Sometimes we only need broad control, think 8s or 12s...but other times we require much more control for intricate actions.
Generally when animating on 3s and 4s you are dealing with more complex motion that needs to be broken down and controlled to a fairly tight degree.
Imagine animating a character dancing like Gene Kelly. The footwork involved can be highly complex as well as moving very fast. In order for us to block out that movement and include the proper key poses, you have to control it to a fine degree by setting keys 3-4 frames apart.
You are trying to keep the computer from splining too much so when keying fine actions like this you are only giving the computer the freedom to create 1-2 frames that are not specifically set by you. In traditional animation animating on 3s and 4s would take on a different meaning, though generally it would only be used in very limited animation, such as the Flintstones. Traditional limited animation would try to reuse a frame or parts of a frame of animation to reduce the amount of work necessary usually for budgetary or stylistic reasons. In CG it refers more to how often we key and object to define or control its motion.
Guest blogger David Breaux