High-speed sync (HSS) lets a flash work at shutter speeds faster than the camera’s sync speed, which is usually 1/160 to 1/250 second. Above that speed the shutter exposes the sensor through a moving slit, so the flash fires a rapid train of pulses for the whole time the slit is travelling instead of one burst. The price is power: about two stops straight away, and one more for every stop of faster shutter speed.
The limit that HSS removes is defined in flash sync speed, and its place among the other ways of using flash in daylight is covered in balancing flash and ambient light. This guide explains the mechanism in more detail: what the shutter is doing, why a normal burst leaves a dark band, where the power goes, how HSS compares with an ND filter, why it does not freeze motion the way ordinary flash does, and how to diagnose bands in the frame.
How a focal-plane shutter turns into a travelling slit
Most interchangeable-lens cameras use a focal-plane shutter: two curtains just in front of the sensor. To make an exposure, the first curtain moves across the frame and uncovers the sensor. After the chosen time, the second curtain follows and covers it again. The page on the mechanical shutter describes the hardware.
The curtains cannot move instantly. Each one needs a few thousandths of a second to cross the frame, typically somewhere around 2.5 to 4 milliseconds. That travel time is fixed. What changes with the shutter speed is the delay between the first curtain starting and the second curtain starting.
- At slow speeds the delay is longer than the travel time. The first curtain finishes its trip, the whole sensor sits uncovered for a moment, and then the second curtain sets off.
- At the sync speed the delay is just long enough for the first curtain to finish before the second begins. This is the fastest speed at which the entire sensor is uncovered at the same instant. It is also called X-sync.
- Above the sync speed the second curtain starts before the first has arrived. The sensor is never fully uncovered. The gap between the two curtains forms a slit that sweeps across the frame.
The faster the shutter speed, the narrower the slit. If the curtains take 3 milliseconds to cross a sensor 24 millimetres tall, then at 1/1000 second (1 millisecond) the slit is about a third of the frame height, or 8 millimetres. At 1/4000 second it is about 2 millimetres. At 1/8000 second it is about 1 millimetre. Every row of pixels still receives light for the correct time, 1/4000 second for example, but the top and bottom rows receive it at different moments, and the whole sweep still takes the full 3 milliseconds or so.
For daylight this makes no difference, because daylight is constant. For a flash it changes everything.
Why normal flash leaves a dark band
An ordinary flash burst is very short, around 1/1000 second at full power on a typical hot-shoe flash and much shorter at low power. At or below the sync speed that suits the shutter perfectly: the camera fires the flash at the instant the sensor is fully uncovered, and the whole frame catches the whole burst.
Above the sync speed there is no such instant. When the burst happens, only the rows under the slit can see it. Rows the first curtain has not yet uncovered, and rows the second curtain has already covered, get no flash at all. The result is a frame where a strip is lit by the flash and the rest is lit only by the ambient light. Indoors the unlit part is black. Outdoors it is less obvious: one part of the subject has fill and the rest does not.
Just above the sync speed, at 1/320 second on a camera that syncs at 1/250 for example, the effect is a narrow dark bar along one edge of the frame: the shadow of the second curtain, which had already started to close. As the speed rises, the bar grows until only a thin lit strip remains.
Cameras protect you from this when they can. With a compatible flash in the hot shoe, most cameras refuse to set a shutter speed above the sync speed unless HSS is switched on. The band usually appears when the camera cannot tell a flash is attached, as with simple radio triggers, studio strobes on a sync cable, or older manual flashes.
How high-speed sync covers the whole sweep
In HSS mode the flash does not fire one burst. It fires a very rapid series of small pulses, tens of thousands per second, that begins just before the first curtain starts to move and continues until the second curtain has finished. The pulses come so fast that they merge into what is effectively a steady light lasting a few thousandths of a second. As the slit sweeps across, every row sees the same brightness for the same time, so the flash exposure is even from top to bottom.
