Frame rate, (expressed in frames per second or FPS) is the frequency (rate) at which consecutive images called frames are displayed in an animated display. The term applies equally to film and video cameras, computer graphics, and motion capture systems. Frame rate may also be called the frame frequency, and be expressed in hertz.
Frame rate and human vision
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 Hz through 90 Hz. 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. 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. Silent films were often intended to be shown at higher frame rates than those used during filming. These frame rates were enough for the sense of motion, but it was perceived as jerky motion. By using projectors with dual- and triple-blade shutters, the rate was multiplied two or three times as seen by the audience. Thomas Edison said that 46 frames per second was the minimum needed by the visual cortex: "Anything less will strain the eye." In the mid to late 1920s, the frame rate for silent films increased to between 20 and 26 FPS.
When sound film was introduced in 1926, variations in film speed were no longer tolerated as the human ear is more sensitive to changes in audio frequency. Many theaters had shown silent films at 22 to 26 FPS which is why 24 FPS was chosen for sound. 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 for 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.
Digital video and television standards
There are three main frame rate standards in the TV and digital cinema business: 24p, 25p, and 30p. However, there are many variations on these as well as newer emerging standards. Frames per second are often expressed as hertz (1 Hz = 1 FPS).
24p is a progressive scan format and is now widely adopted by those planning on transferring a video signal to film. Film and video makers use 24p even if they are not going to transfer their productions to film, simply because of the on-screen "look" of the (low) frame rate, which matches native film. When transferred to NTSC television, the rate is effectively slowed to 23.976 FPS (24×1000÷1001 to be exact), and when transferred to PAL or SECAM it is sped up to 25 FPS. 35 mm movie cameras use a standard exposure rate of 24 FPS, though many cameras offer rates of 23.976 FPS for NTSC television and 25 FPS for PAL/SECAM. The 24 FPS rate became the de facto standard for sound motion pictures in the mid-1920s. Practically all hand-drawn animation is designed to be played at 24 FPS. Actually hand-drawing 24 unique frames per second ("1's") is costly. Even in big budget films, usually hand-drawn animation is done shooting on "2's" (one hand-drawn frame is shown twice, so only 12 unique frames per second) and some animation is even drawn on "4's" (one hand-drawn frame is shown four times, so only six unique frames per second).
25p is a progressive format and runs 25 progressive frames per second. This frame rate derives from the PAL television standard of 50i (or 50 interlaced fields per second). Film and television companies use this rate in 50 Hz regions for direct compatibility with television field and frame rates. Conversion for 60 Hz countries is enabled by displaying every third frame for three fields, while the remainder of frames are held for 2 fields. This is similar to pulldown, and the result looks identical to a typical film transfer. While 25p captures half the temporal resolution or motion that normal 50i PAL registers, it yields a higher vertical spatial resolution per frame. Like 24p, 25p is often used to achieve "cine"-look, albeit with virtually the same motion artifacts. It is also better suited to progressive-scan output (e.g., on LCD displays, computer monitors and projectors) because the interlacing is absent.
30p is a progressive format and produces video at 30 frames per second. Progressive (noninterlaced) scanning mimics a film camera's frame-by-frame image capture. The effects of inter-frame judder are less noticeable than 24p yet retains a cinematic-like appearance. Shooting video in 30p mode gives no interlace artifacts but can introduce judder on image movement and on some camera pans. The widescreen film process Todd-AO used this frame rate in 1954–1956.
48p is a progressive format and is currently being trialled in the film industry. At twice the traditional rate of 24p, this frame rate attempts to reduce motion blur and flicker found in films. Director James Cameron stated his intention to film the two sequels to his film Avatar higher than 24 frames per second to add a heightened sense of reality. The first film to be filmed at 48 FPS was The Hobbit: An Unexpected Journey, a decision made by its director Peter Jackson. At a preview screening at CinemaCon, the audience's reaction was mixed after being shown some of the film's footage at 48p, with some arguing that the feel of the footage was too lifelike (thus breaking the suspension of disbelief).
50i is an interlaced format and is the standard video field rate per second for PAL and SECAM television.
60i is an interlaced format and is the standard video field rate per second for NTSC television (e.g., in the US), whether from a broadcast signal, DVD, or home camcorder. This interlaced field rate was developed separately by Farnsworth and Zworykin in 1934, and was part of the NTSC television standards mandated by the FCC in 1941. When NTSC color was introduced in 1953, the older rate of 60 fields per second was reduced by a factor of 1000/1001 to avoid interference between the chroma subcarrier and the broadcast sound carrier. (Hence the usual designation "29.97 FPS" = 30 frames (60 fields)/1.001)
50p/60p is a progressive format and is used in high-end HDTV systems. While it is not technically part of the ATSC or DVB broadcast standards yet, reports suggest that higher progressive frame rates will be a feature of the next-generation high-definition television broadcast standards. In Europe, the EBU considers 1080p50 the next step future proof system for TV broadcasts and is encouraging broadcasters to upgrade their equipment for the future. Many modern cameras can shoot video at 50p and 60p in various resolutions. YouTube allowed users to upload videos at 60 FPS in June 2014. YouTube also allowed full HFR videos previously uploaded before 2014.
72p is a progressive format and is currently in experimental stages. Major institutions such as Snell have demonstrated 720p72 pictures as a result of earlier analogue experiments, where 768 line television at 75 FPS looked subjectively better than 1150 line 50 FPS progressive pictures with higher shutter speeds available (and a corresponding lower data rate). Modern cameras such as the Red One can use this frame rate to produce slow motion replays at 24 FPS. Douglas Trumbull, who undertook experiments with different frame rates that led to the Showscan film format, found that emotional impact peaked at 72 FPS for viewers.
90p, the HTC Vive and Oculus Rift are virtual reality headsets that refresh at 90 Hz.
100p / 119.88p / 120p are progressive-scan formats standardized for UHDTV by the ITU-R BT.2020 recommendation. Various cameras including GoPro, Sony and JVC can shoot video at 720p and 1080p at 120 FPS. There is an array of modern monitors that have refresh rates capable of 120 Hz which allows users to view content at 120 FPS. PlayStation VR (former Project Morpheus) which is a virtual reality headset released in October 2016, is capable of displaying content at 120 FPS.
144 Hz - 240 Hz, CRT displays and modern PC gaming monitors can display 144 Hz through 240 Hz.
240 FPS, Modern smartphones such as the iPhone 6s and action cameras including Sony and GoPro can take 240 FPS as a maximum in 720p with audio. 240 FPS when played back at normal speed on a 240 Hz monitor is also near the limits or about of perceivable smoothness.
300 FPS, interpolated 300 FPS along with other high frame rates, have been tested by BBC Research for use in sports broadcasts. 300 FPS can be converted to both 50 and 60 FPS transmission formats without major issues. 300 FPS is also the maximum frame rate for the HEVC format.
