Quote Originally Posted by fotch View Post
Flash bulbs meant for focal plane shutters had a longer burn time to accommodate the moving slit. Electronic flash cannot do this, AFAIK. I think the bulbs were labeled FP or something similar.

If I remember correctly, FP bulbs were bayonet-base, gas filled fast peak bulbs designed to be used with X sync at faster shutter speeds. Regular flash bulbs didn't work well at high shutter speeds because you only got to use a small part of the burn (and thus got a low guide number).

There were long peak bulbs designed for focal plane shutters; I think they were all screw-base.