Flash Duration & Motion Freeze Calculator
Enter a subject speed, flash duration, and image scale to estimate motion blur in pixels. The calculator also solves backward for the flash duration or subject speed that fits your blur limit.
Predicted motion blur1
- Subject movement during flash (mm)
- 0.05
- Maximum recommended flash duration (us)
- 50
- Equivalent shutter speed
- 1/20,000 s
- Maximum subject speed (m/s)
- 1
- Duration safety margin
- 0%
- Motion-freeze assessment
- Pass: predicted blur is within the selected limit.
Object-space pixel size is 0.05 mm/pixel.
Transverse speed is 1,000 mm/s.
The result uses lateral image motion only; motion toward or away from the lens can still change focus.
How to use this calculator
- Choose field-of-view mode if you know the real subject width covered by the image.
- Choose magnification mode if you know optical magnification and sensor pixel pitch.
- Enter subject speed, motion angle, flash duration, and the blur limit you can accept.
- Compare the predicted blur with the recommended duration and maximum subject speed.
How the flash motion result is calculated
Flash freezes motion because the useful exposure time is often the flash pulse, not the camera shutter speed. This calculator treats the subject as moving at a constant speed during that pulse and converts the lateral movement into pixels on the image.
The first step is finding how much real-world subject distance is represented by one pixel. In field-of-view mode, the object-space pixel size is:
q = W / P
W is the subject width covered by the image in millimeters, and P is the number of pixels across that same direction. In magnification mode, the same value is:
q = (p / 1000) / M
p is sensor pixel pitch in micrometers, and M is optical magnification. The motion component across the image is:
v_perp = 1000 × v × sin(theta)
The predicted movement during the flash is d = v_perp × t / 1,000,000, where t is flash duration in microseconds. Pixel blur is then:
B = d / q
The reverse answers use the same relation. Maximum duration is t_max = 1,000,000 × A × q / v_perp, where A is the acceptable blur in pixels. Maximum subject speed is v_max = A × q / (1000 × sin(theta) × t / 1,000,000).
What changes the result most
Subject speed, flash duration, and image scale have direct effects. Doubling speed or flash duration doubles the blur. Doubling the field width for the same pixel count halves the blur because each pixel covers more subject distance. Increasing magnification makes the same real movement cover more pixels, so close-up and macro work often requires very short t0.1 durations.
The angle also matters. Motion straight across the frame uses the full speed. Motion along the lens axis has no lateral pixel blur in this model, although it can still change focus or size during the exposure.
What this calculator leaves out
The estimate assumes the flash provides nearly all recorded subject light and that the full pulse happens while the shutter is open. It does not model ambient ghosting, camera shake, rolling shutter, trigger delay, acceleration, rotation, defocus, diffraction, lens aberrations, sensor blooming, or sharpening in post-processing. Use a measured t0.1 duration when possible; t0.5 values can make a flash look faster than the visible tail of the pulse.
Worked example
Suppose a subject moves sideways at 1 m/s, the flash t0.1 duration is 50 us, and a 300 mm field width is recorded across 6,000 pixels. The image scale is 300 / 6,000 = 0.05 mm per pixel. The subject moves 1000 × 50 / 1,000,000 = 0.05 mm during the flash, so the predicted blur is 1 pixel.
With a 1 pixel limit, the maximum recommended duration is 50 us, the equivalent shutter notation is 1/20,000 s, the maximum subject speed is 1 m/s, and the duration safety margin is 0%.
Common questions
What flash duration is needed to freeze a moving subject?
It depends on the subject speed, motion angle, image scale, and how many pixels of blur you can accept. Use t0.1 duration when you have it, because it includes more of the light-producing tail than t0.5.
Should I use a flash's t0.1 or t0.5 duration?
Use t0.1 for a conservative motion-freezing estimate. A t0.5 number measures only the central part of the pulse and can understate blur from the longer tail of light.
Why can flash freeze motion at a slow camera shutter speed?
If the flash is the dominant light source, the subject is recorded mainly while the flash emits light. The shutter only needs to be open for the flash, although bright ambient light can still add a separate ghost trail.
How many pixels of motion blur are acceptable?
One pixel is a strict technical target for a sharp sample. Larger values may look acceptable for small displays, prints viewed at a distance, or subjects without hard edges.
How do I estimate optical magnification or field of view?
For field of view, photograph a ruler at the working distance and measure the real width covered by the crop you will judge. For magnification, divide the image size on the sensor by the real subject size; camera pixel pitch is usually listed in specifications or can be calculated from sensor width and pixel count.