MOST OF THE LATEST
digital still and film cameras have a built-in electronic flash, which
at first glance seems great. The trouble is that it's almost impossible to take
a good professional photo with only a single flash. They're OK for "happy snaps"
but that fixed flash, right next to the lens and pointing in the same direction
is a big problem. It gives very "flat" lighting and very dark shadows.
Fig.1: the camera flash is picked up by photodiode PD1 and this drives transistor Q1 which in turn clocks IC1. IC1
is wired as a programmable counter and the output of gate IC2c (pin 10) will go low only when the right number of pulses have been counted. IC2c then triggers SCR1 (via IC2b & Q2) to trigger the slave flash unit.
For much better modelling and control of shadows, you really
need at least one additional source of light and/or a system of light diffusion.
But neither of these options is easy with most digital cameras, not only because
of their fixed forward-facing internal flash but because they generally don't
have a "hot shoe" or conventional flash contact socket to trigger an external
So the only way to trigger a second flash with these cameras is
to use a slave flash trigger unit. This has an optical sensor which detects when
the camera's own flash operates, to trigger an external "slave" flash.
But there is a further complication with many new digital
cameras. Their internal flash often operates only in "red-eye reduction" mode,
where the flash gives not just one single pulse of light but multiple flashes.
There may be one, two or even a bunch of short pre-flashes shortly before the