focal plane

noun Optics.

1. a plane through a focal point and normal to the axis of a lens, mirror, or other optical system.Compare principal plane.
2. the transverse plane in a telescope where the real image of a distant view is in focus.

noun

1. the plane that is perpendicular to the axis of a lens or mirror and passes through the focal point
2. the plane in a telescope, camera, or other optical instrument in which a real image is in focus