frame rate

Rate and apparent motion

A motion picture is a series of still frames presented in rapid succession; the frame rate is how many appear per second. Because the impression of movement arises from apparent motion rather than from any real motion on the strip, the rate must be high enough for successive frames to fuse into continuous action. A rate that is too low yields visibly discrete or jerky images, while higher rates render fast movement and camera movement more smoothly.

Frame rate is distinct from the perception of flicker. Flicker depends on how often the screen goes bright and dark, which is set by the shutter rather than by the number of distinct images alone. Early projectors used multi-bladed shutters to interrupt each frame two or three times, raising the flicker frequency above the threshold of visibility without increasing the number of frames.

Film standards

In the silent era there was no single standard. Cameras and projectors were often hand-cranked, and rates varied roughly between 16 and 24 fps, sometimes changing within a single film; projection speeds frequently differed from the speed of capture. The introduction of synchronized sound at the end of the 1920s required a fixed rate, because a soundtrack read at a constant linear speed cannot tolerate variation. The figure adopted, 24 frames per second, became the standard for sound film and remains the conventional rate for theatrical cinema.

Video and display rates

Television and video adopted rates tied to the electrical supply and the scanning systems of their regions. In PAL and SECAM regions the rate is 25 fps, and in NTSC regions it is approximately 30 fps — precisely 29.97 fps in the color standard, a value chosen to keep the color signal compatible with existing monochrome broadcasts. Transferring 24 fps film to these systems requires speed changes or the insertion of repeated fields to fill the difference.

A further distinction applies to digital display: the capture rate (how many distinct images are recorded) may differ from the refresh rate (how often a screen redraws its image). A display may repeat each frame several times, or generate intermediate images by interpolation, so that its refresh rate exceeds the rate at which the source was actually shot.

High-frame-rate cinema

High-frame-rate (HFR) cinema records and projects at rates above the 24 fps convention, commonly 48, 60, or 120 fps. The higher rate reduces motion blur and the strobing of fast movement. The resulting image has been associated with a reduction in the appearance long connected to 24 fps projection, an effect sometimes labeled the “soap opera effect.” Because 24 fps is the established rate for theatrical film, frame rate functions as a variable in cinematography as well as a technical parameter.