Flash Sync and High-Speed Sync: How to Freeze with Light

What is flash sync speed?

Flash sync speed is the fastest shutter speed at which a camera can take a normal flash picture without the shutter blocking the flash. With a mechanical focal-plane shutter, exposure is made as a pair of curtains move across the sensor. At slower shutter speeds, the first curtain finishes opening before the second curtain starts to close, so the whole sensor is uncovered for at least an instant. That instant is when the flash fires.

At faster shutter speeds, the second curtain follows the first so closely that no moment exists when the entire sensor is visible. The flash, which lasts for a very brief burst, illuminates only the part of the sensor that is currently uncovered. The result is a dark band across the frame.

The sync speed is the point where the camera can still fire the flash while the full frame is exposed. It is listed in camera specifications, and it depends on the shutter design. If you want to understand how the shutter itself controls exposure, start with shutter speed and shutter types.

Why faster shutter speeds create a dark band

Imagine the shutter as a slit that travels across the frame. At the camera's sync speed, the slit is as wide as the sensor, or the full sensor is open, so the flash sees the entire image area. Above the sync speed, the slit is narrower than the frame. If the flash fires while the slit is still moving, the curtains themselves act like a shadow, and part of the frame never receives flash.

This is why a flash picture taken at an unsupported fast shutter speed often has a horizontal black edge or stripe. The stripe is not a lens problem; it is the shutter curtain blocking the flash. The same principle applies whether the flash is built into the camera or attached to the camera hot shoe.

Different shutter designs handle this differently. A leaf shutter, found in many compact and medium format cameras, opens like an aperture and can sync with flash at higher shutter speeds because it does not use a traveling slit. Cameras with electronic shutters read the sensor progressively, so their flash behavior depends on the sensor readout; see rolling shutter for more.

What is high-speed sync?

High-speed sync, often abbreviated HSS, is a flash mode that lets you use shutter speeds faster than the normal sync speed. Instead of firing once while the sensor is fully open, the flash fires in a rapid series of pulses while the shutter slit travels across the frame. To the shutter, the flash appears to be on for the entire sweep, so the slit does not record a dark band.

Some camera systems call this FP flash, for focal plane flash. The idea is the same: the flash output is distributed over time so that every part of the sensor receives light while the shutter exposes it. Because the flash must stay active during the whole travel, the system needs a camera body and flash unit that both support the mode.

HSS matters most when bright ambient light forces a fast shutter speed but you still want to use flash. For example, a daylight portrait with a wide aperture may need a shutter speed above the normal sync speed to avoid overexposing the background. Without HSS, you would have to stop down, lower the ISO, or use a neutral density filter. With HSS, you can keep the aperture you want and add fill flash.

The cost is effective reach. Because the same amount of flash power is spread over a longer time, the light striking a distant subject is weaker. In practical terms, HSS works best close to the subject or when the flash can be powered up to compensate. This is tied to flash guide number.

• Outdoor portraits with wide apertures
• Action shots in daylight where you want both fast shutter and flash
• Backlit subjects that need fill light
• Any situation where normal sync speed is too slow to control ambient exposure

Normal sync vs. high-speed sync: choosing an approach

Normal sync is usually the right choice when the flash is the main light and the shutter does not need to be very fast. It gives you the full rated power of the flash, because the entire burst is used in one brief moment. That means more range and often better battery life.

High-speed sync is a control tool. It lets you reduce ambient light with a fast shutter while still shaping the subject with flash. Choose it when the problem is too much sunlight, not too little flash power. When the shutter is at or below the sync speed, normal sync will almost always give you more flash reach.

Some cameras offer an automatic HSS mode that switches on when the shutter speed exceeds the sync limit. The exact menu name and trigger method vary by brand, so check the camera's help section. You also need to know the camera flash modes your body supports before relying on a particular mode.

How shutter type changes flash sync

Focal-plane shutters, common in interchangeable-lens cameras, create the sync speed limit described above. The exact sync speed depends on how fast the curtains travel and how the camera triggers the flash.

Leaf shutters, often built into compact cameras and some medium format lenses, do not use a traveling slit. The shutter blades open from the center, which means a flash can fire while the full sensor is uncovered even at very fast shutter speeds. That is why some studio photographers choose leaf-shutter systems for high-speed sync work without HSS.

Electronic shutters are a separate case. Because the sensor is read out line by line, the camera may restrict flash use, lower the sync speed, or disable flash entirely in fully electronic mode. Some mirrorless cameras use an electronic first curtain with a mechanical second curtain to improve flash compatibility. For more, see shutter types.

What to look for when buying a camera for flash work

Start with the body. If HSS is important, the camera must support it over its flash hot shoe or built-in wireless control, and you need a compatible flash. If you only use a built-in flash, HSS is usually not available because the small built-in unit cannot sustain the pulsing output.

Check the listed sync speed in the camera's specifications. A body with a higher sync speed is more flexible for daylight fill flash even before you add HSS. Standards bodies such as CIPA coordinate common specifications across the industry, but the exact flash behavior still appears in each camera's specification sheet.

If you mainly photograph people indoors or at events, conventional sync and a powerful external flash may be enough. If you want outdoor portraits at wide apertures or action photos in bright light, look for a system with HSS support. Our full-frame mirrorless cameras guide, mirrorless cameras guide, and DSLRs guide all include models with different flash systems. For a smaller carry-everywhere camera, compare the flash features in the compact cameras guide.

Practical steps to freeze motion with light

Decide what the flash is doing. If it is the main light and the shutter is at or below sync speed, set the camera to manual or aperture priority, place the flash where it needs to be, and use flash exposure compensation to fine-tune the balance.

If sunlight forces a faster shutter, switch the flash to HSS. Select the shutter speed you need, keep the aperture you want, and watch the flash range indication in the viewfinder or on the flash. Move closer or increase flash output if the range warning appears.

Remember that a flash burst can freeze motion even at a shutter speed slower than you might expect. In a dark room, the flash's brief burst is often the light that records the subject, while the shutter speed mostly controls the ambient background.

What to pick for your use

Questions

What is flash sync speed?

It is the fastest shutter speed at which the entire sensor is exposed while the flash fires. Above that speed, a focal-plane shutter's traveling slit blocks part of the flash, creating a dark band.

What is high-speed sync?

High-speed sync (HSS) makes the flash fire in a rapid series of pulses while the shutter slit crosses the sensor. The flash behaves like a continuous light, so you can use a shutter speed faster than the camera's normal sync speed.

Why does my flash photo have a dark band across it?

You probably used a shutter speed above the flash sync speed. The shutter curtain was covering part of the sensor when the flash fired. Use the sync speed or slower, or switch to HSS if your camera and flash support it.

Does high-speed sync reduce flash power?

Yes, in practical terms. The flash output is spread over the time it takes the shutter slit to travel, so a distant subject receives less light. You may need to move closer or raise the flash output.

Do all cameras and flashes support HSS?

No. HSS requires compatible hardware on both sides. Built-in camera flashes often do not support HSS, and some external flashes only work with specific camera systems. Check the specifications before buying.

Can I use HSS with an electronic shutter?

Not always. Fully electronic shutter modes may change or disable flash sync because the sensor reads out progressively. Some cameras handle this by using a mechanical shutter or electronic first curtain for flash work. Check the manual.