Things to consider before using this "old school" technique.
The Importance of Context
The interesting thing about video is that the way something appears is experienced differently by the audience according to it's supposed context. A camera movement can look great if done so with purpose, but the same digital video technique may appear misplaced if it was done so by accident. The same thing is true with certain stylistic choices, and if you are going to make one that will reveal the technical functions of the equipment, then it has to have a clear purpose. This is the kind of dilemma that goes into choosing to use rack focus in a given video clip.
What is Rack Focus?
Racking focus is when the videographer uses a change in focal length to shift the audience’s attention from one object to another. An example of this is if you have one person closer to the camera and on the left side of the screen, and then a person farther away and on the right side of the screen, and the camera’s focus changes from one person to another. This switches the objects that are blurry and forces the audience to switch their attention.
The decision to do this is purely a stylistic one because it is easy to make both images be in focus at the same time. Likewise, it is difficult to make the rack focus smoothly on a home digital video camera because you will most likely need to be turning the focus knob on the camera manually. To make this shot effective you need to have already framed an image perfectly and have two distances represented.
Reasons to Use or Not Use It
Rack focus is a very dramatic technique and should only be used if you are utilizing a melodramatic theme or satire. The rack focus should not be used as a back and forth between two people talking because it will end up more distracting than anything else. Often times it is a great way to shift the way a theme is being presented. You can have a person talking, then rack focus to an object or event behind them that illustrates their point.
Record on Purpose
Film is such a literal medium that every aspect and choice needs to be well thought out, planned, and have a clear purpose. Rack focus will look sloppy and over dramatic if it is not used sparingly and with a full reasoning behind it.