I'm a focus puller in the industry, and can tell you a few basics. During rehearsals (if we get a rehearsal anymore), we take measurements of where the camera is in relation to the marks that the actor hits. It's a choreography if you will. As the actor and camera moves through the set from mark to mark, we have measured each of those distances to know in relation, where the subject is from the focal plane of the camera, using a follow focus or wireless FIZ (Focus Iris Zoom) controller to match those distances on the lens. We have an onboard monitor that allows us to see what the operator is pointing at, at any given time, and must make minor adjustments to the best of our ability to compensate for an actor or camera op, missing a mark.
Anymore though, with the onslaught of the digital revolution, we are not given rehearsals as much because a lot of young DP's, directors, and producers don't understand how focus works, and we are expected to pull off the monitor because so many 1st AC's now pull off the monitor. There's this myth that we don't need a rehearsal or marks anymore with pulling from the monitor, which is completely false. This usually leads to the first take having a few buzzes here and there, especially if the 1st hasn't had a ton of experience pulling off the monitor. More experienced DP's will make sure we still have our marks before allowing the scene to roll, especially on longer lenses. For those of you in the video game generation, monitor pulling becomes much of the same eye-hand coordination that you use when playing your favorite video game. Knowing where you are on the lens, either with the follow focus or FIZ, becomes muscle memory after awhile, and getting a feel for the movements and distances becomes second nature. On larger shows, there has been times when I'm not even in the room of the camera. I will sit in the next room at a 17" full 1080 OLED monitor (full 1080 is important, because there is no degradation of the image, and allows us to see full sharpness) and wireless Preston FIZ. Before that can happen though, I still go in and take measurements of the room and grid it out. Measuring the size of the room, and objects in the room such as a couch or table or counter, help to give spatial awareness and make sure you don't pull to 15' when it's really only 12' away, etc. Also with HD, critical focus is so much more apparent, and the roll off of the depth of field is substantially more noticeable than on film. It's either in focus or soft. There is no "kind of sharp" anymore that you could get away with occasionally on film.
One problem that many people experience is not using proper cinematic lenses with good, clear, witness marks. This can screw over even the most talented of 1st AC's. I've turned down jobs because they were using canon still lenses before (the barrels continuously spin with no hard marks). In that situation, there is no way for me to guarantee focus to someone, and it makes me look like a terrible AC if everything is in and out of focus. I would rather turn down the job, than not be able to deliver quality, in focus images. People know you by your work in this industry, and the last thing you want is a show you worked on, playing in a theater, and half the movie is soft, followed by your name in the credits.
Hiring a good 1st AC is critical in the digital world of cinema. We are usually one of the least noticed and hardest working people on set, as we rarely get to leave the camera, and are responsible for knowing everything about how the cameras works, how to fix it, as well keeping the image sharp. A good 1st can save you missing that critical take where the performance was incredible, but you missed it because it was soft, or even get the camera back up and running ASAP after it has had an issue (though some glitches and failures are not repairable on set). We may not get the credit and fame of cinematographers and directors, but we are an important part of the filmmaking process. Visually, you'll only know we are there if it's out of focus, which if we are good, should rarely happen.
Joshua R. Turner
1st AC - IATSE Local 600