What is the focal plane of photography?

The focal plane is a general term for the first focal plane and the second focal plane.

The plane passing through the first focal point (front focal point or object focal point) and perpendicular to the main optical axis of the system is called the first focal plane, also called the front focal plane or object focal plane. The plane passing through the second focal point (back focal point or image focal point) and perpendicular to the main optical axis of the system is called the second focal plane, also called the back focal plane or image focal plane.

The light emitted (or directed) from a point on the first focal plane passes through the optical system and becomes a beam of parallel light with a fixed included angle with the main optical axis. The direction of the parallel beam depends on the position of the luminous point on the first focal plane. A parallel beam with a certain angle to the main optical axis passes through the optical system, and the outgoing light (or its reverse extension line) intersects a point on the second focal plane.