What is a frame rate?

In the early days of movies, film wasn’t responsive enough to capture the short exposures needed to show motion fluidly. That's why photography subjects in the 1800s had to stand still for long stretches of time to get their picture taken. In the late 1880s, technical progress in film meant a higher number of frames could be captured by hand cranking a roll of film through the camera. This led to different frame rates being used throughout the industry, with films ranging from 14 to 26fps. This meant that real-time movement was not captured consistently. Mechanical cranks were eventually added to film cameras to stabilize the recording process. However, many filmmakers preferred to shoot specific scenes in different frame rates for different cinematic effects, like the super fast motion of a Charlie Chaplin movie, leading to industry-wide irregularities.

Two significant factors prompted the adoption of 24fps as the industry standard: the advent of sound synchronization and TV broadcasts. Earlier attempts to incorporate sound into film proved unfruitful, but by the late 1920s, the phonograph and similar inventions allowed Hollywood to sync audio during playback, starting with 1927’s The Jazz Singer. As image and sound in film became synonymous, filmmakers began to move away from the 16fps of the silent movie era to 24fps, which was the best frame rate for sound comprehension while using the least possible amount of film. 

In the fifties, 30fps became the norm for analog TV broadcasts in North America, Japan, and South America. At the same time, Europe and Africa adopted 25fps due to the different video formats based on hertz power, the NTSC and PAL, respectively. Modern television shifted away from these formats in recent times due to digital conversion, but the NTSC and PAL standards are still in use in the film and television industry. “A lot of times when I work on a project that’s European-based, they’ll say, ‘Just make sure to shoot in PAL mode,’” cinematographer Hiroshi Hara says.

Frame rate and shutter speed are often mistaken as interchangeable. They aren’t, but they do share a close relationship. Shutter speed is a measurement in seconds of how long the shutter (which controls how much light is allowed into the camera) is open. The faster the speed, the lower the amount of light exposed to the film or digital sensor. There are many different types of shutters, from focal-plane shutters (found on DSLRs) to leaf shutters (found on medium to large cameras). However, for video, the most common shutter is an electronic one, though film cameras still use rotating discs for their light control. Electronic shutters have the ability to set higher or lower shutter speeds, in comparison to manual shutters, which must be physically attached. As a rule, to obtain realistic movement that the human eye is used to seeing, the shutter speed needs to be twice the frame rate.

The rotating disc is the most often used type of shutter in the film industry. It has a single constant shutter speed and, instead of changing the speed, filmmakers adjust the shape or angle of the shutter by 15-degree intervals. The larger the angle, the more light can pass through.

Similar to shutter speeds, the ratio between angle and light is interrelated. Divide or multiply the angle and it will do the same to the light. Most film cameras use shutter angles ranging from 0 to 180 degrees. At 180 degrees, you get the cinematic look we recognize from the big screen, as it correlates to precisely half the time that each frame takes to play in one second. “They realized at 24fps, a 180-degree shutter angle gives you the best motion blur that looks realistic,” Hara says.

Shutter angle is also the reason that, when filming at a higher frame rate, creators will need to account for more lighting to achieve the same exposure as shooting at a lower frame rate.