The film plane is the precise flat surface at the back of a camera where the film is positioned during exposure. It is the point at which the lens projects a focused image, and it is essential that the film lies perfectly flat and parallel to the lens to ensure that the entire image is rendered sharply and evenly across the full frame.
The position of the film plane is a critical factor in camera design and lens calibration. When a photographer focuses a lens, they are effectively adjusting the optics so that the sharpest point of focus falls exactly on the film plane. Most cameras feature a film plane indicator marked on the outside of the camera body - typically represented by a circle with a horizontal line through it - which allows photographers to calculate the precise distance between the film plane and the subject when accuracy is critical, such as in close-up or macro photography.
In modern digital cameras, the film plane has been replaced by the image sensor plane, which serves exactly the same optical function. The term focal plane is often used interchangeably with film plane, and is also referenced in the name of the focal plane shutter - a type of shutter mechanism positioned immediately in front of the film or sensor plane that is commonly used in SLR and mirrorless cameras.