Keeping focus on a subject at any focal length is imperative to filmmaking. With a properly calibrated lens and focus marks, this is usually achievable. However, even if focus is accurately set on the barrel of the lens, it is sometimes possible for the focus to soften as the lens is zoomed in or out. If this happens on a parfocal lens, which should keep focus as the focal length changes, it is likely there’s an issue with the back focus of the lens. This does not mean that the lens is inoperable, it simply must be calibrated to the camera that you are currently using.
Back focus refers to the distance between the rear element of the lens and the camera’s sensor plane. This differs between types of camera, such as mirrorless and SLR, and also sometimes between cameras of the same type, such as two cinema cameras. Therefore, it is likely that at some point, a filmmaker will have to adjust the back focus of a lens. Also, re-setting the back focus might be necessary if there are large changes in ambient temperature.
The back focus is also sometimes referred to as focal flange distance. If the focal flange distance is off, the lens will soften as zoomed in or out even if it is parfocal. The back focus of the lens is easily calibrated on many lenses via a back focus ring, which can be properly adjusted while testing the lens optically at home or on set. All of Fujinon’s broadcast and cinema lenses have a back focus ring, allowing for the focal flange distance to be adjusted when necessary.
In order to properly back focus a lens, the focus of the lens must be examined and tweaked by doing an optical test. The entire process of back focusing a lens, once the camera is set up for the test, can take as little as five minutes:
Place the camera on a tripod and lock the tilt and pan so that the image will remain stable throughout the whole test. Use a focus chart such as a Siemens star chart if possible. The radial spokes of the star give fine detail that assist with finding the proper focus of the lens. If a focus chart is not available, find an object with fine detail, such as a fence or brick wall.
For a wide-angle lens, place the camera six feet from the focus chart or object you will be focusing on. For a long telephoto lens, place the camera fifteen to twenty feet away from the focus chart or object you will be focusing on. But typically, especially for zoom lenses, a distance of 10 feet is good.
After setting the camera distance, open the aperture as wide as possible on the lens. If the image becomes too overexposed, use an ND filter or alter the light in the room in order to adjust exposure, or adjust the shutter speed, same as you would to properly expose any other image.
It can sometimes help to adjust your monitor, turning up the contrast and lowering the brightness. Peaking assist may also help with finding the proper back focus, as long as it is not turned up too high.
If your lens has an extender, make sure that it is set to “1x”.
Once the lens and monitor are set, zoom in to the longest focal length on the lens. Adjust the front focus ring to find focus on the chart or object. Once you have gotten focus on the object, zoom out to the widest focal length on the lens without changing the front focus. Note if the sharpness of the image changed.
Loosen the screw on the back focus ring and adjust the ring to regain the sharpness of focus that was lost as you zoomed out. Repeat this process several times until there is no discrepancy between the focus when zoomed in or out. You should now be able to focus on an object using the front focus ring alone, and that focus should be retained through a zoom in or out.
Remember to lock the screw on the back focus ring once back focus is set properly. Do not over tighten the screw, as this may damage the lens.
Repeat the process as necessary when changing cameras or lenses.
Watch this quick tutorial on how to back focus a cinema lens.
