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. The focal plane mark on the outside of the camera body identifies the location of the focal plane surface inside. The focal plane is also called the image plane, sensor plane or film plane as this is the surface on which the image is projected and recorded.
Various aspects of critical-focus work can be measured from the focal plane mark. It gives a consistent point for measurement. The focal plane remains in a fixed position regardless of which lens is in place. Thus, the focal plane mark acts as a fixed measurement point.
For example, the minimum focus distance is given on most good lenses. Measuring from the focal plane mark to the point of minimum distance establishes the edge of the acceptable sharpness (start of the depth of field) when it is at the minimum distance. This is very useful, especially for close work and macro work.
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) for points of measurement or where components line up with each other.