High-speed sync, HSS, FP sync and Auto FP all describe the same thing (FP stands for focal plane). On most systems you enable it once, on the flash or in the camera’s flash menu, and it engages only when the shutter speed goes above the sync speed. Below that, the flash returns to a normal single burst.
HSS needs three things that all support it: the camera, the flash, and anything in between. A radio trigger that only passes a simple fire signal cannot do HSS, because the flash has to start before the shutter opens. Built-in pop-up flashes generally do not offer it.
The power cost, in numbers
HSS is expensive for two reasons.
- Most of the light hits the curtains. The flash has to stay lit for the whole sweep, about 3 milliseconds plus the exposure time, but each row only collects light while the slit is over it. At 1/4000 second that is a quarter of a millisecond out of more than three. Over nine-tenths of the output lands on the shutter curtains.
- Pulsing is less efficient. A flash tube delivers the most light in one unrestrained burst. Holding the output level for several milliseconds means running well below that peak.
The practical outcome on a typical hot-shoe flash: switching into HSS just above the sync speed costs about two stops compared with a normal full-power burst. From there, each time you halve the exposure time, each row collects half as much flash, so every further stop of shutter speed costs another stop of flash. In guide number terms, the figure roughly halves on entering HSS and then drops by a factor of 1.4 for each stop.
| Shutter speed | Approximate guide number (flash rated 60 in normal use) | Flash lost compared with normal sync | Aperture for full sun at ISO 100 | Distance for a full flash exposure |
|---|---|---|---|---|
| 1/250 (normal sync) | 60 | None | f/10 | 6 m |
| 1/250 in HSS | 30 | About 2 stops | f/10 | 3 m |
| 1/500 | 21 | About 3 stops | f/7.1 | 3 m |
| 1/1000 | 15 | About 4 stops | f/5 | 3 m |
| 1/2000 | 10.5 | About 5 stops | f/3.5 | 3 m |
| 1/4000 | 7.5 | About 6 stops | f/2.5 | 3 m |
| 1/8000 | 5.3 | About 7 stops | f/1.8 | 3 m |
These are representative figures in metres, not a specification. Flash manuals list the real HSS guide numbers. The apertures come from the sunny 16 rule: each gives the same daylight exposure at its shutter speed.
The last column holds the most useful lesson. If you keep the daylight exposure constant, the working distance in HSS does not change as the shutter speed climbs. Each stop of faster shutter costs a stop of flash, but it also lets you open the aperture a stop, which gives that stop back. So the choice between 1/1000 at f/5 and 1/8000 at f/1.8 costs nothing in reach. What costs reach is entering HSS in the first place: about half the distance you would have at the sync speed.
Shutter speed no longer separates flash from ambient
With normal flash, shutter speed changes only the ambient exposure, which gives you an independent control for the background. In HSS that rule stops working. The flash now behaves like a continuous light, so shutter speed, aperture and ISO all change the flash and the daylight by the same amount. The only controls that shift the balance between the two are the flash power and the flash distance. If the subject is too dark against the background, a faster shutter will not help. Move the flash closer or turn it up.
Getting stops back
- Move the flash closer. Halving the distance gives two stops. Taking the flash off the camera and bringing it to one metre is the most effective fix.
- Remove modifiers. A softbox or diffuser costs one to two stops on top of the HSS loss.
- Zoom the flash head to a longer setting to concentrate the beam on the subject.
- Add flashes. Two identical flashes side by side give one stop more than one. Four give two stops.
- Put the subject in shade or turn their back to the sun. The flash then competes with ambient light two or more stops weaker than direct sun.
When high-speed sync is the right tool
- Wide apertures in bright light. A portrait at f/2 in sun needs something like 1/6400 second at ISO 100. HSS is the simplest way to add flash to that frame and keep the shallow depth of field.
