This is a trick question, since it can be interpreted in two ways; how many frames do I need for a video of a certain duration or what is the frame rate needed for a certain amount of motion to be clearly portrayed?
For both questions to be answered satisfactorily, we first need to talk about two very important properties of videos; interlacing and frame rates.
The first important choice when shooting a video is whether we will shoot it in interlaced or progressive mode. Interlaced videos contain two kinds of frames; those containing odd lines and those containing even lines. When displaying these frames consequently (first odd then even) at a high enough frequency, the viewer cannot distinguish the empty lines and therefore perceives the video as sequential full frames. Progressive (or non-interlaced) videos on the other hand, display full frames (all lines appear at each frame). From the description of these two types of videos, it can be deduced that a progressive video will demand twice the size of an interlaced video of the same length and resolution. Another interpretation of this property is that the frame rate of an interlaced video that the human spectator perceives, is double that of an equally sized progressive video. Which brings us to the frame rate question; what is it and what does it affect?
The concept of frame rate denotes the number of frames that will be shot (and consequently displayed) in a second and thus it is measured in frames per second (fps). The number of frames per second defines the quality of motion capturing in a video, as high frame rates can capture more changes per second than low frame rates. This is particularly useful in videos containing lots of motion, for example, sports videos.
The frame rate of a video is highly dependent on the camera used to shoot it and the type of media it is shot for. Another very important feature of videos that affects the way the frame rates are described, is whether they are progressive or interlaced. The most widely used frame rates are:
- 24 fps progressive, or 24p, as it is more commonly known. A frame rate of 24p means that every second, 24 frames will be shot (or displayed). This frame rate is the typical standard for the film industry.
- A similar frame rate, used mostly in PAL video, is 25p. As its name denotes, this is a 25 fps frame rate of progressive video. This format is preferred in countries with an electrical current of 50Hz, as it is divides the frequency evenly. A common practice is to shoot PAL video at approximately 23.98 fps, slowing it down by a factor of 1000/1001. This leads to an easier transfer process of the video to the NTSC format, which is described next.
- In the U.S.A. and Canada, as well as some other countries which have a 60Hz power grid, the usual frame rate used is 30 fps progressive, that is, 30p. This video format is called NTSC and it is usually a better choice when the video will be viewed on computer monitors, as they are likely to have a refresh rate of 60Hz, which is divided evenly by 30. It is worth noting that the actual frame rate used for most NTSC videos is 29.97 fps. This should be taken into account when processing a video, because in these tasks accuracy matters a lot.
- Lately, Hollywood moviemakers started experimenting with a frame rate of 48p, with the first movie to be filmed at this frame rate being The Hobbit.
- The interlaced equivalents of PAL and NTSC videos mentioned previously are called 50i and 60i respectively. While they are generally better than the progressive alternatives at half the frame rate (for example, 60i over 30p) in depicting fast movements (such as in sports videos), they tend to blur details because only half of the information exists in any given frame.
- Higher frame rates are also feasible in modern systems such as 50p, 60p (or 59.94 fps for better compatibility with NTSC), or even 120p. These formats are slowly being spread beyond the field of industrial applications where they were first used because of their property of capturing high speed processes with great detail.
Now, let's get back to one of our original questions; how many frames do I need for a video of certain duration of, let's say 20 minutes?
Well, assuming that we shoot in 25p (PAL), the number of frames will be equal to the number of seconds multiplied by the number of frames per second (25). Since each minute consists of 60 seconds, we will need:
(20 minutes) x (60 seconds/minute) x (25 frames/second) = 30000 frames.
Now, if we were to shoot our video in classic NTSC format (29.97 fps), we'd need:
(20 minutes) x (60 seconds/minute) x (29.97 frames/second) = 35964 frames.
This is quite a lot of frames, right? This is why video processing is one of the most time-consuming tasks you can use to stress test a PC. It is also the reason why compression is a definite must when it comes to storing videos, if you want to avoid having to buy hard disks regularly.
Selecting the proper frame rate for your video has a lot to do with its intended use. Common sense dictates that the higher the frame rate, the better the quality of the video in terms of temporal smoothness (capturing motion details). This is the reason that older silent movies that were shot at frame rates below 24 fps appear to have an unnatural and jerky motion at times.
However, even 24 or 30 fps do not necessarily guarantee a perfect, fluid capturing and display of motion. The speeds of the depicted objects, as well as the detail in each of them also play important roles in choosing a high enough frame rate for your videos and generally speaking, when having a choice you should go with the higher frame rates, always minding the peculiarities of conversions between formats.
On the other hand, when dealing with animated sketches, the choice is usually very common; most of them are drawn to be played at 24 fps. However, the process of drawing 24 sketches for each second of an animated film is extremely time-consuming and exhausting. The solution to this issue, is for the designers to draw half, or even a quarter of the images needed and repeat them as many times as needed to fill the missing frames (for example, when drawing 12 frames for a second, each frame is shown twice to achieve a 24 fps frame rate).
Another important consideration in the aforementioned problem is the sensitivity of human visual perception. The visual system that we possess is quite complex and cannot be analyzed easily in a way that would favor a certain frame rate over others. While our eye-brain combo can be fooled into believing that a hand-drawn cartoon with only 12 unique frames per second moves smoothly, on the other hand it can detect light flashes that last way less than 1/25th of a second. This perception can also be affected by the viewing angle; that is, peripheral vision is even more sensitive to light flickering.