Frame rate, also known as frame frequency, is the frequency (rate) at which an imaging device displays consecutive images called frames. The term applies equally to film and video cameras, computer graphics, and motion capture systems. Frame rate is expressed in frames per second (FPS).
The human eye and its brain interface, the human visual system, can process 10 to 12 separate images per second, perceiving them individually. The threshold of human visual perception varies depending on what is being measured. When looking at a lighted display, people begin to notice a brief interruption of darkness if it is about 16 milliseconds or longer. Observers can recall one specific image in an unbroken series of different images, each of which lasts as little as 13 milliseconds. When given very short single-millisecond visual stimulus people report a duration of between 100 ms and 400 ms due to persistence of vision in the visual cortex. This may cause images perceived in this duration to appear as one 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. Persistence of vision may also create an illusion of continuity, allowing a sequence of still images to give the impression of motion.
Early silent films had stated frame rates anywhere from 16 to 24 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.
However recent studies have shown that the Retina actually juggles when processing information. In 2014, it was shown during research that the human eye could see at various frame rates varying from person to person. In 2011, written on a forum was said that the retina takes about 5 to 12 milliseconds for an electrical impulse to fire and reset, 100 to 1000 rods depending on where in the retina you are, can fire every 7 milliseconds on average or around 140 fps. Another website said that the human eye on average could see up to 150 fps. It is still unsettled on what the average "frame rate" of the human eye is, but so far based on recent studies within the last decade, show the human eye seeing anywhere between 75 to 150 fps with an average of about 140 fps.
Motion picture film
In the motion picture industry, where traditional film stock is used, the industry standard filming and projection formats are 24 frames per second (FPS). Shooting at a slower frame rate would create fast motion when projected, while shooting at a frame rate higher than 24 FPS would create slow motion when projected. Other examples of historical experiments in frame rates that were not widely accepted were Maxivision 48 and Showscan, developed by 2001: A Space Odyssey special effects creator Douglas Trumbull.
The silent home movie film frame rate was 16 FPS or 18 FPS for 16 mm and standard 8 mm, 18 FPS for Super 8. Sound speed was normally 24 FPS for all formats.
Digital video and television
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.
- 24p is a progressive 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-draw 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 slowing down the media to 24p then converting to 60 Hz systems using pulldown. 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 60fps 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 90Hz.
- 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 120p. There are also an array of modern monitors that now have refresh rates capable of 120 Hz which allows users to view content at 120 FPS. Project Morpheus which is a virtual reality headset to be released in early 2016, is capable of displaying content at 120 fps. 120 fps may become the new standard of HFR video by 2018.
- 144 fps, various gaming monitors can display 144 Hz. At 144 fps, motion appears continuous and there are no frames visible.
- 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. 300fps is also the maximum frame rate for the HEVC format.