Scenario 1: You’re taking someone’s photo outdoors with the sun behind them. Or maybe you’re subject is indoors, a window behind them with bright daylight outside. Easy, just pop a bit of fill flash in to light the subject. But the background is too bright; the sky is a washed out blue and too bright in general.
Scenario 2: You’re using flash outside, but want a nice shallow depth of field. You’re in aperture priority mode at ISO 200 and the widest aperture you can use is around f18 at 125sec), but you need f1.8 or f2.8 to get that blurred background you’re looking for.
Solution and Set-up: Use high speed sync! For this (the tip you never see in books), you will need to set-up your camera first. Go to your camera Custom Settings Menu > Bracketing & Flash > Flash Sync Speed. Now choose either1/320s (Auto FP) or 1/250s (Auto FP). I use the former, but any one will do (the important thing is the Auto FP). That’s it, you can leave it set that way forever.
Here comes the science bit: When you use a shutter speed of 250th or slower, the front curtain moves across to the end of the frame, and then the rear curtain follows. This leaves a point in the middle where the curtain is fully open allowing one big pop of flash light to expose the whole frame. If you use a very fast shutter speed though, the rear curtain follows the front one so soon that there is only a thin slit moving across the frame. But in high speed sync mode the flash will fire rapid bursts of light that will expose the full frame in stages.
Aperture priority mode will give you nice even results, but try using the camera’s manual mode and you can up the shutter speed as high as you like (8000th of a sec if you like) which will darken the background, still keeping your subject evenly exposed. You can almost change your background from day into night. Try experimenting with a combination of shutter speed, aperture, exposure compensation and flash compensation. You won’t always get the result you expect.