High Speed Sync is a term used when a flash outputs light during a shutter speed that is faster than the cameras maximum native sync speed. In order to have a better understanding of this, we must first have knowledge of how a cameras shutter and a flash work together.
When using flash, shutter speeds can vary but are most commonly set in the range of 1/60th to 1/250th of a second. In comparison, a burst of light from a flash takes a lot longer. When the shutter opens the flash illuminates the subject in front of the camera and the shutter closes. The quickest time a camera can perform this operation is approximately 1/250th of a second.
As the illustration above shows, at higher shutter speeds, both the front and the rear shutter curtain work together. Using flash at a higher shutter speed than that of the cameras native sync speed will result in dark bands across the sections of the image. These bands are actually shadows created by the shutter curtains.
It is possible however to take a photograph whilst using flash at any shutter speed you wish, and there are great advantages of wanting to choose a higher shutter speed. Portrait photographers tend to prefer using a large aperture as this will create a shallower depth of field, ie. isolating the subject by blurring the background. By choosing a wide aperture, (eg. f2) he or she now has too much light entering into the camera. To compensate for this, a higher shutter speed, such as 1/4000th (ISO 100) would be required.
One option would be to use a neutral density filter. Placed in front of the lens an ND filter would cut down on the amount of light entering through the lens whilst maintaining a wide aperture. The downside to this is that the light in the viewfinder is also reduced thus hampering the photographers vision for focusing, composition, etc.
A much better solution is high speed sync. A flash enabled with HSS is able to rapidly fire light in pulses rather than the conventional burst of light. The flash fires these ‘pulses’ in such quick succession that they merge together in one continuous beam of light for a very brief period. They require far less time and can match the shutter speeds of up to 1/4000th of a second and beyond. HSS can be applied to any genre of photography such as wildlife or sports where high shutter speeds are essential in freezing fast action.
It is important to note, the shutter speed controls the ambient (natural) light and the aperture controls the artificial (flash) light. ISO controls both. In theory, a portrait could be photographed in daylight using a wide aperture, a high shutter speed and flash and it would appear that the photograph was taken at night, (see below). This is because the shutter was not open long enough to expose the ambient light yet it let in the light from the flash lit subject, exposing it to the sensor.
Personally I like to use this technique while in aperture priority mode. I can choose my aperture and control my depth of field whilst keeping an eye on the shutter speed as it changes accordingly to the light. I can adjust my aperture until I have attained a desired balance of artificial and ambient light.
There is a downside to HSS, and that is loss of power. As the flash fires continuous pulses of light it uses more battery life, this is due to the fact that it is working throughout the whole time the shutter is open rather than giving the ‘regular’ burst of light. On average a stop of flash light is lost while in HSS mode. In addition a further stop of flash light is lost for every increase of a stop in shutter speed. Most photographers however are in agreement that this is a small price to pay in comparison to the benefits gained in their flash photography.