Though they seem simple to do, racking focus is an art that requires precision and finesse.
Having a favorite rack focus from a film is something reserved primarily for big cinephiles and filmmakers, perhaps because it requires one to pay attention to the little, but integral things that occur on screen often without much ado. At first glance, focus pulling is as simple as turning a ring to change the focus from one subject to another, but it's so much more than that, technically, aesthetically, and narratively. In this Fandor video essay, Philip Brubaker not only shows you some of the greatest rack focuses in cinema, but also explains how 1st ACs and other pros do it so beautifully.
Video is no longer available: vimeo.com/212359038
Still think focus pulling is simple? Well, in some shooting situations it might be, but for others the task gets much more complicated and challenging. The first thing that comes to mind is how cinematographer Matthew Rosen pulled off an excellent focus pull for a detergent commercial, which was not only shot at 1500 fps with a high-speed Phantom Flex, but was also shot with a very shallow depth of field. Translation: there was zero margin for error.
There are many tricks DPs and 1st ACs use to pull focus correctly so they hit their marks, including something as simple as using your fingers as a guide, but it's not just about hitting them technically; it's also about hitting them psychologically. Many times there will be numerous rack focuses within a single shot and if the focus puller misses a beat and doesn't pay close attention to the rhythm and flow of a shot, they'll end up with a failed rack focus.
It makes sense considering the fact that they ultimately determine where the audience will focus their attention, and if it's on the wrong thing at the wrong time, then viewers will not be able to appreciate the actors' performances, the scene, or the aesthetic beauty of the shot to the fullest.
So, next time you see a focus puller, give them a pat on the back or a high five. They certainly deserve it!