What is flash synchronization?
Flash synchronization or flash sync is the firing of a camera flash harmonized to the opening of the shutter, admitting light to the image sensor. The flash is connected electrically to the camera via cable or contacts in a hot shoe bracket.
DSLR cameras have focal plane shutters that consist of two parts that travel from top to bottom to reveal the sensor and let in light when the release button is pressed. Above a certain shutter speed, the second shutter begins traveling before the first one closes. This means that the sensor is not fully exposed at a time.
Why is flash sync speed important?
Photographers have to make sure that the shutter is open when the flash goes off, otherwise dark shadows will be cast on the image, ruining it.
In order to avoid this problem, you need to keep your shutter speed at, or below, your flash sync speed. This is the highest speed at which the camera can use flash with the shutter 100% open. Most modern cameras won’t even let you set a higher shutter speed when using the built-in flash.
Flash sync speed is usually 1/200 or 1/250 seconds on recent cameras. This limits the top shutter speed, so that becomes irrelevant.
What is high-speed sync?
High-speed sync is a setting suited for extremely fast shutter speeds. In some lighting conditions, even with the smallest apertures, an exposure time will result in overexposure. Other times to capture action shots in sharp focus, like sports photography, you can’t afford a slow shutter speed.
The high-speed sync pulses low-intensity light instead of the usual burst of flash. This allows even lighting as the shutter curtains travel across the plane, saving your pictures.
Do you want to read more about this technique? Read more about high-speed sync flash on expertphotography.com.