Canon Patent Targets Banding in High-Speed Sync Flash With a Smarter Inductor
High-speed sync has a dirty secret. Push a speedlight past your camera’s flash sync speed and every frame is quietly at risk of faint horizontal stripes. A Canon patent application published in the US this August goes after that problem at the level of a single component inside the flash: the coil that sits between the capacitor and the xenon tube.
The filing is US 2026/0235932, titled “Light-emitting device, flash device, imaging system, and imaging method.” Its Japanese priority date is February 2025 and the inventor is Yasuhiko Sano. It reads like a circuit paper, but the payoff is something any wedding or outdoor portrait shooter will recognize.
Two jobs, one coil
A shoe-mount flash like the Speedlite EL-5 works in two very different ways. In a normal pop it dumps its capacitor into the tube once, in a fraction of a millisecond. In high-speed sync, which Canon’s patent calls flat or focal plane emission, it does the opposite. The flash switches on and off over and over to make a long, lower-power pulse that lasts as long as the shutter slit takes to cross the sensor.
An inductor in the circuit has a role in both modes, and the two roles pull in opposite directions. For a normal flash you want a small inductance, so the current rises and falls fast and the flash cuts off cleanly when TTL says stop. For high-speed sync you want a large inductance, because it smooths out the current wobble caused by all that rapid switching.
That wobble is the problem. Canon’s filing says plainly that if the light flickers during flat emission, the image sensor records it as horizontal stripe noise. The shutter slit sweeps down the frame while the light pulses, so every dip in brightness lands on a different band of the picture.
Most flashes use an air-core coil here, and an air-core coil has one fixed value. You pick a compromise and live with it in both modes.
Letting the magnet do the switching
Canon’s idea is to use a coil wound on a magnetic core, such as ferrite or permalloy, and to exploit a property engineers usually try to avoid. A magnetic core saturates when the current gets high, and its inductance drops.
In a normal flash the peak current is huge, so the core saturates and the coil acts small. That’s exactly what a crisp, fast flash wants. In high-speed sync the average current is much lower, so the core stays out of saturation and the coil acts big. That’s what smooth, even light wants. One part, two values, with no switch.
The numbers in the filing make the point. In the first worked example the coil measures about 9.1 microhenries in flash mode and 21 in flat mode. Flash-mode current still peaks at 200 amps after 17 microseconds, the same as the air-core comparison, so normal flash behavior doesn’t suffer. In flat mode the current ripple drops by 28 percent, and Canon says that cuts the stripe noise.
A second version adds a small air-core coil in series. That combination reaches 8.3 microhenries in flash mode and 26.8 in flat mode, and Canon claims the ripple falls by 30 percent or more. It also lets the engineers narrow the core’s air gap, which shrinks the stray magnetic field. Canon notes that a leaking field near the camera can itself put noise into images.
Why it matters on a shoot
High-speed sync is how you overpower the sun at f/2 or freeze a fill-flashed portrait at 1/2000s. It’s also the mode where speedlights are weakest. They lose a lot of power, and evenness across the frame gets harder at very fast shutter speeds. If you shoot a lot of HSS, our speedlight versus battery strobe guide covers when a bigger light makes more sense.
This patent doesn’t promise more power. It promises cleaner light from the same power, with a part that’s small, cheap and passive. The filing also notes the idea isn’t limited to xenon tubes and could apply to LED light sources, which matters as hybrid flash and video lights blur together.
As with any patent, there’s no guarantee this ends up in a product. But a cheaper way to get stripe-free high-speed sync is exactly the kind of quiet fix that shows up in the next Speedlite without a word on the spec sheet.
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