Flash photography can be a mystery, but if done right, can add character and depth to an image that would otherwise not be possible. One of the more mysterious Flash technologies is called High Speed Sync (HSS), which may be magical – but that's a trick you can master.
HSS allows the use of flash at very high shutter speeds (up to whatever the fastest camera setting is). This is useful for outdoor portraits when a shallow depth of field is desired, which requires a fast shutter speed.
The effect is well demonstrated in the above video by Chicago photographer Manuel Ortiz (via PetaPixel). This demonstration works well because Ortiz does not focus on the usual difference between flash and non-flash, but specifically on HSS flash versus non-HSS flash. Without HSS, its shutter speed is limited to 1
The indirect effect of HSS is therefore a small depth of field that pulls the subject out of the background. But you may be wondering: why is taking pictures over 1/250 seconds really different than taking pictures at a slower speed when it comes to lightning? To understand this, one must first understand how a slit shutter works (that is the type of shutter used in DSLRs and mirrorless cameras).
A slot shutter has two parts: a front (or first) curtain and a rear (or second) curtain. The front curtain will fall to start exposure, and the back curtain will follow to stop the exposure. Each camera has a so-called "maximum synchronization speed" (usually 1/200 or 1/250 second). This is the shortest shutter speed available on a non-HSS flash. Above this speed, the rear curtain begins to close before the front curtain has exposed the entire sensor, making the shutter more like a scanner and guiding a light bar across the sensor. (If it's helpful to have a picture, this process is well illustrated on Fstopper.)
This is a problem for a standard flash that emits a flash of light that often lasts 1/1000 of a second or less. At a high shutter speed, this is not enough to illuminate the entire frame, as the shutter curtains at some point block some of the sensor. HSS solves this problem by moving the flash extremely fast over the entire shutter speed. In essence, a flash in HSS mode behaves more like a constant light.
The disadvantage of HSS is that extending the flash duration decreases the maximum output power, which can be a problem when trying to overload the sun or using a large light modifier. HSS is a standard feature of most third-party external flash units such as Sony, Canon and Nikon, but is also included in some high-end studio lights. The Flashpoint Xplor 600 TTL, which Ortiz uses in the video, offers significantly more power than a Sony flash and is therefore more flexible in high-speed synchronization.