FP Modes (also called High Speed/FP flash synch on Canon)
Now that you know that the shutter needs to get all the way open to let the instantaneous pop of an electronic flash expose all of the film or sensor, what if instead we made the flash stay on long enough to let the curtains of a focal plane (FP) shutter complete their travel from one side to the other? We could let the flash expose the film or CCD at any shutter setting during the time it takes the curtains to travel from one side to the other.
You can do that as a trick mode with many high-end flashes and cameras. It's called the FP mode.
There are many disadvantages, which is why I don't ever use these modes.
1.) FP mode often reverts to totally manual exposure calculation. If so, it's only useful for shooting things that stay at the same distance so you can calculate it. I saw this feature illustrated in a Nikon brochure, and lo and behold, the two examples were 1.) a water skier shot from the boat (the rope stays the same length) and 2.) a shot of a race car made from a camera bolted to the car and fired remotely. Some newer cameras like the D2H have automated this, finally.
2.) The flash always pops at full power on the flashes I've seen. Thus as above you lose battery life, have long recycle times, no high frame rates and all the other disadvantages above.
3.) Since only a fraction of the light at any time is exposing the film or CCD you lose a lot of light, again getting you back to the problems of limited flash range. The loss of light also depends on the shutter speed you use, thus the flash guide number used in manual calculations in 1.) above changes with shutter speed! You lose most of the light at the faster speeds and lose less at slower speeds.
At least FP mode gives you the flexibility to use any shutter speed and also because you lose a lot of light you can get to the larger apertures.
This can be a handy feature in limited applications, however there are so many limitations you see why I don't consider it anywhere as useful as a healthy true sync speed.
Some of the very latest cameras like the Nikon digital SLRs and Minolta Maxxum/Dynax 7 have FP modes that are TTL and do vary the power down from full. This way you have only the disadvantage of 3.) above. I have not researched these; believe it or not I'm too lazy.
