The back focus is the distance from the rear glass of the lens to the focal plane, when the lens is focused on a distant object.
Back focusing is the process of adjusting the distance between the rear element of a broadcast lens and the image plane in order to ensure that the image is in focus. This is an important step in setting up a camera system for broadcast or video production.
To back focus a broadcast lens, follow these steps:
- Mount the camera on a tripod and point it at a target that has clear and distinct lines or patterns.
- Set the lens to its widest aperture and zoom in to the maximum focal length.
- Adjust the focus until the image appears sharp and in focus on the camera’s viewfinder.
- Zoom out to the shortest focal length and check the focus again. If the image is not in focus, adjust the back focus of the lens.
- To adjust the back focus, locate the back focus ring on the lens. This is usually a small ring located on the back of the lens near the mount.
- Loosen the back focus ring lock screw, and then turn the back focus ring to adjust the distance between the rear element of the lens and the image plane.
- Tighten the back focus ring lock screw and check the focus again. Repeat this process until the image is in focus at all focal lengths.
- Finally, check the focus again after the camera has been moved or bumped to ensure that the back focus has not changed.
It is important to note that improper back focusing can result in blurry or out-of-focus images, which can be costly to correct in post-production.