When you apply an effect to a clip, you may want the effect to appear only for part of the clip, or you may want the effect to change as the clip plays. In iMovie, you can make an effect appear or disappear gradually. More advanced video-editing programs like Adobe Premiere and Apple Final Cut Pro control effects by using keyframes. A keyframe is simply a frame of the movie that you set as a landmark (as you would the points you create and adjust on audio rubberbands when you edit audio). In advanced video-editing programs, you can set as many keyframes as you like, and you can change the properties of effects at each keyframe.
For example, suppose you use a rain effect on a video clip, and you want the rain to increase and decrease in intensity throughout the clip. Several seconds into the clip, you can set the rain to be very heavy. A few seconds later, create another keyframe where the rain is very light. The software automatically adjusts the Rain between those keyframes to make the change appear gradual.
Using keyframes effectively take a while to learn, and if you use a program like Adobe Premiere, I recommend you get a book that more fully describes the features of that programBut the keyframe feature does give you almost infinite control over the effects in your movies, so it’s definitely worth seeking out if you plan to move up to a more advanced editing program.
