I am lucky enough to have stumbled across the website of Sam Garwood, a director of photography and a camera assistant in the industry. One of the sections on his site is titled “A Really Simple Really Difficult Job” that is full of tips and theories about camera assisting “aimed at people who are already in the camera department and probably already doing the job.”
There was one piece of advice about pulling focus on longer lenses that I found particularly enlightening and useful.
Garwood claims this is the “most effective” method for focusing on long lenses, especially with subjects traveling to or away the barrel of the lens. Since he does such a good job explaining the method himself, I will let the tip stand on it’s own:
You’ll need some time at the eyepiece. And you’ll need to ask the DoP for the approximate stop.
Send a loader/trainee with a bunch of marks to the furthest point of the shot. You might both need walkie-talkies. Get a focus mark (my preference is always on the lens barrel) for that point, wide open, looking through the lens.
Stop the lens down to the shooting stop, or somewhere around it. Looking through again, bring the assistant forward until they go unacceptably soft. Stop them at that point, get them to drop a mark (they’ll need to number them, because they’ll be calling them out to you during the shot), open up the lens and get a focus. Stop the lens back down to the shooting stop.
Repeat the above, stopping the assistant each time they go unacceptably soft, until you have them as close as you’ll use the shot (for safety, get just one more…). Obviously as they get closer, you’ll be stopping them more and more frequently.
On the barrel of the lens you’ll now have a bunch of numbered marks, each pair closer together than the previous two. Concentrate on these marks during the shot, with your assistant leaning over your shoulder watching the subject and whispering the numbers to you (or if they can’t see all the marks, standing somewhere where they can and relaying them to you on the walkie-talkie). If the subject is moving at a constant speed, the marks will be called to you faster and faster, but you’ll feel the speed increase in a natural way. Getting the assistant to say ‘and…’ before each mark sometimes helps with the rhythm of it. It’s harder to do this with a shot of a subject going away, because you have to start fast and slow down, but console yourself with the thought that unless it’s an actor backing away from you, it’s hard for the audience to see their face so you’ve got more leeway – if there aren’t any eyes to look at, humans will generally look at the brightest part of the shot, then the part that’s moving, then the part that’s in focus (score!)
While having an assistant lay down intermittent marks for a pull like this is nothing new to most AC’s, the aspect that I hadn’t thought of was to lay the marks whenever the subject went unacceptably soft. This means that even if you are off your marks by a little bit, there will be a bit of leeway in the focus to get back on track when you hear that number come up in your ear. Sure, it doesn’t make this kind of focus pull easy but it sure is a big step away from making it really hard.
I highly suggest you head over to Sam Garwood’s website to read the other tips he have. For some more advice, I also have posted before about the art of pulling focus, general tips for working on film sets, and of course, how to make the RED camera go. Some of this stuff may be old news, but others may be helpful, especially for those just starting out having to deal with those finicky DSLR’s.