Canon Patent Fixes Broken Light Trails in Multi-Shot Virtual ND Exposures

In-camera “virtual ND” sounds like a free lunch. Instead of screwing a dark filter onto the lens for a long daytime exposure, the camera shoots a burst of short frames and averages them into one smooth picture. Moving water goes silky, crowds blur away, and no filter needed. It mostly works. A Canon patent application published in the US in August 2026 deals with the case where it doesn’t: bright moving lights.

The filing is US 2026/0247041, “Image processing apparatus that processes images captured by split exposure.” Its Japanese priority date is February 2025, and it names Takao Sasaki as inventor. Canon’s term for the technique is split exposure: shooting a run of frames across one long exposure period, adjusting brightness with ISO and averaging the results to mimic the slow-shutter look of an ND filter.

Why averaging breaks headlights

Canon’s example is a car with its headlights on, moving through the frame. With a real ND filter and one long exposure, the headlights draw a continuous bright streak. Light simply piles up on the sensor for as long as the shutter is open.

Split exposure is different under the hood. Each short frame catches the headlights in a different spot, and in each spot they’re bright enough to clip to white. When the camera averages, say, four frames, every part of the trail is bright in only one of them and dark in the other three. So the average pulls those clipped pixels well below their true brightness. The patent describes what you get: a dimmer trail, a hue shift, and breaks along the line where a smooth streak should be.

As Canon puts it, a long exposure is really an addition of light, and split exposure performs an average instead. For a waterfall in soft light the difference hardly shows. For a headlight, a street lamp moving in the wind or the sun glinting off a wave, it shows a lot.

Canon notes that earlier fixes reduced the color shift and dimming, but in its words the problem was never fundamentally solved.

Treat the bright parts separately

The fix is simple to describe. As each frame comes in, the camera sorts its pixels into saturated areas, which are clipped or close to it, and unsaturated areas, which are everything else.

It then builds two composites. One combines only the saturated areas from every frame; the other combines only the unsaturated areas. Finally it merges the two into the output image.

The trick is that each composite can use whatever blending method suits it. Ordinary scenery can keep the averaging that gives the smooth ND look. For the saturated parts, the patent names lighten-only blending as one option: compare frames pixel by pixel and keep the brighter value. That’s the same method star-trail and light-painting photographers use when they stack frames by hand. It gives a headlight its continuous, full-brightness streak back.

A second version adds a map of where saturated areas appear across the burst and uses it to correct both composites before they’re merged, which should help the edges where a light trail meets the background.

Where you’d notice it

Virtual ND already exists in some cameras. OM System’s Live ND made it famous, and the appeal is obvious for travel and street shooters who don’t want to carry filters for every lens. The weak spot has always been scenes with moving lights, which is exactly when a long exposure looks most dramatic: city traffic at dusk, fairground rides, boats with navigation lights in a harbor.

For now this is a patent application, not a feature on a Canon EOS R6 Mark III or any other body. But it’s all processing, with nothing new in the hardware, so it’s the sort of thing a firmware update or the next generation of bodies could pick up quietly.

If you still prefer the real thing, glass ND filters remain the clean way to get one continuous exposure. For video, our ND filter guide for drone footage explains how strength and shutter angle work together.

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