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.