A slave flash is ANY photographic flash attached to a slave unit. A slave unit has a sensor built into the unit that is flash sensitive. The slave unit can "sense" when a photographic flash has been fired. The slave unit then closes its circuit and activates whatever flash is attached to it. Most are so sensitive that they can be activated 100 feet away in full sunlight!
The photographer uses an auxiliary camera handle that has an attached
hot shoe. A screw 
mounts
the camera to the handle. The slave unit fits into the hot shoe handle
and the flash is mounted on the slave unit. In this configuration the flash
on the camera activates the slave flash. When the camera flash goes off
so will the slave flash, all happening within 1,000 to 10,000 of a second,
depending on the flash.
The PhotoPC digital camera has an ISO/ASA rating of 130 ISO/ASA with a permanent f-stop of 5.6. This f-stop is the limiting factor in the distance a flash will work with this camera. Since the f-stop is fixed, so then is the flash distance to object. All flashes have flash guide numbers. These guide numbers divided into the film speed will give you the maximum distance for the flash.
A Vivitar 283 flash has the following specifications according to the owners manual. The flash guide number for an ASA of 125 is 135. Dividing the flash guide number of 135 by the fixed f-stop of 5.6 the answer is 24.1 feet. Therefore, using a Vivitar 283 flash set on manual, the photographer can be up to 24.1 feet away from the subject. If the Vivitar 283 flash is set on automatic then the light sensor in the flash can automatically adjust to the distance of the subject. Again referring to the owners manual of this flash, the manual states the 5.6 f-stop is compatible to the "red" setting on the flash. This setting will automatically adjust the light intensity of the flash from 4 feet to 30 feet from the subject. I would suggest the automatic mode be used for most lighting conditions. Most photographic flashes are rated for 100 ASA film. You will need to read their individual manuals to determine what their flash guide rating is for an ASA/ISO of 130. The "better" the flash the more distance you can be from the subject, within limitations. Even jumping to a high end flash unit, the most you will get is about 40 feet from the subject with this camera.
Taken 30 feet away with only the camera's flash.
Slave flash from same distance.
.
Taken on a bright sunny day!
Camera's flash about 60 feet from street.
Using a slave flash, same distance.
Late afternoon sun!
Taken only with the camera's flash.
Taken with a slave flash.
Using a slave flash is not restricted to outdoor use only. It should also be used on any indoor photos that the subject is greater than 10 feet from the camera. This extra lighting will greatly improve your photographs. This technique is good for all point & shoot cameras, including the Polaroid type cameras with their "instant" pictures.
REVISED 06/99: I have been informed that the new digital cameras
that have "red eye reduction" or commonly refereed as "pre-flash" requires
a special digital flash unit to act as a slave unit., see "Digital
Slave Flash". The older digital cameras without pre-flash will still
work with slave units.
If you have comments or suggestions, email me at
hbb@feinc.net