quite. Most non-TTL flashes nowadays are thyristor controlled. They have a sensor on the flash body itself (somewhere near the IR thingie for AF flashes) to detect when there's enough exposure and shut off the flash. No preflash is required. Of coz, this is also calculated based on ISO and aperture selected.Originally posted by Flare
In this case, since its not TTL... the flash needs to know how much light to output. The flash need to know the sensitivity of the film and the amount of light that is entering the camera. Then it fires a preflash (I guess) and judge the reflection of the subject and then emitts a flash that should correctly expose the pic taking the sensitivity of the film and the aperture into consideration.
i.e If the aperture is larger, the flash out put should be lower compared to a case where the aperture is larger. And a High ISO film will require less light, so the flash has to emit a lower output.