A nodal plane is an imaginary flat surface passing perpendicularly through the optical axis of a lens at the position of one of the lens's nodal points, extending infinitely in all directions perpendicular to the axis. Because a compound photographic lens has two nodal points - the front nodal point and the rear nodal point - it correspondingly has two nodal planes, one associated with each nodal point, oriented perpendicular to the optical axis at their respective positions along it.
The nodal points of a lens are specific positions on the optical axis defined by the optical behaviour of the lens system as a whole. The front nodal point is the position from which incoming light rays appear to diverge after refraction through the lens, while the rear nodal point is the position from which the refracted rays appear to converge towards the image. In a simple thin lens, both nodal points coincide at the optical centre of the lens and the two nodal planes merge into a single plane. In a complex compound lens - such as a modern photographic objective containing multiple elements - the two nodal points and their associated planes are typically separated along the optical axis by a distance that varies with the specific optical design, and may in some designs fall outside the physical extent of the lens barrel entirely.
The rear nodal plane has particular practical significance in photographic optics because the focal length of a lens is defined as the distance from the rear nodal point - and therefore from the rear nodal plane - to the focal point where parallel rays of light from infinity are brought to focus. This means that the focal length is measured not from the physical rear of the lens or from the lens mount, but from the rear nodal plane, which may be located at a position quite different from either of these physical reference points depending on the lens design. In a telephoto lens, the rear nodal plane is located in front of the physical rear of the lens - sometimes well in front of the lens barrel - allowing the lens to have a physical length shorter than its focal length. In a retrofocus or reverse telephoto wide angle lens, the rear nodal plane is located behind the rear of the lens, providing a longer back focus distance than the focal length and ensuring that the rear element clears the mirror box of a single lens reflex camera.
The concept of the nodal plane is also critically important in panoramic photography, where rotating the camera around the front nodal point - rather than around the tripod socket or any other arbitrary physical point on the camera - ensures that the perspective relationships between near and distant objects remain consistent across all the overlapping frames of a panoramic sequence. Rotating around any point other than the front nodal point introduces parallax error - apparent shifts in the relative positions of near and far objects between adjacent frames - that produces misalignment artefacts in the assembled panorama that cannot be fully corrected by stitching software. Panoramic heads and nodal slide accessories are designed to allow precise positioning of the camera's front nodal point directly over the axis of rotation of the tripod head to eliminate parallax error in panoramic photography.