Understanding slow motion, frames per second and percentage of playback speed

Understanding slow motion, frames per second and percentage of playback speed

I recently read someone online asking how to convert between overcranked Frames Per Second for slow motion and getting the percentage of reduced speed in their final NLE. This is a pretty simple equation, but one than can be confusing to those just starting to understand how your capture frame rate equates to your final project frame rate.

Here are the factors that you have to consider:

  • The frame rate that your camera’s sensor was set to output
  • The frame rate of your video stream that is being recorded in the camera (can be different than sensor output!)
  • The frame rate of your final NLE sequence or timeline

You will set your camera to record in a high speed frame rate (generally considered 48fps and above) in order to record slow motion footage in the field.

Some cameras will also allow you to choose a separate frame rate for the video stream that’s actually being recorded (when the camera’s frame rate is faster than the recorded stream’s frame rate, it’s called “overcranking” or in-camera slow motion, the opposite is called “undercranking” or in-camera timelapse).

Overcranking will result in a slow motion file that will already match the frame rate of your final project (usually 24p, 25p or 30p). When you use overcranking, you will end up with a clip that already plays back in slow motion, and the percentage of slow motion will depend on your camera’s frame rate and the frame rate of the video stream. For example, if your camera was set to 60fps, and recorded that into a 24p stream, that would result in footage that is 2.5x slower than real time, or 40% of the actual speed. If your camera was set to 60fps and recorded into a 30p stream, that would be 2x slow motion, or 50% of the actual speed.

cSome cameras do not allow you to overcrank in-camera, and in that case your camera will be set to record for example at 60fps into a 60p stream. When you play this back in your NLE or media player, the clip will play back at what appears to be 100% of the actual speed. You will have to tell your NLE to use a slower frame rate for the clip, resulting in slow motion playback. In Adobe Premiere, right click on the clip in the project bin and go to Modify > Interperate Footage and choose “Use this frame rate” and enter the frame rate of your final project (usually 24p, 25p or 30p).

When you do this you will tell your NLE to playback the clip at a lower frame rate than the footage was captured in, resulting in slow motion playback. Whether your camera does this, or your NLE, it’s important to understand the basic equation at work:

Cameras with Overcranking:
Camera Frame Rate ÷ Stream Frame Rate = the amount of slow motion
(ie: 60 ÷ 23.976 = 2.5x slow motion)
100 ÷ Amount of Slow Motion = percentage of slow motion
(ie: 100 ÷ 2.5 = 40%)
100 * Amount of slow motion = increase necessary to playback footage at 100% speed
(ie: 100 x 2.5 = 250% playback in NLE for 1x speed)

Cameras without Overcranking:
Camera Frame Rate ÷ NLE Project Frame Rate = the amount of slow motion
(ie: 120 ÷ 23.976 = 5x slow motion)
100 ÷ Amount of Slow Motion = percentage of slow motion
(ie: 100 ÷ 5 = 20%)

The final percentage of slow motion could be used to make other clips match, or used to reduce audio recorded separately, as well as for time remapping so you know how much you can reduce or speed up clips to match actual speed or be the slowest possible without introducing frame interpolation.

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