Flash FP Mode
This article discusses the Nikon FP flash mode, which is often derided by photographers as ‘beneath contempt’. That’s not my opinion.
What is it? FP mode causes your (external) flash to emit a bunch of rapid light bursts instead of one big one. These rapid bursts keep the xenon in your flash continuously energized, so the light output stays at an almost-constant level. The upside to this is that you can use a shutter speed like 1/4000 and still use flash. BTW, “FP” is short for “focal plane”, and is perfectly descriptive to nobody outside of Sendai Japan. Think “flash bulb” for those of you born during the Pleistocene epoch.
What is it good for? FP mode enables you to use high shutter speeds in conjunction with your flash, so you can do things like fill in shadows outdoors in sunlight. You can fill in those terrible dark blobs under the eyes and nose, and you can get that sparkle back in the eyes. You can use those large apertures you paid for while outdoors, and still get fill-in flash. FP mode isn’t something you would want indoors; regular flash mode will work better there. It’s really just available so you can get beyond the “sync speed” of the shutter while outside.
It's true that you can use something like "D-Lighting" while postprocessing the picture to fill in those shadows, but this is really second-best to getting it right in the beginning. You don't want to add shadow noise when there's any easy (better) option. If you're too far away from your subject you may have no choice, but your flash can have a positive effect from farther away than you might think.
There is still no such thing as a free lunch. The light output from your flash will be less (about 2 1/3 stops). And you have to stick that flash on top of your camera. I suppose you could rig up a “light tube” around your flash to project a narrow beam when you use that telephoto and regain some of those lost stops. It’s possible to use “commander mode” with your camera internal flash to invoke FP mode, but it gets too complicated.
How do I get it? You have to set “Auto FP” mode in your (Nikon) camera. I’m clueless if Canon has it. You need a separate Nikon flash (I use the Nikon SB600, but the 700,800,900 series have it too). When you set a shutter speed faster than your internal flash can handle (like faster than 1/250), the external flash enters the FP mode. If you’re inside, you can just change the shutter speed to the flash sync speed or lower and the flash will automatically exit FP mode and act like a regular flash.
A related topic is using your camera in ‘manual’ mode with a flash. Guess what? Your flash is still in ‘automatic exposure’ mode, even when your camera is in ‘manual’! Is that great or what?
A nice effect to try is to under-expose in manual mode by about 1 ½ stops and let your flash do the rest. The flash will expose your subject correctly (if you’re close enough), but the background will be nicely under-exposed by just the right amount. Indoor shots are somewhat disturbing when the background goes black, and it screams ‘amateur’; use manual mode to get the ambient light levels up, and the flash can correctly light your subject.
Nearly every picture is improved when you use fill lighting. Jacking up the ISO isn’t always the best solution to low light, either.