24, 30, 60 … What Does it all Mean?
When planning for a video production, you may have been asked a number of questions. Let me reassure you that these questions are not to satisfy some strange curiosity. There are actually specific technical reasons for asking a number of questions. They have to do with choosing the proper video format for your project.
Choosing the Right Frame Rate
For example, we can film and produce video with various frame rates. The frame rate of a video is the number of single frames or images displayed each second. The standard video frame rate for cinema has been 24 frames per second (fps) since the introduction of sound film in 1926.
Analog video broadcast made it necessary for television cameras to lock onto the frequency of their respective electric grids. This means that for most of the world they adopted frame rates that matched the 50 Hz of their electricity. In North America, Japan and South Korea, as well as some parts of South America, instead, the frame rates matched the 60 Hz frequency of their grids. However, a problem that occurred in displaying color television images in older black and white TV sets made it necessary to drop the frame rate from 60 to 59.94 fps. This is still the current video format in the US.
As color TVs developed, three standards emerged: NTSC, PAL and SECAM.
In the areas marked in green, where the NTSC standard was adopted, the typical frame rates used are: 23.976, 29.97, and 59.94 fps. These are often referred to, respectively, as 24, 30 and 60 fps. In the other areas of the world, which adopted the PAL or SECAM standard, the frame rate is 25 fps.
23.976 (24) fps
Footage is usually recorded at this frame rate to be included in programs recorded on film or for cinema. A number of TV shows are also recorded with this frame rate to give a “cinematic look” to their production. This can be useful for including new footage with footage from film archives. It is often used for documentary work, independent film production and other similar uses. The key concept to keep in mind is that this frame rate will give a “cinematic” look to the video.
29.97 (30) fps
This frame rate is one of the most commonly used. It is ideal for streaming or downloading the video online, for forensic or legal documentary videos, and most other productions. It still gives a slightly “cinematic” look, but not as perceptible as with the slower 23.976 rate. You should not use this frame rate, however, if you need to convert the video to film or to the PAL system.
59.94 (60) fps
This frame rate produces video that is clearly and immediately identified as any other video camera with no “cinematic look.” It is the NTSC rate standard. So, it is the most familiar in terms of broadcast or TV viewing in the US. We can convert this to PAL if necessary. This rate is quite useful for sports, local news, industrial training and other similar productions.
Some of the cameras that we use can also record at higher frequencies, like 120 fps. This can create a slow-motion effect when played back at normal rates, like 29.97 or 23.976 fps. Footage taken at 120 fps but played back at 23.976 fps will show motion about five times slower; if played back at 29.97 fps it will appear about four times slower. Similarly, recording video at a slow rate and playing it back at a higher rate will produce a fast pace effect. This is like the very old movies, where everything seemed to move faster than normal.
Interlaced or Progressive?
In addition to all this, video can also be recorded as progressive or interlaced. An interlaced video displays two fields of video captured at different times. The odd and even lines of the video are displayed on alternated passes. This enhances the perception of motion while reducing the bandwidth necessary for the transmission of the signal to about half of what is necessary for a progressive image. Interlaced video was necessary in the early days of television, when the bandwidth available for the transmission of a TV signal was quite limited. It is a standard video format, however, that has remained even to our days. In progressive scan, instead, each line of video is displayed in progressive order, rather than in an alternate order.
While some uses, like the possible need to extract still images from the video, forensic documentation, or motion analysis, require a progressive scan, other uses call for interlaced video. This, however, is not quite as important in terms of decision making as the frame rate. In fact, a video recorded with progressive scan can easily be encoded as interlaced if needed. However, going the other way around does not produce good results.
Video Format and Decision Making
While all this “technical stuff” may seem boring, it is quite important in the process of decision making and planning for a job. For example, choosing which camera or artificial light sources to use for a given project may depend largely on the video format needed.
Certain scenes need to be planned quite differently at slower frame rates. For example, in a project filmed at 23.976 fps, in order to avoid a strobe-like effect, a pan needs to be considerably slower (and longer) respect to high frame rates. At 24p (23.976 fps with progressive scan), with a wide angle a 90 degree pan may require something like 15 seconds. The same pan with a telephoto lens may need as long as 120 seconds or even more.
Selecting a certain video format may depend on the target audience. If your intended audience is both in the US as well as in Europe, for example, you will need to select a video format that can be encoded to the European PAL standard of 25 fps as well as the US standard.
Does your video need some slow motion or time-lapse? If so, the decision — in terms of video format — affects not only the frame rate at which the scene is filmed or recorded, but also the rate at which it is played back. There are more complex situations, as well, that require a great deal of decision making before the scene is even set up. One challenging example (and certainly not the only one) is combining a slow motion effect with a time-lapse scene.
So, don’t be turned off by some initial questions. It is always better to plan ahead than to be sorry after the fact.