Object space and image space

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The object space of an optical system (for example an objective ) is understood to mean the set of all object points that the system can map, and the image space is the set of points onto which it can map an object point. The object points located in a plane form an object plane . The image scale of the optical image results from the lens equation .

The term object space means that the points contained therein are viewed as object points. An object point is the intersection of light rays that are in front of the optical system (object space rays ) and hit its first surface ( lens surface or mirror surface) from the front . When the rays are lengthened to straight lines , they intersect at the object point. Likewise, the term image space summarizes the points that are generated by the rays coming out of the system. An image point is where the straight lines in the image space rays intersect.

An object point does not have to be in front of the optical system, but can also be in the middle of the system or behind it, and accordingly an image point can also be in or in front of the system. The object and image space are not spatially separated, but rather overlap.

An object with a given object size , in a convergent beam path from the object plane (respectively object plane ) in the object space (on the left) from the object distance on a principal plane squarely with appropriate focal distance and image size in the image plane in the image space shown (right). There is both an object-side and an image-side focus . Principal rays go through the main point , are not deflected and connect an object point in a straight line with an image point. Light rays that hit the main plane through one of the focal points are deflected parallel to the optical axis (dash-dot line) and vice versa.

An object point is called real when it lies in front of the first surface, i.e. a physical object (object) can be placed there, which is then mapped by the system. Otherwise it is called virtual , that is, it only exists as an imaginary point where the rays hitting the system intersect. The object points can only be virtual if an object is generated by another, upstream optical system (than its image).

Correspondingly, a pixel is real if it lies behind the last surface of the system, for example if a screen can be arranged there to capture the image. Otherwise it is virtual . A virtual image point can only be used to be further mapped by a downstream system (as its object point).

Examples of real object points are the points of a subject in photography . If the object is a physical thing, then its points are always real. The image points must also be real points, otherwise the images could not be captured with a film or digital image sensor .

For example, the image that is generated by the objective of a Galilean telescope is virtual . The eyepiece is arranged in front of the plane of this intermediate image and picks up the rays before they intersect in the image point. This intermediate image is in turn a virtual object for the eyepiece. The image generated by the eyepiece, which is at infinity, is also virtual and is in turn viewed by the eye of the user. A real image is only created on the retina of the eye .


  • Ludwig Bergmann , Heinz low : optics. Wave and Particle Optics, 10th edition, Walter de Gruyter, Berlin 2004, ISBN 3-11-017081-7 .
  • JW Classen: Mathematical Optics. GJ Göschensche Buchhandlung, Leipzig 1901.
  • AA Rusterholz: electron optics. Volume 1, Basics of Theoretical Electron Optics, Springer Basel AG, Basel 1950.

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