The Time Warp Effect: Frame Rate Conversion
In movies, a filmmaker has the ability to alter time thanks to a technique called frame rate conversion. A typical video plays back at a speed of 30 frames per a second (fps), which is also the speed which is normally used for recording. When we want a video to play back at high speed, we record it at a frame rate that is less than 30 fps, and when we want slow motion, we record at a speed greater than 30 fps. So, when a shot is recorded for ten seconds at 15fps, it will take five seconds to play back that clip. If the video was shot at 60 fps, then it will take twenty seconds for the entire clip to play back.
Now, there are two ways to create high speed and slow motion effects. One is to do it with your camera when shooting. The other way to is to alter the frame rate in post-production.
Changing the Frame Rate in Camera
The traditional way of changing the frame rate is to do it in camera. If you’re going for a high speed effect, then whichever method you choose is going to produce the same results. But, if you’re going for a slow motion effect, then getting it in camera will get the best results. The reason why slow motion is best to be done in camera is that you will have unique and distinct frames in the video clips as opposed to duplicate frames that are created when a clip’s playback speed is altered in editing.
The biggest challenge that a filmmaker has to over come when altering the frame rate in camera is to compensate for the lighting. The frame rate is altered because the shutter speed is changed. Video being shot at 15fps gets twice as much shutter time than 30fps video. You need to reduce the light levels of the shot or else you’re going to have an over exposed image.
If you’re shooting slow motion, then you need more light than normal because the shutter speed gets reduced. If you do not adequately compensate, you will have an underexposed frame.
Changing the Frame Rate in Post-Production
Thanks to the digital age, it is possible to easily alter a clip’s playback speed in post-production. To create a high speed effect, the program just removes frames from the clip to shorten it’s length. For slow motion, the program duplicates and adds frames to extend the clip’s running time.
You can easily chop up your clips in editing and alter them to create a time warp effect. If you plan on doing this in your movie, then plan out all of the details before you shoot so you know what will happen where. If you have the ability to change the frame rate in your camera, then you should take advantage of it because slow motion looks better that way. Otherwise, you can just alter the frame rate in post.