# Motion blur and QR scanning

> Motion blur smears modules across the sensor when the code or camera moves during the exposure. At 50 km/h a passing car covers 14 m per second, so a typical 1/60 s exposure smears the image about 23 cm — far wider than any module. Codes must be read from a stop or near-stop.

Source: https://useqr.app/docs/scanning/motion-blur-and-qr-scanning · Last reviewed 2026-08-21 · UseQR is free forever, MIT licensed, no signup.

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## The one-line physics

During an exposure, relative motion drags every edge across the sensor:

**smear = speed × exposure time.**

A decode survives when the smear stays under roughly **half a module width** — edges
stay distinguishable and the sampler still lands on the right side of the threshold.
Beyond that, adjacent dark and light modules average into grey and the grid dissolves.

## The drive-by arithmetic

Codes aimed at moving traffic fail on arithmetic, not luck. A car at 50 km/h covers
**13.9 m per second**. A phone in daylight shoots around 1/60–1/125 s once you account
for a hurried, moving subject:

| Speed | 1/500 s (bright, still hand) | 1/60 s (typical) | 1/30 s (dim) |
|---|---|---|---|
| Walking, 5 km/h | 0.3 cm | 2.3 cm | 4.6 cm |
| Cycling, 20 km/h | 1.1 cm | 9.3 cm | 18.5 cm |
| Driving, 50 km/h | 2.8 cm | **23 cm** | 46 cm |
| Motorway, 100 km/h | 5.6 cm | 46 cm | 93 cm |

Even a [billboard](/qr-codes-on/billboards)-scale code with 10 cm modules is smeared
past recovery by the 23 cm swipe of a 1/60 s exposure at 50 km/h. And that ignores the
part where the driver should be driving. Codes for drivers are a design error — one of
the clear cases in [when not to use a QR code](/docs/basics/when-not-to-use-a-qr-code).
The honest pattern for [vehicle wraps](/qr-codes-on/vehicle-wraps) and roadside media
is a code that works **when parked or stopped**, backed by a short memorable URL for
everyone in motion.

The walking row explains everyday experience: a pedestrian who pauses half a second
scans fine; one who scans mid-stride in a dim station concourse does not.

## Handheld shake

Even standing still, hands tremble. At normal exposures (1/100 s and faster) the
tremor moves the image far less than half a module and scanning feels instant. The
trouble starts in dim light, where exposures stretch to 1/30–1/15 s and the same
tremor becomes a multi-module smear — which is why low light and motion blur are
really one combined failure, covered from the light side in
[QR scanning in low light](/docs/scanning/qr-scanning-in-low-light). Brace the phone,
hold for a second, and let autofocus settle. Optical stabilisation helps but does not
repeal the arithmetic.

## How industry freezes motion

Parcel sorters read codes on belts moving metres per second — seemingly impossible by
the table above. They cheat on exposure: industrial
[2D imagers](/docs/scanning/hardware-barcode-scanners-and-qr) use global-shutter
sensors with intense pulsed illumination, cutting effective exposure to tens of
microseconds. At 1/10,000 s, a belt at 3 m/s smears just 0.3 mm — nothing. The lesson
transfers: motion tolerance is bought with light and shutter speed, which a phone in a
dim room has neither of.

## Design margin

If your code will plausibly be scanned by people in motion — transit riders, queue
walk-pasts — buy margin the same way you do for distance:

- **Bigger modules**: print larger than the
  [distance minimum](/docs/scanning/scanning-distance-limits) via the
  [size calculator](/size-calculator), or shorten the payload.
- **Light the placement** so exposures stay short.
- **Give people a reason to stop** — a caption and a spot to stand beat any amount of
  oversizing.

Then confirm the artwork itself is sound with the [validator](/validate), so motion is
the only variable left.

## FAQ

### Can you scan a QR code from a moving car?
Effectively no. At 50 km/h the car moves 14 m per second, so an ordinary 1/60 s exposure smears the image about 23 cm — wider than even billboard-scale modules. Roadside codes only work for stopped vehicles, and inviting drivers to scan is irresponsible anyway.

### How much movement can a QR scan tolerate?
The smear during the exposure must stay under about half a module width. At bright-light exposures near 1/500 s that permits slow walking; at dim-light exposures near 1/30 s even handheld tremor can exceed it.

### Why do my scans fail at night but work in daytime?
Exposure time. In dim light the camera exposes for 1/30 s or longer, so the same hand shake drags the image many times further across the sensor than it does in daylight. Brace the phone or add light.

### How do warehouse scanners read codes on fast conveyor belts?
With global-shutter sensors and bright pulsed illumination that cut effective exposure to tens of microseconds. At 1/10,000 s a belt moving 3 m/s smears only a third of a millimetre, so the code is frozen mid-flight.

## Try it

- https://useqr.app/size-calculator
- https://useqr.app/validate