Conjugate focal plane

In optics, a conjugate focal plane of a given plane, P, is a plane P′ such that points on P are imaged at P′. In a telescope, the subject focal plane is at infinity and the conjugate image plane, at which the image sensor is placed, is said to be an infinite conjugate. In microscopy and macro photography, the subject is close to the lens, so the plane at which the image sensor is placed is said to be a finite conjugate. Within a system with relay lenses or eyepieces, there may be planes that are conjugate to the aperture.