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In optics, a conjugate plane or conjugate focal plane of a given plane ''P'', is the plane ''P′'' such that points on ''P'' are imaged on ''P′''. If an object is moved to the point occupied by its image, then the moved object's new image will appear at the point where the object originated. In other words, the object and its image are interchangeable. This comes from the principle of reversibility which states light rays will travel along the originating path if the light's direction is reversed. The points that span conjugate planes are called conjugate points.''Fundamentals of Optics'', Fourth Ed. (1976) Francis A. Jenkins, Harvey E. White p. 48
It comes from the mirror formula 1/v+1/u=1/f where if u and v are interchanged then the equation remains same.
In a telescope, the subject
In optics, a conjugate plane or conjugate focal plane of a given plane ''P'', is the plane ''P′'' such that points on ''P'' are imaged on ''P′''. If an object is moved to the point occupied by its image, then the moved object's new image will appear at the point where the object originated. In other words, the object and its image are interchangeable. This comes from the principle of reversibility which states light rays will travel along the originating path if the light's direction is reversed. The points that span conjugate planes are called conjugate points.''Fundamentals of Optics'', Fourth Ed. (1976) Francis A. Jenkins, Harvey E. White p. 48
It comes from the mirror formula 1/v+1/u=1/f where if u and v are interchanged then the equation remains same.
In a telescope, the subject focal plane
In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the '' focal points'', the principal points, and the nodal points. For ''ideal'' ...
is at infinity and the conjugate image plane, at which the image sensor is placed, is said to be an infinite conjugate. In microscopy
Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...
and macro photography
Macro photography (or photomacrography or macrography, and sometimes macrophotography) is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is grea ...
, 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 lens
In optics, a relay lens is a lens or a group of lenses that receives the image from the objective lens and relays it to the eyepiece. Relay lenses are found in refracting telescopes, endoscopes, and periscopes to optically manipulate the light ...
es or eyepieces, there may be planes that are conjugate to the aperture.
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