Before we can talk about high-speed synchronization (sync), we need to discuss what flash sync is. Flash sync is the computer-controlled feature in which the flash and the shutter release are synchronized such that the flash light output illuminates the subject for the specific moments that the shutter exposes the image sensor. The flash instantaneously lights up the subject (it does travel at the speed of light), so the flash doesn't last nearly as long as the shutter remains open even though this is very fast (1/60s, 1/125s 1/250s, etc.). There is a limit to the shutter speed, and this is the camera’s native sync.
What is High Speed Sync Flash?
High-speed sync flash is your DSLR’s ability to use a flash at shutter speeds faster than the camera’s native sync. Most cameras have a native sync of 1/250th of a second, and anything faster than that is beyond the camera’s ability to sync the shutter with the flash. But if you happen to be in a situation that requires faster shutter speeds to effectively capture the action, or for other aesthetic reasons (like a wide aperture), then you’ll over-exposure your image. However, high-speed sync flash/camera combinations allow you to use the flash at higher shutter speeds.
How to use High Speed Sync Flash?
High-speed sync flash is used when you want to use a shutter speed that is faster than your camera’s native flash sync speed, or when you want to use a wider aperture setting that requires a higher shutter speed as is often the case with outdoor daylight shooting. For example, you may want to take an outdoor portrait and your TTL meter tells you that the f-stop should be set at f/16 with a 1/125s shutter. Those settings will give you too much depth of field, way too much actually. Nearly everything in sight will be in focus. Instead, what you want is a sharp subject, but a soft, blurred background, which would be achieved with an aperture of about f/2. That’s six stops of light difference, which means that shutter speed needs to go up to 1/5000s. This is easily achievable by setting the flash to High Sync Speed. When you take your photo, you’ll have that beautiful, soft background that the pros get when shooting outside. You’ll want to use High Shutter Sync when you’re shooting with a telephoto lens, trying to capture fast action, using a high shutter speed as well as a high f-stop. So in sports photography High Shutter Sync is ideal, and in some wildlife situations as well.
How High Speed Sync Works
With a high-speed sync flash-capable camera and dedicated flash unit, all you do is set the camera to that setting. But how does it really work? Basically, at high shutter speeds the rear curtain starts to close before the front curtain fully opens. This way only a sliver of exposure moves across the image sensor. It is within this moving sliver of exposure that the flash fires, and voila! A high-speed shutter speed is synchronized to the flash. The flash does fire longer than in standard flash mode. In standard flash mode, the flash duration is much shorter than the time it takes for the shutter to move across the image sensor, and the partially opened shutter will cover part of the frame. This would leave large sections of black in your image. The underexposed black in the image is not good, to say the least.
What is Rear and Front Curtain Sync?
The shutter on your DSLR consists of two curtains; the front and rear that open and then close in the time designated by the shutter speed (i.e. at 1/500s, the front shutter opens and then the rear moves to close all within 1/500th of a second). The default setting coupled with a flash is “front curtain sync”, in that the flash fires as soon as the front curtain begins to move, thus illuminating the subject for the duration of the shutter speed. However, many 35mm and DSLRs give you the option to have the flash fire just before the rear curtain (or second-curtain) begins to move. You can achieve arresting, creative motion-blur and streaming light effects by deftly manipulating the front and rear curtain sync, especially with longer shutter speeds (1 second or more).
The high-speed sync gives you more artistic control over your photos, particularly in aperture-priority mode. When you master this use of the flash, you’ll be able to take photos that will grab people’s attention and have them wondering how it was done. High-speed sync overcomes the limitations of the camera’s native flash sync. That’s a boon to those photographers who always wished for just one or two more speeds on the shutter.