eli5 how old camera flashes were timed


flash powder. there’s been movies where the camera flash was a powder, ignited on a stick the camera operator is holding.

the powder burns very fast, so how did they have the photo to be in time with the flash? wouldn’t the long exposure time also make it difficult?

In: 2

AFAIK, the long exposure time is what made it work. By “long” we’re still only talking a couple of seconds, but this enabled the photographer’s procedure to be 1. open the shutter, 2. count a number of seconds determined by the ambient light conditions, i.e. more for dark, fewer for bright, 3. ignite the flash, and then immediately 4. close the shutter. By not having an exposure time of a fraction of second (which you could do with the film speeds used after the powder flash era), there was ample time to start to record the image and then ignite the flash.

If your film is slow enough, the ambient light does not matter much.

That means that you can open the shutter, fire the flash, close the shutter.

To compensate for the film being really slow, the flashes were quite powerful, too — the guide number of a typical electronic flash is 20 – 60, the guide number of a magnesium bulb was easily around 200-300. This means that the ambient light would matter much less.

Camera flashlights are designed to produce light for a defined response time and duration once triggered, all you have to do is to design the camera shutter to respect this definition, at least for a certain shutter speed.

That’s why on old camera there is a specific setting of shutter time to be used with flash, usually 1/60. The shutter operation is calibrated so the trigger time and duration of the flashlight lands right in the full open time of the shutter.