Choosing the right frame rate for your footage is a crucial decision to make when preparing for a project. The problem is that frame rates are often confused thanks to preconceived ideas. We have therefore outlined some points that will hopefully help to choose the right frequency.
What is the Frame Rate?
The frame rate of a captured image is simply the number of images that will be captured and therefore recorded in one second. If we were talking about 25/50p, this would mean 25/50 ‘progressive’ images per second, whilst 50i would be for ‘interlaced’. These rates can be modified according to any value, dependent on the camera that you are using – from 1 frame to several thousand per second. Frame rates are always related to that of broadcasting, which in Europe is generally 25 frames per second.
If you decide to record 1 frame per second and play it back at 25 frames per second, you will have an X25 (1×25) speed, comparable to a timelapse. On the other hand, 100 frames per second played at 25 frames per second will give a slow motion of X4 (100/25).
We can therefore only shoot at the same frame rate as the one planned for broadcasting, but we can decide to go off piste in order to free ourselves from certain limits.
The rate of image: history and perception of the eye
Historically, the frame rate is a succession of photos taken over a second. This introduced us to animation, which is the craft of making sure that our eyes can no longer see that they are watching a series of still images, and is therefore an illusion related to retinal remanence.
When silent cinema was first introduced, we were watching moving images at a rate of between 14 and 20 frames per second, because it depended on the speed of rotation of the operator’s crank! This is what gave the jerky aspect, as our eyes perceived the rest of the image.
When talking movies came to the gold, a magnetic strip was glued to the film, however the problem with this was that at this slow speed, the voice couldn’t be recorded properly. It was therefore decided that the rate of 24 frames per second was the perfect compromise between sound restitution, jerky images and the cost of film (which was extremely expensive at the time).
When television was introduced, in Europe the current frequency was set at 50 Hz, since cathode televisions consist of an electrical beam which scan the screen from top to bottom. It was therefore decided to send 50 half images (interlaced), so this beam had time to scan the screen from top to bottom between two images and two AC signals, because at the time we could not send full, progressive images.
Since the digitization of cameras and the revolution of consumer screens which can support full, progressive images, we can theoretically send any frame rate to any device. Despite this, you will hear that all cinema releases have to be in 24p, that TV is 50i and that 50p is only used to slow down the images, that everything in 50 gives a video text to the images and is therefore not cinematic, and on it continues. All of this however is largely false.
Why choose this or that frame rate instead of staying on the broadcasting value?
All directors have at least once been confronted with the following question: why is my panorama jerky when playing? The answer is that too low a frame rate was chosen i.e. 24/25 frames per second. As the camera makes a large movement, each of the 24/25 images in the second are too different from the next to fool our vision, and we therefore perceive the jerks. Each cadence offers its own advantages and disadvantages that we will list:
- 24p: Standard historical cinema
- Advantages – you can shoot everything in 24p without having to comply if you are shooting for a film release. Some cameras only offer Cinema 4K (4096 x 2160) at this frame rate. 24p is also the only frame rate that is universal between PAL and NTSC.
- Disadvantages – impossible to make fast camera movements without jerking, and to shoot action shots precisely. There is also a difficulty to comply with the frequency of the lights (50 Hz in Europe, 60 Hz in the USA), because 24 is not a multiple in the frequency of the 180d rule
- 25p: European standard for many TV channels
- Advantages – a good format to do everything as the rendering is strictly the same as that of the cinema. It is also a multiple of 50 Hz, meaning the shutter can manage artificial lighting.
- Disadvantages – again, it is impossible to make fast camera movements without jerking, and to shoot action shots precisely.
- 50p: Recent format, supported on the Web in diffusion.
- Advantages – far more precise, and you can pan the camera without jerking. This frame rate can also be used to produce slowdowns when conforming to 25p for example
- Disadvantages – greater use of data, as we are recording 2X more images that in 25p. TV channels do not support this format yet. The rendering is supposed to be less cinematic because the image is very precise, but this idea is wrong as everything is played with the shutter speed
- 50i: TV broadcasting standard in Europe
- Advantage -supported by almost all TVs, with no jerks in the image
- Disadvantages – the rendering is ‘video’ because we are recording 50 overlapping half images per frame, known as the comb effect when we freeze the image. It is therefore not recommended to be used for filming, and instead better to comply with a delivery of 50i if requested
- Higher speeds (100/120/240 and +, frames per second)
- Advantages – used to produce slow motions. Some films for example are almost shot entirely at 120fps to allow for slow-motion in post-production.
- Disadvantages – the level of light required, as artificial lighting can lead to flickering, as the camera is capturing more images than the frequency of the electric current. The image is also often limited to 2K unless you are going very high-end. Sound management is also extremely difficult to work around
The shutter speed – guilty for all the confusion
Viewpoints of frame rates are tenacious, because they are often based on the famous popular cinema rendering with motion blur, shallow depth of field and so on, and the famous 180 degree rule, which consists of using a shutter speed twice that of the frame rate in order to achieve a good compromise between precision and blurred.
The problem is that this rule is often applied with new formats, such as 50p for example. In other words, a 1/100th shutter is then used, which is not appropriate. Doubling the shutter speed is like asking for 2X more light, since the sensor will be 2X less exposed, and it is this than gives this type of image a ‘surgical’ effect. If you try to shoot in 50p at 1/50th, you will see that the motion blur is the same as in 24/25p, but that the rendering will be slightly different as there are 2X more images and therefore more fluidity.
Not all films are shot in 24p. “The Lord of the Rings” is an excellent example of this without you feeling like you’re watching TV. You must therefore feel free to choose your frequency and shutter speed according to your project, and make sure you ask the right questions during preparation!
So which rate should we use?
- Always ask yourself what the final broadcast medium will be as this will greatly determine your frame rate. Internet? Anything is possible. TV? We will start from 25/50P or 50i (or 25/50P that we will conform in post-production in 50i)
- Comply with the current frequency of the country in which you are shooting. When choosin your speed, unless you are shooting in the middle of nature, you will always be bothered by artificial lighting and flickering. In other words, if we shoot in the United States to broadcast in France, we will use a shutter to the 60th of a second (60Hz country) and use a frame rate of 25/50 frames per second for example. If we shoot in France and broadcast in the USA, we will take a shutter to 50Hz and use a frame rate of 30p.
Flickering samples due to bad frame-rate or shutter speed.
- Make your own artistic choice based on what you’re going to shoot. A football game? A high throughput and a fast shutter. A short film? A slower shutter etc.
- Consider post-production work. A common problem is mixing several frame rates on the same timeline. For example, a drone that only does 30p whilst the rest of your scenes are in 24p, or a mix of 24p and 25p interviews. Not all editing software is equal in this field – some will simply skip the excess images i.e. taking 30p down to 24p which of course creates jerks, whilst others will create missing images by mixing. The latter will analyse a whole sequence to recreate ‘real images’, following a very long process. Sound must also be considered. A 30p sound confirming to 24p will be accelerated and will therefore have to be corrected. Make sure you test thoroughly so you can make your own choices.