The minimum rate of presentation of successive images of a motion picture that allows motion to seem smooth, rather than jerky. The fusion frequency is almost always lower than the flicker frequency.

As it applies to the rate at which images are presented, rather than the rate at which they were shot, material that appears to be at or above the fusion frequency when viewed at normal speed may be below it when viewed in slow motion. Techniques to smooth motion presented at a rate below the fusion frequency have been developed for such purposes as computer-assisted animation; these are sometimes called in-betweening techniques. See also Judder.