Flash Duration Motion Freeze Calculator
Motion and flash inputs
Motion-freeze result
Using the entered t0.1 duration. The estimate assumes the flash dominates the moving subject's exposure.
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Flash duration and blur comparison
The table keeps the calculated subject speed and movement angle constant. The highlighted row is closest to the current flash duration.
| Flash duration | Time | Subject travel | Across-frame blur | Compared with current |
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How to use the flash duration motion freeze calculator
- Choose the unknown. Calculate blur distance for a known setup, maximum speed for a chosen blur limit, or the duration required to hold motion within that limit.
- Check the duration definition. Select t0.1 when the specification gives it. If only t0.5 is published, the result describes that shorter interval and may understate visible trailing blur.
- Enter instantaneous subject speed. Rotating tips, splashes, wheels, limbs, and projectiles can move at very different speeds even within one scene.
- Set the movement angle. Use 90° for lateral travel across the camera's view. An oblique path has a smaller across-frame component; the calculator uses the sine of the viewing angle.
- Interpret the physical displacement. The result is distance at the subject. Whether it looks sharp also depends on reproduction scale, camera distance, magnification, focus, and resolution.
- Test the real setup. Use the exact flash model, mode, and power setting, suppress ambient exposure when flash should do the freezing, and inspect a captured detail at the intended output size.
Flash duration, speed, and blur formulas
Let v be subject speed, t the entered flash duration, θ the angle between the subject's path and the camera's viewing direction, d total subject travel, and b the component projected across the frame.
required t = b ÷ [v × sin(θ)]
Example: 36 km/h is 10 m/s. During a 1/2,000-second flash, the subject travels 10 × 0.0005 = 0.005 m, or 5 mm. With movement directly across the frame at 90°, the projected component is also 5 mm. At 30°, it is 2.5 mm because sin(30°) = 0.5.
Why t0.1 matters
Electronic-flash output rises and falls rather than switching on and off instantly. The t0.5 duration spans the interval above 50% of peak intensity; t0.1 spans the longer interval above 10%. Sekonic's flash-duration explanation describes t0.1 as the more useful estimate of the total pulse for motion freezing. Broncolor's cut-off technology guide warns that there is no generally valid factor for converting t0.5 to t0.1 because pulse shapes differ.
Published values can vary substantially by power level and operating mode. For example, Profoto publishes separate t0.1 and t0.5 ranges for the Profoto A2. Use the duration for the exact setting you will fire, not only the shortest headline specification.
Accuracy, ambient light, and practical limits
Planning estimate: this is a constant-speed, straight-line subject-space model. It does not predict sensor-plane blur, sharpness, or a guaranteed keeper rate.
The calculation treats the selected flash-duration interval as equally capable of recording motion, although real light output varies across the pulse. It does not model acceleration, rotation, changing direction, subject deformation, multiple flash pulses, rolling-shutter effects, focus error, diffraction, lens aberrations, or camera movement. A rotating object has different linear speeds at different radii, so enter the speed of the detail that must appear sharp.
When flash dominates the moving subject and the camera is at or below its normal sync speed, the short pulse can become the effective exposure time for that subject. Ambient light remains active for the full shutter interval and can add a separate ghost or trail. Reduce ambient contribution with aperture, ISO, shutter speed within sync limits, or scene control. HSS and long-tail/Hi-Sync techniques distribute light differently; use equipment-specific measurements rather than treating the nominal duration here as a conventional single pulse.
A 1 mm physical displacement may be invisible in a wide environmental photograph yet unacceptable in high-magnification product or macro work. Confirm with a test frame viewed at the final crop and reproduction size. For hazardous high-speed subjects, use appropriate remote triggering, containment, and specialist safety procedures; this calculator only estimates motion.
Flash duration and motion freeze FAQ
What duration should I enter: t0.1 or t0.5?
Enter t0.1 when available and select t0.1 in the calculator. It includes more of the decaying light tail and is a more conservative indicator of motion stopping. A t0.5 value usually produces a smaller calculated displacement but can omit light that still records blur.
Can t0.5 be converted to t0.1?
Not reliably with a universal multiplier. The ratio changes with flash design, power level, pulse shaping, and cut-off mode. Find the t0.1 specification for the exact setting or measure it with suitable equipment.
Why does lower flash power sometimes freeze motion better?
Many speedlights and IGBT-controlled strobes reduce power by cutting the pulse short, so lower settings can have much shorter durations. Some conventional voltage-controlled studio flashes behave differently. Always check the model's duration table.
Does a 0° angle mean there is no motion blur?
It means this simple model predicts no lateral subject-space displacement across the frame. Motion directly toward or away from the camera can still change magnification and focus, and real subjects rarely move as perfect points on a straight optical axis.
Is 1 mm of calculated blur acceptable?
There is no universal physical threshold. Acceptability depends on magnification, camera distance, focal length, resolution, crop, print or display size, subject detail, and creative intent. Use the result to compare setups, then inspect a test capture.
Does shutter speed still matter?
Yes. It controls the period during which ambient light can register and must remain compatible with the flash synchronization method. A short conventional flash can freeze the flash-lit component, while ambient exposure creates a longer trail.
Are my settings saved or uploaded?
No. The calculator runs entirely in your browser. Copy and CSV controls also work locally.