Slow motion: what it is and how it is achieved

Slow motion, also known as rallenty or ralenty, is a video technique, performed during filming or editing, that consists in shooting the subject in motion at slow speed.

The father of slow motion is August Musger, an Austrian priest and physicist. He invented an instrument that, when applied to the camera, changed the speed of the film. This was a complex mechanism that included the use of sixteen mirrors. It first appeared in the early twentieth century. 

Let’s discover the secrets of slow motion!

How does slow motion work?

The human eye perceives a video as fluid if it flows at least 24 frames per second (fps). These are frames that our eye cannot perceive individually, but that identify as a movement as they flow fast. This is why videos are usually recorded at speeds ranging from 24 to 30 frames per second, so that the human eye perceives movement. However, if in a video camera you increase the frames from 24 or 25 fps to, say, 120, the human eye wouldn’t be able notice the difference.

It would still perceive the video at normal speed, only more fluid and detailed. The slow motion effect is based on this perception anomaly: the video that is recorded has many more frames per second, but is still played at the “standard” speed. With a video of many frames per second but played at 30 frames per second, the end result for the eye is an expansion of the movement, which is perceived slower than normal. This results in the characteristic slow effect.

How is slow motion achieved?

Slow motion can be achieved during filming, with the help of professional cameras called “High Speed Cameras”. They record up to 1000 fps per second and slow motion up to 43 times its normal speed.

Another way to achieve slow motion is to intervene in post-production. In video editing, frames are redistributed over a certain amount of time. The video will be less fluid, though. 

Smartphones capable of recording medium-high quality video can also produce slow motion video. However, it should be borne in mind that the quality may drop due to the high video size and the maximum resolution allowed by the camera. The slo-mo effect is also applicable on a video recorded “normally” (24 frames per second) with special software, such as Adobe Premiere Clip or Slow Motion Video FX.