When buying an external hot shoe flash for a digital camera you must remember some very important points. If you buy a flash exclusively for your camera type like a Canon Speedlite for a Canon camera or Nikon flash for Nikon camera then fine they were designed to be used with them however, some facts you need to know.
1) Old SLR cameras used a mechanical switch to trigger the flash so they were pretty bomb proof.
2) Digital Cameras use electronic circuitry switches to trigger the flash so they are much more sensitive
3) If you plan to use any other flash than one designed for your camera YOU MUST check the trigger voltage your camera can handle
4) Make sure the flash you buy falls below your cameras max trigger voltage. EG my camera can handle up to 50v trigger
5) If you pick up an old flash from a boot sale or the attic before attaching it to your precious camera do the following
6) Get hold of a multimeter and set it to DC volts. Start high like 100v range
7) Switch on flash and let it charge so the ready light illuminates.
8) Put the tip of the mutimeter probes: One on the largest part of the flashes hot shoe and one on the centre pin. (Manual flashes)
9) My flash read 5v so it was safe you may have to adjust multimeters range to get a reading
10) Some really old flashes can have a trigger voltage of a few hundred volts
11) Failure to check this first if you do not have the info for the flash and the camera could fry your cameras trigger circuitry