Has this ever happened to you?
You want an outside picture with a blurred background. So you set your aperture to a low number, and click, a nice enough picture.
But your subject is a little dark, so then you realize “wait – I should have lit my subject with a little flash”. So you simply turn on your flash and change nothing else. And then this happens:
Whaa? What happened?
Ah. In the first picture, you were at 1/1600th of a second, say. But you forgot that you have a maximum flash sync speed – usually 1/200th to 1/250th second. So for the second picture, the moment you actiavted the flash, your camera said “oh, my owner is using flash. I am slowing down the shutter to the sync speed, whether he/she likes it or not”. The result: a grossly overexposed image.
Solution: either use a slower shutter and a higher f-number and forget the blurry background, or activate “high speed flash” (“Auto FP flash”, as Nikon calls it), where the flash emits a 40 kHz pulse of little flashes, so you can go beyond the sync speed, and you can keep your f-number low. Now you get this (shot at 1/1600th second, f/2.8, 100 ISO):
Note: That “fast flash”mode is only available on speedlights (like the 600 EX/SB910). And there is a drawback: your range is significantly reduced. In the previous picture your flash might reach 10 metres; with fast shutter speed and high speed flash it can be as little as a metre or less. So it’s good when you are close, as I was here.