A really simple method I use in news stories is inserting 4-6 frames of pure white. Add some nat sound of a camera click and you're done. Kind've ghetto and I'm not sure if that's what you mean, but its worth a try
Keyframe an effect in Vegas according to a point where you move the camera (you can cut out a part of the middle of the movement out to make it more jerky). Keyframe an effect to make the screen go from normal to 100% white and back to normal again. You might even want to keyframe a lightray effect to go along with this.
*Edit*....As I read the other reply, I forgot that you said what programs you were using...Hm...Well, if you happen to get Vegas I guess you have another idea....
Usually in a flash, the attack is really fast, and the decay is a little slower. So you might want to cut to white immediately, then take 3-5 frames to fade back to the picture. Also, usually flashes are point sources, so you may want to use an iris effect for 1 frame at the beginning.
If you wanna give the idea that somebody took a photo on the set, personally i think that applying a total white makes it a little fake.
I rather adjusted the levels, sliding the white reference down a lot along the brightness scale. So while all tones above a certain brigthness will become practically white, the darker areas will still stand, so not all the screen will go white, but different areas of the screen will go white depending on their original brigthness.
I agree with the concept of using keyframes and allow a very fast attack (instantaneous will be ok) and slower decay (2-3 frames).
you could also get a copy of Nattress film effects @ www.nattress.com
I love these filters and they're inexpensive. There's a camera flash effect that works as both a filter and a transistion in Final Cut. :thumbup:
The demo movies they have on their site are keyframed really really really slow... personally I don't much care for that. But if you just keyframe the effect a little faster it looks very nice (obviously depending on what you're doing).
free is always good, and it looks nice - but it looks more like a burn out than flash - even sped up. When I think of camera flash, I think of the effect you get on Super 8 home movies - there's a lot of flicker as well as burn out.