The frames per second of an animation, or fps, can determine a lot about how the final animation looks. An animation done at 48 fps will be extremely smooth, while an animation done at say, 6 fps, will be quite chunky. The most common fps for animation are 24 and 12.
As a general rule, higher fps isn’t always better either. Each person will have their own opinion, but slight choppiness can be a good thing. If you’ve ever seen a film scene or animation where AI is used to smooth everything out and raise the fps, it can feel so off, almost like all the movements lack power and things are moving too slow, even though the length of the footage isn’t changed at all.
So, 12 or 24 fps are both good to avoid animation that is too smooth or too choppy. But what I found is that if you change the fps throughout an animation, the combination of choppiness and smoothness makes it significantly more appealing.
I recently did a short animation study of a shot in “The Legend of Korra,” which has really awesome animation. When I created this animation project file, I chose 24 as the set fps, and as you can see in my timeline, I had some frames playing just once and some holding for a little longer, repeating as many as three times.
When is it okay to let frames hold for a little longer, and when should they go by fast? What you should consider is weight and gravity. When Korra does relatively fast movements like pushing off of the ground, the frames play twice (at a rate of 12 fps). When she is hovering in the air, the delay before gravity takes over, the frame rate slows down as she does, and each frame is played three times (aka afbs). Then, the fire rushes so quickly that each frame is only played once, which is reaching our maximum of 24 fps.
Still having trouble understanding? Let’s use a simpler example, a bouncing ball. This animation is done at a consistent 12 frames per second, and the other is done at a fluctuating fps ranging from 8 to 24. Same as the Korra animation, you may prefer one or the other for different reasons. But at least I prefer the fluctuating one because of the energy and liveliness it has.
When the ball loses momentum, the movements are more subtle, and the fps decreases. This happens when the ball hits the ground, and when it reaches its maximum height. The fast, more extreme movements, when the ball is rising and falling, are at 24 fps, each frame playing just once.
Once you have a good understanding of this, the application is easy. I hope this all made sense, and if it didn’t, feel free to ask questions!