Focal Plane; Image Plane; Film Plane; Sensor plane
The focal plane in a camera is the flat surface onto which the light is focused after it has passed through the photographic lens. In digital cameras the focal plane is the same as the surface of the digital image sensor. Hence the alternative name of sensor plane.
The term “image plane” also refers to the surface that is commonly called the focal plane in a camera. In fact the terms are used interchangeably in general conversation. However, in optics the focal plane may be technically different to the image plane. This is because a lens does not project a flat image owing to the lens curvature. In film cameras (especially fixed focus lens cameras) the image plane was often curved to compensate for the curvature in the focal plane. The general improvement in photographic Lenses has tended to reduce the curvature effect. Any distortions created by specific lenses can now be compensated by the software in the camera or in the editing software used in post processing. So while there is no practical difference between the terms “Focal Plane” and “Image Plane” there may be optical differences.
In film cameras, the film plane was the surface created by the film on to which the light was focused. Modern DSLR manufacturers have labelled the surface of the digital image sensor the ‘image plane’. It may also be called the rear focal plane although that is incorrect.
The light that falls on the sensor is bigger than the sensor area itself. Light that comes from the edges of a lens tends to suffer from exaggerated aberrations, fall off of light intensity (vignetting) and optical distortion. Light that has passed through the centre of the lens is more controlled, so the lens is designed to focus the quality part of the image onto the sensor. The rest of the light is lost/ignored by the camera.
It should be noted that the term “focal plane” as used in photography is not used identically in other fields. You should ensure you are using the correct terms in context outside of photography.
A focal plane shutter (see: Shutter) lies very close to, and just in front of, the focal plane.