The flange or backfocus of a camera is extremely fickle. Since it is the distance from the rear of the lens to the film or sensor plane, if offset only be a couple millimeters, your lens markings could be off by a few feet or more.
With the advent of HD and digital cinema technology, camera assistants have been accustomed to checking the back focus of a camera and servicing it as well.
I’m no stranger to this myself. On a feature, the first thing I had to do every morning was double-check the backfocus of the RED One camera.
But what about film cameras? How do you service those?
Film cameras are wildly different beasts than their video brethren. They require different types of servicing, different types of tests done during prep, and are more mechanical than they are electronic.
A few days ago, reader Dave sent me an e-mail about this:
Well my question is something I thought of when I was prepping a camera package last week. It’s easy to spot when your flange back is off [with a video camera] on a monitor and the usual charts. But how do you go about doing this on a film camera? I guess the videosplit or optical viewfinder isn’t good enough for critical focus?
It is easy to spot when a video/HD/digital cinema camera is acting up on backfocus because you get the advantage of crisp monitoring and you can even service most of them on set.
Film cameras, meanwhile, require a more experienced hand with a collimator. To better explain it, let me quote The Filmmaker’s Handbook
Back focus is measured with an optical device called a collimator. For a film camera, a test image is projected through the front of the lens and bounced off the film and viewed through the collimator. When the lens focus ring is set at infinity, the test target should be in sharp focus, indicating that the lens is properly collimated. With film cameras, collimation is generally tested and adjusted by a lens technician.
The book goes on to recommend you do a backfocus check in a similar way you would with a video system, but roll on some film and have it projected to get a true sense of focus.
Backfocus Off = Focus All Soft
I have a friend who once told me how he shot an entire 5 day short film on 16mm film as 1st assistant camera. He said they did multiple rehearsals, he got plenty of time to get marks, and overall felt pretty good about the focus pulling part of the job.
You can imagine how horrified he was when all the footage came back soft and out-of-focus.
Immediately he called up the rental house and found out something had gone wrong with the collimation of the camera after they had serviced it.
The moral of the story is to always double or triple check gear when you’re renting. Though it is the rental house’s responsibility to maintain their equipment, it is your responsibility to make sure it was done properly.
It’s much better to find out issues during prep than on set.
With a film camera, if your lens markings aren’t focusing where they’re supposed to, well, it’s going to be hard to pull focus that way.
P.S.
The Arri Alexa, though a video camera, is similar to a film camera in that it must be serviced by a qualified technician to adjust backfocus. It uses a shim system that is much more sturdy and reliable than a self-servicing system. The price this reliability comes at is the inability to DIY.
What sort of tests do you perform during prep to make sure the collimation is correct? Have you had any issues with the backfocus of cameras before, digital or film?