- Fill outdoors at close range. Lifting the shadows on a face one to three metres away needs little power, so the HSS loss is easy to absorb. The settings are covered under fill flash.
- Backlit subjects with a rim of sun. Put the sun behind the subject for a rim light, expose for the background, and use HSS flash from the front to light the face at a wide aperture.
- Action in daylight that needs fill. The fast shutter freezes the movement and the flash opens the shadows, within a few metres.
HSS is the wrong tool when the subject is far away, when the flash has to light a group through a large modifier, and indoors or at night, where the sync speed or slower does the job at full efficiency.
High-speed sync or an ND filter
An ND filter solves the same problem from the other side. Instead of raising the shutter speed above the sync limit, it cuts the light entering the lens so that a wide aperture gives a correct daylight exposure at the sync speed. A 5-stop filter turns f/10 at 1/250 into f/1.8 at 1/250. The filter dims the flash by five stops too, but the wider aperture returns exactly those five stops, so the flash works as hard as it would at f/10 with no filter: a normal burst with no HSS penalty.
| High-speed sync | ND filter at the sync speed | |
|---|---|---|
| Flash reach at a wide aperture in sun | About half | Full, roughly two stops better |
| Setup | A menu setting | A filter to fit, and to remove when the light drops |
| Viewfinder and autofocus | Unaffected | Darker optical viewfinder, and autofocus can struggle with strong filters |
| Recycle time, heat and battery use | Worse: the flash often runs near full output | Normal |
| Motion | A fast shutter freezes subject movement in daylight | The shutter stays at the sync speed, so fast movement can blur |
| Equipment needed | A flash and trigger that support HSS | Works with any flash, including manual units and studio strobes |
Choose HSS for speed and convenience at short range. Choose the filter when you need every bit of output: a flash in a softbox several metres from the subject, a group, or a sky you want to darken.
Why HSS does not freeze motion better
Ordinary flash freezes movement because the burst is brief. In a dim room, a low-power burst can be over in 1/10,000 second or less, far shorter than any shutter speed that syncs, and the subject is recorded only during that instant. The guide to flash duration explains how to use this.
HSS throws that advantage away. The flash is on for the whole sweep of the slit, so it has no short burst to freeze anything. Each row is exposed for exactly the shutter time, by flash and daylight alike. At 1/4000 second the picture is frozen as well as any 1/4000 second daylight picture, and no better. The flash adds brightness, not sharpness.
For splashes, dancers and other fast subjects in a dark space, use normal sync at low flash power, not HSS: the burst is shorter than the fastest shutter speed and the flash is many stops more efficient.
Leaf shutters and electronic shutters
Leaf shutters
A leaf shutter sits inside the lens and opens from the centre outward like an iris. There is no slit: the whole frame is exposed at once at every speed, so a normal flash burst syncs at any speed the shutter offers, often 1/500 to 1/2000 second. Leaf shutters are found in some fixed-lens compact cameras and in some medium and large format lenses. One limit remains. At the fastest speeds the shutter can be open for less time than a full-power burst lasts, so it clips the end of the burst and some flash is lost.
Electronic shutters
A fully electronic shutter on a conventional sensor reads the rows one after another, and the scan usually takes far longer than a mechanical curtain’s trip. That makes the flash problem worse, not better, so many cameras disable flash in this mode or allow it only at slow shutter speeds. Sensors with very fast readout narrow the gap and can sync at speeds close to a mechanical shutter.
A global shutter sensor starts and ends the exposure for every pixel at the same instant. With no slit and no scan, a normal flash burst can sync at any shutter speed without HSS. At extremely short exposure times the same clipping applies as with a leaf shutter: the exposure can be shorter than the burst. The comparison in mechanical vs electronic shutter covers the other trade-offs.
