Frame rate

Frame rate, most commonly expressed in frame/s, frames per second or FPS, is typically the frequency (rate) at which consecutive images (frames) are captured or displayed. This definition applies to film and video cameras, computer animation, and motion capture systems. In these contexts, frame rate may be used interchangeably with frame frequency and refresh rate, which are expressed in hertz (Hz). Additionally, in the context of computer graphics performance, FPS is the rate at which a system, particularly a GPU, is able to generate frames, and refresh rate is the frequency at which a display shows completed frames. In electronic camera specifications frame rate refers to the maximum possible rate frames could be captured, but in practice, other settings (such as exposure time) may reduce the actual frequency to a lower number than the frame rate.

The temporal sensitivity and resolution of human vision varies depending on the type and characteristics of visual stimulus, and it differs between individuals. The human visual system can process 10 to 12 images per second and perceive them individually, while higher rates are perceived as motion. Modulated light (such as a computer display) is perceived as stable by the majority of participants in studies when the rate is higher than 50 FPS. This perception of modulated light as steady is known as the flicker fusion threshold. However, when the modulated light is non-uniform and contains an image, the flicker fusion threshold can be much higher, in the hundreds of hertz. With regard to image recognition, people have been found to recognize a specific image in an unbroken series of different images, each of which lasts as little as 13 milliseconds. Persistence of vision sometimes accounts for very short single-millisecond visual stimulus having a perceived duration of between 100 ms and 400 ms. Multiple stimuli that are very short are sometimes perceived as a single stimulus, such as a 10 ms green flash of light immediately followed by a 10 ms red flash of light perceived as a single yellow flash of light.

Early silent films had stated frame rates anywhere from 16 to 24 frames per second (FPS), but since the cameras were hand-cranked, the rate often changed during the scene to fit the mood. Projectionists could also change the frame rate in the theater by adjusting a rheostat controlling the voltage powering the film-carrying mechanism in the projector. Film companies often intended for theaters to show their silent films at a higher frame rate than that at which they were filmed. These frame rates were enough for the sense of motion, but it was perceived as jerky motion. To minimize the perceived flicker, projectors employed dual- and triple-blade shutters, so each frame was displayed two or three times, increasing the flicker rate to 48 or 72 FPS and reducing eye strain. Thomas Edison said that 46 frames per second was the minimum needed for the eye to perceive motion: "Anything less will strain the eye." In the mid to late 1920s, the frame rate for silent film increased to 20–26 FPS.

When sound film was introduced in 1926, variations in film speed were no longer tolerated, as the human ear is more sensitive than the eye to changes in frequency. Many theaters had shown silent films at 22 to 26 FPS, which is why the industry chose 24 FPS for sound film as a compromise. From 1927 to 1930, as various studios updated equipment, the rate of 24 FPS became standard for 35 mm sound film. At 24 FPS, the film travels through the projector at a rate of 456 millimetres (18.0 in) per second. This allowed simple two-blade shutters to give a projected series of images at 48 per second, satisfying Edison's recommendation. Many modern 35 mm film projectors use three-blade shutters to give 72 images per second—each frame is flashed on screen three times.

In drawn animation, moving characters are often animated on twos, meaning one drawing is displayed for every two frames of film. Since film typically runs at 24 frames per second, this results in a display of only 30 drawings per second. Even though the image update rate is low, the fluidity is satisfactory for most subjects. However, when a character is required to perform a quick movement, it is usually necessary to revert to animating on ones, as twos are too slow to convey the motion adequately. A blend of the two techniques keeps the eye fooled and controls production cost.

Animation for most "Saturday morning cartoons" firstly introduced in the mid-1960s was produced as cheaply as possible and was most often shot on "threes" or even "fours", i.e. three or four frames per drawing. This translates to only 8 or 6 drawings per second respectively. Anime is also usually drawn on threes or twos.

Due to the mains frequency of electric grids, analog television broadcast was developed with frame rates of 50 FPS (most of the world) or 60 FPS (Canada, US, Mexico, Philippines, Japan, South Korea). The frequency of the electricity grid was extremely stable and therefore it was logical to use for synchronization.

The introduction of color television technology made it necessary to lower that 60 FPS frequency by 0.1% to avoid "dot crawl", a display artifact appearing on legacy black-and-white displays, showing up on highly color-saturated surfaces. It was found that by lowering the frame rate by 0.1%, the undesirable effect was minimized.