Focal Plane Mark
Calculate the focal distance to an object
The focal plane mark is the point from which distance is measured for various focus calculations. However, the focal plane is found inside the camera. It is this mark, on the outside of the camera body, that identifies the location of the focal plane surface inside the camera. The focal plane is also called the image plane, sensor plane or film plane. It is the surface onto which the image is projected and recorded (by the sensor or film).
Various aspects of critical-focus work can be measured from the focal plane mark. The focal plane remains in a fixed position regardless of which lens is in place. Thus, the mark acts as a fixed measurement point.
Consistent point for measurement
In fact, this little mark gives a consistent point for measurement to items in the field of view. This can be particularly useful when you want to merge images. It is especially useful when items in the shot are moved or changed between shots. When merging shots in processing or creating composite pictures, it helps to have the image proportions correct. Use the mark as a measurement point from camera to changed item. In a series of shots you can maintain correct focus and proportion. This helps in subsequent post processing.
For example, the minimum focus distance is given on most good lenses. Measuring from the mark to the point of minimum distance establishes the edge of the acceptable sharpness. That is where the depth of field) is at its minimum distance. This is very useful, especially for close work and macro work.
Similar marks used in other contexts
The focal plane mark is similar to the letter ‘Phi’ in the Greek alphabet. Similar marks are used in other disciplines such as engineering (witness mark) and shipping (plimsoll mark or legal waterline mark) for points from which a measurement is taken or where components line up with each other.