What is High Speed Sync flash and how do I use it?
Canon and Nikon speedlights, otherwise known as accessory flashes, have the ability to do high speed flash syncronization. Sound complicated? Why do you need it?
What it is?
High speed sync, or Auto FP sync, allows you to use shutter speeds faster than the sync speed by extending the length of the flash pulses long enough for it to appear across the whole of the picture.
Camera flash can only normally be used up to the maximum flash sync speed of your camera, usually around 1/200 or 1/250 of a second. This is because modern shutters are of the focal plane variety, where one curtain on the shutter slides open to let light in to the sensor and a second curtain follows the first curtain in the same direction in order to cut light off again. This two-curtain approach to shutter design, called a focal plane shutter, allows your camera to attain very fast shutter speeds, up to 1/8000 of a second on advanced model DSLRs. Slower shutter speeds, under the flash sync speed, allow the first curtain to open to reveal the complete sensor, before the second curtain starts to close.
The faster the shutter speed, the sooner the second curtain will start to close after the first curtain has started to open, so that at very fast shutter speeds, the shutter is never completely open, but works like a slot of light moving across the plane of the sensor, much like the head of a digital scanner moves across the scanning bed. If you are not using flash, you would not notice the effect of using a focal plane shutter. However, the light duration of a flash is incredibly short, much shorter than any shutter speed you set in your camera. If the flash was fired at a high shutter speed, only the part of the picture revealed by the open shutter would be lit by the flash. A dark band would therefore appear in your flash pictures, as shown in the diagram below.
How do I set it?
First implemented by Olympus, Canon and Nikon accessory speelights now carry the feature. Turning it on for a Canon simply involves pressing the little lighning bolt button with a letter H next to it on the back of the flash unit until the icon appears on the flash screen. For a Nikon Camera, select Auto FP from the Bracketing/flash menu > Custom Settings menu, as shown below.
Once set in a Nikon's camera menus, the FP icon shows on the screen of the speedlight.
Why do I need it?
Ha, well, now you have high speed sync turned on, your fill flash becomes a much more usful tool in the bright sunshine. As you know, bright sun causes strong shadows and is not a particularly great look for portrait subjects. With portraits, we usually want to blur the background by using a wide aperture for a shallow depth-f-field. But, if I want to use fill flash, my shutter speed would be restricted to the flash sync speed, somewhere around 1/200 of a second depending on the camera model. On a bright sunny day, at 200 ISO, my camera needs an aperture setting of around f/16 at 1/200 of a second, or f/11 at 1/200 of a second if I use 100 ISO. That's far too small an aperture to get a pleasing out-of-focus background. If I set a wider aperture, my photo will be everexposed. By using the high speed sync (auto FP) feature, my shutter speeds are no longer restricted and I can therefore use a much wider aperture for a shallower depth-of-field.
The picture below on the top picture was exposed at 1/4000 of a second at f/2.8 and taken with no fill flash, leading to dark shadows on the subject's face. The bottom picture was taken at 1/4000 of a second with an aperture of f/2.8, but with high speed sync fill flash bounced off the window to camera right. This softens the shadows on the subject's face and the wide aperture blurs the background. You need to remember that there's no free lunch though, since turning your camera flash to high speed sync drastically limits the effective range of the flash.