Frames per second (fps) is a unit that measures frame rate, which is the frequency at which consecutive images called frames appear on a display. The frame rate deeply impacts the quality and viewing experience of video and animation. The higher the frame rate, the smoother the motion in the video appears.
24 fps is a standard frame rate used for filming and projecting movies. It was originally chosen for practical reasons related to the cost of film and projector technology in the early 20th century, but has persisted as a cinematic standard due to aesthetics and convention. But what determines the relationship between frame rate and how long each individual frame is displayed for?
What is Frame Rate?
Frame rate is defined as the frequency at which consecutive images called frames are displayed. It is expressed as the number of frames displayed per second. The most common video frame rates are:
- 24 fps – Used for films projected in theaters.
- 25 fps – The PAL television broadcast standard.
- 29.97 fps – The NTSC television broadcast standard.
- 30 fps – Used for HD video and broadcasts.
- 60 fps – The standard for modern HD video.
Higher frame rates result in a smoother viewing experience, as more frames are displayed per second to create the illusion of natural, continuous motion. Lower frame rates may cause choppy or jittery motion.
The choice of frame rate represents a trade-off between smoothness and data rate/file size. Higher frame rates require more storage, bandwidth, and processing power. This is why different standards emerged for film, television, and video based on available technology and cost constraints.
History of 24 fps for Film
The 24 fps standard emerged in the late 1920s as a result of both artistic and technological considerations. Film projectors at the time typically had a shutter that blocked the projected light while the film advanced and a new frame was put in place. With a shutter that was open for 50% of the time, a frame rate of 24 fps resulted in smooth enough motion while keeping film costs manageable.
Lower frame rates around 16 fps resulted in visibly choppy motion, while higher rates put unnecessary strain on the mechanics of film cameras and projectors. 24 fps struck the optimal balance and quickly became the universal standard for both shooting and projection.
Decades later, even with vast technological improvements, 24 fps endures as the standard for a few key reasons:
- It provides adequate motion rendering while minimizing film usage.
- The equipment infrastructure for 24 fps is well established.
- Artists prefer the distinctive “film look” it produces.
- Audiences associate 24 fps with the cinematic experience.
The 1/24 Second Frame Time
At 24 fps, each frame is displayed for 1/24 of a second. This can be calculated as:
Frame Time = 1 / Frame Rate
So with a frame rate of 24 fps:
Frame Time = 1 / 24 fps = 0.0416666667 seconds = ~0.04 seconds
This means that at 24 fps, each individual frame lasts for roughly 0.04 seconds on screen. The quick succession of frames displayed for 0.04 seconds each creates the illusion of smooth, continuous motion.
Why 0.04 Seconds Per Frame?
Is there something special about 0.04 seconds specifically that makes 24 fps work for film? Not quite. It was simply the shortest frame time that allowed for smooth motion rendering within the technological constraints of the 1920s.
However, the 0.04 second frame time has persisted due to factors like:
- Visual aesthetics – Audiences are accustomed to the “look” of 24 fps film.
- Cinematic qualia – The 0.04 second time evokes the feel of movies.
- Economics – Infrastructure and workflows are optimized for 24 fps.
- History – It is hallowed tradition in filmmaking.
While not an absolute technical necessity, the 0.04 second frame time has become intertwined with the cinematic experience.
How Frame Rate Impacts Motion Rendering
The central reason frame rate matters for film is its impact on recording and perceiving motion. Some key considerations include:
Temporal Resolution
The frame rate sets the temporal resolution, which determines how finely video captures motion over time. Higher frame rates enable more temporal resolution and smoother motion.
Exposure Time
At 24 fps, each frame has an exposure time of around 0.04 seconds. Fast motion may become blurry or choppy if the exposure time is too long to capture action cleanly.
Flicker Fusion Threshold
The flicker fusion threshold is the minimum rate at which a sequence of still frames takes on the appearance of continuous, smooth motion. 24 fps sits comfortably above the 15-20 fps threshold for flicker fusion.
Judder
Judder is strobing or jerky motion caused by inconsistent frame times. At a steady 24 fps, motion rendering has minimal judder.
Cinematography Rules
Over the history of filmmaking, directors have developed techniques and rules around composing shots, camera motion, editing, lighting, and more optimized for 24 fps.
In summary, 24 fps provides an effective balance between temporal resolution, motion rendering, aesthetics, and cinematic conventions.
Higher Frame Rates for Film
While 24 fps has been the norm for around 100 years, higher frame rates are sometimes used for films. Reasons for using higher frame rates include:
- Smoother slow motion – High frame rates can enable extended slow motion.
- 3D films – Minimizes strobing that can cause headaches.
- Hyper-realism – Extra smoothness provides immersive realism.
Notable examples of films shot at higher frame rates include:
| Film | Frame Rate |
|---|---|
| The Hobbit: An Unexpected Journey | 48 fps |
| Gemini Man | 120 fps |
| Billy Lynn’s Long Halftime Walk | 60 fps |
However, reviewers and audiences have often criticized the hyper-real “video look” of higher frame rates. As a result, the trend has not completely overtaken 24 fps as the cinema standard.
24 fps for NTSC Video
The 24 fps film rate also carried over into some early television standards to facilitate transferring films to video. Specifically, the NTSC television standard supports a framerate of 29.97 fps. This was chosen so that film frames could be easily converted to NTSC video via telecine by alternating frames in a 3:2 pattern:
- 3 video frames for every 2 film frames
This conversion allows 24 fps film to be broadcast at 29.97 fps video. While not an integer multiple, it provides a suitable workaround for broadcasting film on television.
The Impact of Digital Video
Digital video recording and editing offers far more flexibility with frame rates compared to analog film. Some impacts on 24 fps include:
- Interlaced vs progressive capture – Interlaced video can introduce artifacts.
- Speed manipulation – Frame rates can be changed easily in post-production.
- Digital interpolation – New in-between frames can be computed.
- Variable frame rate – Frame rate can dynamically change.
While 24 fps remains a standard for filmed entertainment, digital technology has expanded creative options around frame rate.
The Future of 24 fps
Despite the new possibilities opened by digital video, 24 fps likely still has a long future ahead as a cinematic standard due to:
- Deep entrenchment across creative workflows and infrastructure.
- Artistic preference for the unique look it produces.
- Audience expectations and associations with the film experience.
For over 90 years, 24 fps has been inextricably linked with cinema. Even with changing technology, this standard frame rate persists both for practical and artistic reasons.
Conclusion
At 24 frames per second, each individual frame lasts for approximately 0.04 seconds on screen. This frame time emerged as the standard in the late 1920s based on factors like flicker fusion rate, motion rendering, and projector technology. While not the only option, 24 fps struck an optimal balance between image quality, motion portrayal, and cost.
The 0.04 second frame time has since become an intrinsic part of the cinematic experience. Artists utilize it to achieve desired visual effects, while audiences associate the 24 fps “look” with films. The frame rate forms a foundation for filmmaking techniques and workflows. While new digital technologies have provided more flexibility, 24 fps endures as the standard frame rate for the art of cinema.