Frame rate set lower than playback speed—compresses time visually. Essential for cloud timelapse, crowds, construction sequences.
You shoot slower, the footage plays back faster—that's the core. Instead of 24 fps, you record at 4, 8, or 12 fps, but then put the reels into the normal editing rhythm. Result: movement becomes acceleration. Clouds race across the sky, a construction site grows in seconds, people appear like ants on amphetamines.
On set, it works like this: You need absolutely stable camera—every shake is amplified by the factor you underexpose. Recording at 8 fps and playing back at 24 fps? The acceleration factor is 1:3. This also means your lighting must remain constant, otherwise the entire footage will flicker. No cloud flight without an ND filter and perfect exposure control. The classic sundial method—a long recording time over hours or days with a fixed camera—uses fast motion as a pure long-term process.
Practically, it gets tricky with editing with real actors. If your protagonist walks up stairs and you speed up time, she appears unnaturally rushed—sometimes desired, often disruptive. Therefore, most professionals use fast motion for pure establishing shots: traffic, sky, nature, crowds from an overview. There, the effect works without psychological irritation.
Related here are slow motion (reverse logic—higher fps rate) and real-time editing, but fast motion remains the best tool for process sequences—when you want to show how time passes without dialogue or character movements becoming a farce. Modern digital cameras do this elegantly; in the past, you had to work with hand-crank cameras or mechanical intervalometers. The technology has become more accessible, but the requirements for stability and exposure have remained.