by Larry Jordan
My goal in this article is to discuss the challenges in converting frame rates. If everything you shoot, edit and output is a single frame rate, then don’t change anything. This is the ideal way to work. However, as you start to integrate elements that originate at different frame rates, frame rate conversion rears its very ugly head.
Think of a video clip as a series of wooden children’s blocks connected by a piece of string. Each block represents a frame of video. As we pull the string, tugging the blocks along in a line, the frame “rate” represents the number of blocks (or images or frames) that pass an observer each second. Frame rate is measured in frames per second; “fps.”
Changing the speed of a clip is NOT the same as changing the frame rate.
- Changing the speed of a clip always speeds up or slows down the action displayed in the clip by repeating or removing frames.
- Changing the frame rate of a clip changes the number of frames passing an observer without changing the perceived speed of the action displayed by the clip.
This difference is significant. The first is easy, the second is hard.
We change the speed of a clip to create a visual effect. We change the frame rate of a clip to match the settings of our clip to the project. If you don’t need to match settings, don’t mess with changing frame rates.
There are two sides to a frame rate discussion:
There is a lot of debate as to which is the “best” frame rate. Some feel that 24 fps is more “cinematic,” while 60 fps is more “real.” As you should know by now, there is no “best.” Just as there is no “best” car, camera, or restaurant; there are simply choices.
Converting to a 24 fps frame rate will NOT make your movie look “filmic.” It will, generally, just make it look worse. The “cinematic look” is a combination of: lenses, lighting, depth of field, shutter speed, shutter angle, motion blur and frame rate. Changing the frame rate only affects the frame rate, not the look.
There are no right answers, just louder voices.
Also, to keep this article to a manageable length, I will ignore:
- Frame blending
- Optical Flow
- Field-based editing
These special cases don’t alter the basic rules of frame rates, though they can complicate understanding.
THE BASIC RULES
- Where possible, always shoot, edit and output the same frame rate.
- Camera-native frame rates always look better than converted frame rates.
- Frame rates are irrelevant on the web. Streaming and downloadable files can play at any frame rate.
- Frames are indivisible. (Think about those children’s building blocks. They can’t be split, stretched, squeezed or discombobulated.)
- The frame rate of your project/sequence takes precedence over the frame rate of your source media.
- The duration of a frame is one frame. Not more, not less.
- Partial-frame durations do not exist for video, though they do for audio. (You can’t display images for half-a-frame, or a quarter-frame. If your project/sequence is 25 fps, then the shortest duration you can display an image is 1/25th of a second.)
- Video editing software automatically “conforms” (converts) clips with different frame rates to match the frame rate of the project/sequence.
- Projects/sequences can only have one frame rate, though they can contain clips that use different frame rates.
- It is “easier” and “better” to convert from a faster frame rate (i.e. 50 fps) to a slower one (i.e. 25 fps), than from a slower frame rate to a faster one.
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