ISO, shutter speed and aperture make up what is commonly referred to as the exposure triangle. It is by intervening on these three parameters that you can quickly manage the exposure of the frame, both in the photographic and video field. However, while the photographer can actually change camera and lens settings with ample room for freedom, in cinema the need to maintain adequate visual continuity makes the situation quite different.
Usually when shooting a film the ISO sensitivity is kept constant at least within each scene. This is to prevent digital noise from varying between one shot and another but also to ensure that the extension and distribution of the dynamic range remain constant, as well as to keep the colors of the images as homogeneous as possible, facilitating the work of post production.
It is normal for an entire movie to be shot using only one or two ISO sensitivities, one for daytime and one for nighttime or low light shooting.
As indicated by DZOFilm, the shutter speed in film shooting must be set between 1/45 and 1/55 “. In this way, the blur created by the movement of the subjects and the camera, called motion blur, appears realistic to us. Traditionally, 1/48 “is used when shooting at 24 fps and 1/50” when shooting at 25 fps. This practice dates back to the days of film and the use of cameras equipped with circular shutters.
If you take faster times when shooting, the movements appear jerky in the movie playback because the moving elements are excessively defined within the individual frames. Using slower times, on the other hand, the moving elements are excessively indefinite within the individual frames and the movie appears unnaturally fluid and lacking in detail. With very slow times, it can even happen that fast-moving subjects leave a trail.
When it comes to shooting for slow motion, however, to obtain pleasantly fluid movements we follow the principle of shutter speed equal to double the frame rate. For example, shooting at 50 fps the recommended shutter speed is 1/100″, while shooting at 100 fps the ideal shutter speed is considered 1/200″. Mathematically it would be correct to say that the shutter speed must be equal to half the frame rate but it is common to use the opposite expression, taking into account the numbers and not what they represent.
Here in exposure triangle, the cinematographic need to maintain sufficient visual continuity imposes decisive restrictions. Normally the working aperture is left constant at least within each scene, so that the relationship between angle and depth of field does not vary between one shot and the next.
Let’s imagine the classic field-reverse: if in one plane the background appeared completely undefined and in the other clearly legible, the effect would undoubtedly be distracting.