Troubleshooting bands and dark frames
| Symptom | Likely cause | Fix |
|---|---|---|
| A hard-edged dark bar along one edge, or most of the frame without flash | Shutter speed above the sync speed with HSS not active | Enable HSS on the flash and the trigger, or return to the sync speed |
| A thin dark bar at exactly the sync speed when using a radio trigger | The trigger adds a small delay, so the second curtain has started by the time the flash fires | Slow the shutter by a third to two-thirds of a stop, for example 1/160 instead of 1/250 |
| The camera will not go above the sync speed | HSS is not enabled, or the flash or trigger does not support it | Look for the HSS or FP setting on the flash, the trigger and in the camera’s flash menu |
| Flash-lit subject too dark in HSS, even at full power | Out of range after the HSS loss | Move the flash closer, remove modifiers, zoom the head or add a second flash |
| Brightness fades gradually from one side of the frame to the other | The flash output sagged during the sweep, common near full power, or a studio strobe is being synced on the tail of a long burst | Lower the flash power and move it closer, or use a slower shutter speed |
| Soft-edged stripes across the frame under indoor lighting, with or without flash | Not a sync problem: the room lights are flickering | See light flicker and banding |
Common mistakes
- Expecting full power in HSS. The subject comes out dark at the distance that worked at 1/200 second. Fix: halve the distance or remove the modifier.
- Using a faster shutter to darken the background behind an HSS-lit subject. Both get darker together. Fix: keep the shutter speed and raise the flash power or bring the flash closer.
- Using HSS indoors or at night. It wastes two or more stops for no gain. Fix: set the sync speed or slower and let the flash fire a normal burst.
- Relying on HSS to freeze fast action in the dark. The exposure time is the shutter speed, and the flash is weak. Fix: normal sync at low flash power.
- A trigger that does not pass HSS. The camera limits the shutter speed, or a band appears. Fix: use a trigger made for your camera system that lists HSS support.
Try this
This takes fifteen minutes with one HSS-capable flash and a white or pale wall, indoors in ordinary room light.
- Put the camera on a tripod two metres from the wall. Set manual mode, ISO 400, f/5.6, and the flash on manual at 1/4 power with HSS enabled.
- Shoot at the sync speed. Check that the wall is evenly lit, and adjust the flash power until it is well exposed.
- Without changing anything else, shoot at one-third of a stop above the sync speed, then at 1/500, 1/1000, 1/2000 and 1/4000.
- Compare the frames. The first frame in HSS is about two stops darker than the frame at the sync speed, and from 1/500 onward each frame is a stop darker than the one before. All of them are evenly lit.
- Now switch HSS off. If the camera still lets you choose 1/500 second (many will with the flash on a basic trigger or cable), shoot one frame and look at the band.
Frequently asked questions
How much power does high-speed sync lose?
On a typical hot-shoe flash, about two stops as soon as HSS engages, then one more stop for each stop of faster shutter speed. At 1/4000 second that is roughly six stops below a normal full-power burst.
Should I leave HSS switched on all the time?
On most systems, yes. The flash only pulses when the shutter speed is above the sync speed and fires a normal burst otherwise. The risk is drifting above the sync speed without noticing and losing power you needed.
Does high-speed sync work with any flash?
No. The flash, the camera and any wireless trigger between them must all support it and be compatible with each other. Most built-in flashes and basic manual flashes do not.
Can I use HSS to darken the sky?
You can darken the sky with a faster shutter speed, but in HSS the flash on the subject darkens by the same amount. The flash must then be close or strong enough to bring the subject back up, which is why this look is usually shot within a couple of metres or with an ND filter instead.
Related guides
- Flash Sync Speed: the limit HSS exists to get past.
- Fill Flash: the most common use of HSS, with starting values and reach.
- Balancing Flash and Ambient Light: how the two exposures combine at normal sync speeds.
- Flash Duration: how a single short burst freezes motion, and why HSS cannot.
- ND Filter: the alternative that keeps full flash power.
- Off-Camera Flash: getting the flash close enough to make HSS practical.