Focal Plane; Image Plane; Film Plane; Sensor plane
The focal plane in a camera is the surface the light is focused onto after passing through a photographic lens. In digital cameras the focal plane is the surface of the digital image sensor. Hence, the term “sensor plane” is used.
Differences between the “Focal” and “Image” Planes
The term “image” plane” also refers to the surface commonly called the “focal” plane in a camera. In fact, the terms are easily swapped in conversation. However, in optics the focal and image planes may be technically different. This is because, a lens does not project a flat image. The lens curvature creates a curved projection. In film cameras (especially fixed focus lens cameras), the image plane was often curved. This adjusted for the curve in the projection of the image. Improvements in lenses have reduced this effect. Actually, distortions created by lenses are often adjusted by in-camera software. By the way, that is a good reason to match the lens and camera body to the same manufacturer. Also, editing software can adjust for distortions. So, while there is no practical difference between the terms, 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 name the surface of the digital image sensor the ‘image plane’.
Light passing through a lens projects a bigger light area than the sensor. However, light from the edges of a lens tends to suffer from aberrations. Examples include, fall-off of light intensity (vignetting) and optical distortion. Usefully, light passing through the centre of the lens is more controlled. So, the lens is designed to focus the quality part of the image onto the active sensor region. As a result, the rest of the light is ignored by the camera.
Be careful to use the right term
Careful, the term “focal plane” in photography is not used the same way in other fields. Remember, you should ensure you are using the correct terms in other fields.
For example, use of the term “rear focal plane” (RFP) is incorrect in the context of a camera image plane. Normally, an RFP is focused on a reticule or reticle, not a surface. Usually, RFP relates to optical targeting or measuring devices like a gun scope.
A “focal-plane shutter” (or ‘Shutter’) lies very close to, and just in front of, the image sensor plane.
The sensor plane position inside the camera is often marked on the outside of the camera body. Consequently, the mark is called the “Focal plane mark“. So, check that article out to find out what the mark looks like and the practical uses of the term!