Best Practices for Shooting Slow Motion Videos How to improve the effectiveness of optical flow when editing slow motion videos

Lewis McGregor recently posted a video for Shutterstock Tutorials on shooting slow-motion videos and using different techniques and methods for making those videos look more realistic and smooth even when not shooting in high frame rates.

As you are probably aware, shooting at high frame rates (i.e. typically above 24 or 30 frames per second) is the best way of getting smooth and creamy slow-motion shots by letting your video editing software “convert” them to a normal (i.e., 24/30 frame timeline). However what if you can only shot in 24p (either your camera does not support higher rates – most cameras don’t support over 30p in 4K yet and sometimes you prefer to shoot in 4K to be able to crop even if you deliver in 1080p and surely if you deliver in 4K).

The answer is that you can still sort of get away with some degree of slow-motion even if your material was shot at 24p but you need to consider a number of things:

  1. The simplest method to make a slow-motion is just to stretch the clip but this means that the software just duplicate frame to fill in the blank space that was created between frames and it looks bad (as if the video freezes of stagers).
  2. There are more advanced algorithms around that can “look” at the previous and next frames and decide what changed and create a frame in between. Optical flow in Premiere Pro (and similar effects on other editing software under different names) is just such a feature and it produces better results, typically but even if you are using it there are some things that you can do to help it create better results.
  3. When using methods such as optical flow you need to try and avoid fast cross-screen movement (slow movement or movement towards or away from the camera is preferable and result in better slow-motion effect).
  4. If you have to shoot cross-screen movement what you can do to improve the way optical flow works is to move together with your subject and try and keep it in the same area of the frame.
  5. You also should look for a scene with good tonal separation so that the optical flow algorithm will have something to work with.
  6. Even with everything is done right at ideal conditions you should typically slow down footage by no more than 25% (50% in extreme cases, you might be able to get by with more in scenes where there are only still subjects and the camera is moving very slowly).

You can watch more HDSLR and video techniques on our dedicated HDSLR channel here on LensVid.

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