To make any kind of animated transition within a video clip your software needs to “key in” on the starting and ending frames to demarcate them. Early black and white films were at these frame rates.
The human eyes starts seeing motion somewhere between 16-20 fps. There is debate as to the merit and need for this level of resolution (similar to the sample rate fidelity debate- 44.1khz compared to 96khz in audio sampling), but that is beyond the scope of this blog.
made movies are shot at 24-30 frames per second (fps), but directors like Peter Jackson are just beginning to shoot at 48fps. Video is shot in frames, and each frame is a picture still. This will let you create an animated pan (the effect of motion within a video clip) to a crop that has a smooth flow from one section of your video to another. In both iMovie and FCPX there is a function to do just this, appropriately named the Ken Burns Effect. Another approach to this is to use zoom as an effect to “crop” into your image while panning in your video clip, an effect if not originated at least made very famous by Ken Burns. In my last post I talked about zooming into and cropping an entire video clip in iMovie and Final Cut Pro X (FCPX).