In film production, a back focus chart is a tool used to check and adjust the back focus of a camera lens. Back focus refers to the distance between the lens mount of the camera and the image sensor or film plane. It is critical to achieve accurate back focus in order to ensure sharp focus and proper registration of the image on the sensor or film.

A back focus chart typically consists of a target pattern with various elements, such as lines, circles, and grids, placed at different distances from the camera. The chart is placed in front of the camera and the lens is adjusted until the image on the monitor or viewfinder appears sharp and the target pattern is in focus.

The process of adjusting back focus involves manipulating the lens elements to achieve the correct focal distance. This is particularly important when changing lenses or using accessories that affect the back focus, such as lens adapters or extenders. Improper back focus can result in soft or blurry images, especially when using shallow depth of field or shooting at longer focal lengths.

By using a back focus chart, camera operators and technicians can ensure that the lens is properly calibrated for optimal focus accuracy. It is a standard practice in film production to perform back focus checks regularly, especially when there are changes in lenses or camera configurations.