How to use a built in Flah on your DLSR?

SO I bought a Vivitar 2800 auto thyristor and when I want to put it in my D5000 or Canon AE-1 it doesn't turn on, I've added batteries to it, but it won't work, how can I get it to work with my Nikon D5000?

Leave the flash on manual. Don't use the Auto or ttl settings you could ef up your camera.

The flash must have a shooting button, so see if it wokrs by itself. When on the camera shoe it has to be synchronized with the camera setting, then have a look at how they work with the flash unit

Do you mean the flash literally does not TURN ON? That has nothing to do what so ever with the camera body. The flash should turn on whether it is mounted on the camera or not. If the flash will not turn on at all, and you are sure that good batteries are installed correctly, then the flash is obviously defective.
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