Back focus is a common term used to describe the Flange Focal Distance (FFD). Or the distance between the rear surface of a lens mount and the film plane or sensor. The most common FFD in my line of work is relative to the Positive Lock (PL) Mount system used on many motion picture cameras and lenses. The FFD for a PL camera is 52mm (2.0472441″) This means that when a lens is mounted to a camera correctly, the image produced by the lens should come to focus at exactly 52mm from the rear of the lens mount. There is a lot of argument about precision calibration in regards to where exactly the image lands. For film and digital, the argument is that the image must land “within” the film or sensor and not on the surface. This is an extremely small amount, approximately 0.02mm or 0.0007″.
Adjustment of the FFD does not usually require moving the actual lens mount but moving the body and glass of the lens forward or backward relative to the film plane. Most mounts are manufactured using reliable solid stainless steel and are precision ground. This allows the mount of the lens to interface with the mount of the camera with little to no variance. Since the locking of the two mounts creates the same distance every time. The easiest way to adjust back focus is to add or remove shims between the lens mount and the body of the lens, effectively moving all of the glass and changing the distance from the rear element to the film plane. This can be tricky because both the camera and the lens have a standard to comply with and sometimes one standard must be broken to obtain proper back focus. All of this is assuming the lens you are using has precise focus marks that require calibration… If not, stop reading now.
Several tools are used to aid in the adjusting lenses as well as cameras. A lens must be set so that it obtains infinity on its mark. This is assuming the lens is marked properly from the factory. To check this a Reflex Autocollimator. I don’t want to geek out too much here, but basically, this tool uses a small test pattern that light passes through onto a corrective lens that makes the light travel in a parallel pattern into the lens in question. The light then travels though the lens and lands on a mirror (film plane) and bounces back through the corrective lens, still parallel, back to a reflex mirror where a technician can inspect the image. Assuming the lens mount of the collimator is proper, and the lens is adjusted properly, a sharp crisp image of the test patter can be seen. Otherwise, adjustment is necessary. The glass of the lens either needs to be moved closer or farther from the film plane so that the focal point is exactly where it should be. As I mentioned before, the easiest most common method for adjusting the distance to the film plane is with shims under the lens mount. Lens manufacturers will design a lens with room to add several shims from the factory so that they can be removed if necessary. The smallest common adjustment is one half thousandth of an inch (0.0005″). A perfectly adjusted lens (relative to infinity) can still be useless if the cameras FFD is improperly adjusted.
In the “old days” of film, adjustment of the FFD on a camera was done only be experienced camera technicians. With the coming of the digital age and cameras such as the RED One, adjusting back focus on the camera has become common practice. A poor common practice in my opinion. There are many tools to determine the FFD of a camera. The most simple and obvious is a depth gauge. Basically a ruler that measures the distance… Easy right? Not so much… I would recommend something a little more scientific. There are a few tools that will be coming to the market soon. Of course you could always use the old wide angle lens trick. This simply requires a wide angle lens that you know is perfectly adjusted. You may ask yourself, why a wide lens? Basically, the depth of focus is far more critical on the film plane with a wide angle lens than it is with a telephoto lens. So the idea is to set the lens on it’s infinity mark and dial in the back focus on the camera while looking through the view finder or a monitor trying to find the best image possible. Sometimes this can be tricky and less than accurate.
All in all, a camera can be adjusted to correct improper back focus in a lens and a lens can be adjusted to correct improper back focus in a camera… But this doesn’t do anyone any good because as soon as you change either variable, you’re going to be searching for a reason why everything is out of focus or your lens won’t reach infinity focus. The camera and lens should be adjusted independently and brought together with proper specs. This is the only way to keep everything calibrated correctly and avoid variance in critical components.