What is flash sync speed?

Flash sync speed, or X-sync, is the fastest shutter speed at which a focal-plane shutter exposes the entire sensor at once. Below it the flash can fire a single pulse while the whole frame is open; above it the shutter exposes through a moving slit, so a normal flash lights only part of the frame, leaving a dark band.

What is High Speed Sync and why does it cost power?

High Speed Sync fires the flash as a rapid train of low-energy pulses for the entire time the shutter slit travels across the sensor, so every part of the frame receives light. Because that energy is spread over time rather than delivered in one burst, usable output drops sharply — commonly two to four stops.

How much power do I lose in HSS?

Roughly one stop per stop above sync speed, plus about a 1.5-stop penalty for entering HSS mode in this model. Going from 1/200 to 1/2000 — about 3.3 stops above sync — leaves only a small fraction of full output, which is why HSS often needs a powerful head or a close flash.

How do I avoid the HSS penalty entirely?

Stay at or below your sync speed and control ambient light another way: lower the ISO, close the aperture, or add a neutral density filter to cut bright sun. An ND filter lets you keep flash at full power and a wide aperture for shallow depth of field without switching to HSS.

Does HSS work with any flash?

Only flashes and triggers that explicitly support HSS can do it, and the camera must be in a compatible mode. Studio strobes often lack HSS but offer hypersync or simply rely on ND filters and sync-speed shooting instead, so check your gear before planning a high-shutter shoot.

Flash Sync Speed & HSS Calculator

To freeze motion or overpower bright sun, you often want a shutter speed faster than your camera’s flash sync speed — and that is exactly where flash gets complicated. This tool tells you whether you have crossed into High Speed Sync territory and estimates how much flash power you will sacrifice, so you can plan output, distance and modifiers before the shoot.

How it works

A focal-plane shutter has two curtains. Up to the X-sync speed (commonly 1/200 or 1/250 second) the first curtain fully opens before the second starts to close, so the whole sensor is exposed at once and a single flash pulse lights the entire frame.

Above sync speed there is never a moment when the whole sensor is open: the second curtain begins closing while the first is still travelling, exposing the frame through a moving slit. A normal flash pulse then lights only the strip that happens to be open, leaving a dark band.

High Speed Sync solves this by firing a rapid train of pulses for the whole time the slit crosses the sensor. Every row gets light — but the energy is spread over time instead of one burst, so usable output falls. The tool models the penalty as:

stops above sync = log2(shutter speed / sync speed)
HSS penalty      = 1.5 + stops above sync
usable power     = 100% × 2^(−penalty)

Worked example

With a 1/200 sync speed and a desired 1/2000 shutter, the shutter is log2(2000/200) ≈ 3.3 stops above sync. The estimated penalty is 1.5 + 3.3 ≈ 4.8 stops, leaving roughly 4% of full output. To compensate you would move the flash much closer, open the aperture, drop a modifier, or reach for a more powerful head.

Tips and notes

To keep full flash power in bright conditions, stay at sync speed and add a neutral density filter instead of switching to HSS — the ND cuts ambient light without the power penalty.
Only HSS-capable flashes and triggers can do this; studio strobes often cannot, so plan around sync speed and ND filters with that gear.
The penalty here is a planning estimate. Real loss depends on the specific flash’s pulse efficiency, but the two-to-four-stop range holds for most speedlights.