On Mon, 11 Aug 2003, Michael Perham wrote: > I saw an advertisement for a flash that was primarily designed to augment > the flash on a point and shoot and had a built in slave that was triggered > by the camera's built in flash. The thing that grabbed my attention was > that the add' suggested the flash was compatible with camera's using a > pre-flash.
Unfortunately, most of these flash units are not as sophisticated as you hope, nor are the P&S cameras themselves. These slave flash units are supposed to ignore the pre-flash and fire on the main flash. > Correct me if I am wrong, but would that not require that the remote flash > fire two equal flashes back to back. The first with the camera's pre-flash > so that the camera can measure the flash TTL and the second, which would > have to be of the same power and duration, for the actual exposure. No, that's not the case. No pre-flashes from the slave flash unit. Actually, in many lower end P&S, pre-flash is only used for setting the white balance. And some cameras like Optio S uses 2 preflashes to set white balance and exposure; and it can fool some of these "pre-flash compatible" slave flash. > My reason for thinking this is that the remote (slave) would have no way of > knowing the formula or algorithms or whatever it is that camera's uses on > readings from the first low power flash to judge the full flash used in the > actual exposure. You are absolutely correct. You can change the auto thyristor settings on the slave unit and the flash output will be added to the built-in flash. And yes, it can result in overexposure. So the slave units work best for background illumination or when the built-in flash is out-of-range (in the latter caes, the slave unit would be the main source of light output). -- --Lawrence Kwan--SMS Info Service/Ringtone Convertor--PGP:finger/www-- [EMAIL PROTECTED] http://www.vex.net/~lawrence/ -Key ID:0x6D23F3C4--
