Slow motion is a ratio: 100 fps interpreted at 25 fps plays at one quarter speed and makes every recorded second last four seconds.
Capture rate versus playback rate
The camera records a number of individual frames each second. Your editing timeline then displays a different number each second. When capture fps is higher than timeline fps and every captured frame is retained, the clip becomes longer and movement becomes slower.
Divide the timeline rate by the capture rate to find playback speed. A 120 fps clip on a 30 fps timeline plays at 25% speed; ten seconds of action becomes forty seconds. This is different from merely changing a clip’s speed with frame interpolation, which may manufacture intermediate frames.
Shutter speed still shapes the look
Higher frame rates usually require shorter exposures per frame. A common starting point is a shutter duration near twice the frame-rate denominator, such as around 1/200 s at 100 fps. It is a creative convention rather than a law, but it gives motion blur that tends to look natural when slowed down.
Shorter exposures demand more light. Before choosing the highest frame rate, check whether the scene, lens and acceptable ISO can support it. Many cameras also crop the sensor, reduce resolution or change codec options in their fastest modes.
Plan the edit before pressing record
Choose the delivery timeline first. UK broadcast-style projects commonly use 25 fps, while web projects may use 24, 25 or 30 fps. The same capture rate produces a different slowdown on each timeline, so calculate the finished length before allocating card space or screen time.
Slow-Motion Duration Calculator
See how long a high-frame-rate clip becomes on a 24, 25 or 30 fps timeline and how slow the resulting playback will be.
Open the calculator