First of all, let's clarify what the fps (acronym for frames per second ) and the shutter speed are. The fps indicate the frame rate, i.e. how many frames each second of a film sequence consists of. Shooting at 24 fps therefore produces a movie made up of 24 frames every second, which means that the maximum time available to the camera to acquire a single frame is 1/24 ?. The shutter speed determines how long the sensor is exposed to light while capturing each individual frame. It follows that shooting at 24 fps the longest usable shutter speed is 1/24 ?. The shutter speed also determines the motion blur present within each frame and consequently in the final movie. Extremely fast shutter speeds can also freeze motion within individual frames and produce jerky-looking movies, while slower shutter speeds can amplify motion blur until the movie is unnaturally smooth and undefined. Cinematic motion blur is produced by a shutter speed between 1/45 and 1/50 “. As for the frame rate, when using speeds below 24 fps the illusion of motion decays and the movie is perceived as a sequence of frames, while using frequencies above 25 fps the movie tends to be too fluid to appear cinematic.
In film cameras and lenses, shutter speeds can be indicated both as fractions of a second and as angular degrees. The latter are based on the concept of a rotating shutter, which is the type of shutter mounted on film cameras and cine lenses. This mechanism has a more or less large portion of the disc that makes one complete revolution for each frame of film that is scrolled. The larger the missing portion of the disc, the longer the frame is exposed to light and the longer the shutter speed. Technically we should call it exposure time rather than shutter speed, but for some reason the counterintuitive definition is usually preferred.
The most used rotating shutter in the cinema field has always been equipped with the so-called crescent, that is a half disc. By covering an angle of 180 °, the crescent leaves the frame exposed to light for half of its rotation, ie for a time equal to half the frame rate. Therefore, 180 ° corresponds to 1/48 “when shooting at 24 fps, to 1/50” when shooting at 25 fps, to 1/1000 “when shooting at 500 fps and so on. Since cinema is traditionally made of 24 fps, the vast majority of films were shot at 1/48 ? and this has become the standard shutter speed of film shooting, which creates the degree of motion blur we are used to. to see at the cinema.In recent years, with the spread of digital, 25 fps and 1/50 ? are also arriving in theaters and it is really difficult to distinguish a projection of this type from a classic 24 fps exposed to 1/48 ?. The difference is minimal in terms of both frame rate and motion blur.
Since the angular degrees indicate the portion of the circle that is absent, the correspondence between angles and shutter speeds is clear. An angle of 90 ° leaves only 1/4 of a circle uncovered and consequently represents a time 4 times lower than the frame rate; an angle of 180 ° leaves 1/2 circle uncovered and therefore corresponds to a time halved with respect to the frame rate; an angle of 360 ° leaves 4/4 of a circle uncovered and therefore indicates a time equal to the frame rate. And so on.
It should be noted that while digital allows a shutter angle of 360 °, working in film this is not possible because the dragging of the film itself, however fast it may be, requires a time that must be subtracted from that available for exposure.
The advantage of using angular degrees instead of fractions of a second to set shutter parameters when using a digital camera is to let the camera automatically set a shutter speed equal to half the selected frame rate, so to produce a movie with the level of motion blur that we are used to seeing in the cinema. However, this is only valid as long as you use the standard fps speeds of 24 or 25 fps or when you use higher frame rates to get slow motion. If you use high frame rates with the intention of playing the movie at the same fps rate at which it was captured, things change.
To obtain a slow motion effect, frame rates higher than that which will be used for reproduction are used during the shooting phase. For example, if you want to make a slow motion at half speed in a film that will be reproduced at 25 fps, when shooting you set 50 fps. By doing this you have a movie with 50 frames within each second and in playback at 25 fps every second captured lasts twice as long. If you want to reduce the speed of the scene to 1/4, set 100 fps and so on. Also in these cases, to obtain a reproduction of the movements as you are used to seeing in the cinema, you use a shutter speed equal to half the frame rate and therefore 1/50 ? for 25 fps, 1/100 ? for 50 fps, 1/200 ? for 100 fps and so on.
However, if you shoot at 50fps for a 50fps projection, the closest motion blur to cinema is always produced by a shutter speed of 1/50 ?. Following the 180 ° rule and using a shutter speed of 1/100 ? would result in a movie in which the movements would appear jerky because the individual frames would be excessively defined.
It is clear that as the frame rate increases, the minimum usable shutter speed becomes higher and higher and that beyond 50 fps it is no longer possible to obtain a cinematic motion blur unless you are making a slow motion.
The 180 ° rule stems from the analogue tradition, from rotating shutters and from the fact that cinema, for about a century, only traveled at 24 fps; consequently, if frame rates higher than 1/48 ? were used during shooting, it was only to obtain a slow-motion effect during projection. Now that it is possible not only to shoot but also to reproduce a film at frame rates higher than 24 or 25 fps, the 180 ° rule reveals its limits.
The 180 ° rule is often declined in video, the shutter speed must be double the frame rate. We talk about double the frame rate instead of half the frame rate because we consider the numbers instead of the temporal measure they indicate: 25 of 25 fps is opposed to 50 of 1/50 “, 50 of 50 fps is opposed to 100 1/100 ? and so on.
So: the 180 ° rule, also called the double frame rate rule, finds reason to exist only when the delivery is at 24 or 25 fps. In any other case, the only factor to consider when it comes to cinematic motion reproduction is that the shutter speed that produces cinematic motion blur is 1/48 ?. It follows that all shutter speeds between 1/45 and 1/50 “are acceptable in order to reproduce motion in a cinematic way as they differ by less than 1/10” from the ideal measurement.
For slow motion. By applying the 180 ° rule, slow motion is more natural as it is characterized by a pleasant degree of motion blur.
For visual effects. Replacing a green screen, stabilizing a shot, tracking, rotoscoping, are all simpler processes when the movie being processed has a low level of motion blur. It is not uncommon for motion blur to be digitally added at the end of post production.
To avoid flickering or rolling bands problems. Artificial lights can create unwanted effects when shooting at certain frame rates, but it’s not infrequently possible to avoid this problem simply by changing the shutter speed.
By expressive choice. In many films, motion blur has been reduced to emphasize the dynamism of an action scene or accentuated to characterize a dream sequence.
