Understanding HyperSync and High Speed Sync

HyperSync and High Speed Sync both allow flash triggering with a camera at shutter speeds faster than X-sync. They function differently, however, and it is important to understand how X-Sync works to understand what is happening with HyperSync.

X-Sync

A flash directly connected to a camera

X-Sync is typically the fastest shutter speed at which your camera can trigger a flash. A camera triggering a flash at X-sync follows this time-line:

1. The camera is triggered by the photographer. Several milliseconds of Lag Time pass.
2. The first shutter opens, exposing the digital sensor. Some microseconds of time pass. This shutter will stay open for several milliseconds when the camera is set to X-Sync.
3. The camera triggers its PC terminal and hot shoe.
4. Some microseconds of time pass, then the flash begins to emit light.
5. The flash emits light for its Flash Duration. This can be any amount of time up to a few milliseconds.
6. The second shutter in the camera closes and both shutters reset.

Shutter speeds faster than X-Sync typically have the first and second shutters moving at the same time, or with no delay between the first shutter fully opening and the second shutter beginning to close. This does not work with normal flash triggering because the second shutter can be moving when the flash is generating light which yields a clipped exposure.

If your flash has a very long duration, or your shutter is very slow moving, you can still get clipping at X-sync.

A flash triggered remotely by a standard radio slave

Adding a standard radio slave to the above time line adds additional microseconds between steps 3 and 4. This additional delay moves the flash triggering moment towards the time when the second shutter will be moving. If your flash has a very long duration, or your shutter is very slow moving, you can get clipping when using a standard radio slave even at X-sync. You may have to use a slower shutter speed.

High Speed Sync

High Speed Sync (HSS), also known as Auto FP Sync, allows for triggering at shutter speeds faster than X-Sync.

"FP" stands for "Focal Plane." When the first shutter starts to open, it is exposing the digital sensor, which is the Focal Plane. FP Sync means triggering the flash just before the focal plane is exposed rather than waiting until shutter is fully open.

HSS uses a pulsed light technique that generates continuous light from before the first shutter begins to move until after the second shutter closes. At faster shutter speeds, both the first and second shutters are moving simultaneously, creating a moving slit across the digital sensor. Since the light appears continuous to the digital sensor, there is no clipping even at these fastest shutter speeds.

HSS requires a special flash like a Speedlite/Speedlight that can do the pulsed light technique. It also requires special timing information from the camera called pre-sync that is communicated through the TTL pins of a camera's hot shoe. A standard non-TTL flash cannot perform the light pulse technique, nor does it have the electrical connections required to use the pre-sync information from the camera's TTL hot shoe pins.

An HSS time line would look like this:

1. The camera is triggered by the photographer. Several milliseconds of Lag Time pass.
2. Just before the first shutter would open, pre-sync occurs. This tells the Speedlight to begin generating pulsed light as ...
3. The first shutter opens, exposing the digital sensor while the flash continuously pulses throughout the exposure.
4. Shortly after the first shutter opens, the second shutter begins to move. The pulsed light continues.
5. The second shutter in the camera closes and both shutters reset. The flash stops pulsing.

The pulsed light technique cannot emit as much light as a normal flash pulse as it uses a lot of energy to make the light continuous. This means that your flash must be much closer to the subject to be effective.