Focus level

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The focal plane is that plane in the object space whose points are sharply imaged by a lens or an objective as points on the image plane in the image space .

If the image plane and the objective plane are parallel to one another, the focal plane is also parallel to the image and objective plane. This is the case with cameras in which the recording medium and the lens are fixed in one housing. Points outside the focal plane are shown as circles or ellipses on the image plane. Up to a certain size, however , these circles of confusion still appear point-like, so objects that are outside the plane of focus are also shown sufficiently sharp. The area in front of and behind the plane of focus in which a point is still sufficiently sharp is the depth of field .

Which plane is sharply imaged in the object space can be determined by positioning the image plane in the image space. With the distance setting of a lens, the image distance, i.e. the distance between the lens and the image plane, changes and thus the distance to the focal plane (the object distance ) also changes.

View cameras and tilt-and-shift lenses also enable the optical axis of the lens to be pivoted. The focal plane is thus tilted relative to the image plane in accordance with Scheimpflug's rule . As an example, focusing on a road / railroad running from the foreground to the background or a targeted defocusing is possible.

Not all lenses have a focal plane, fisheye lenses e.g. B. have a spherical (or parabolic) focus area. In practice, imaging errors also lead to a certain distortion of the sharpness and image plane relative to one another.

literature

  • Kristian Bredies, Dirk Lorenz: Mathematical Image Processing . 1st edition, Vieweg & Teubner, Wiesbaden 2001, ISBN 978-3-8348-1037-3 .
  • Jürgen Beyerer, Fernando Puente León, Christian Frese: Automatic visual inspection . Springer Verlag Berlin Heidelberg, Berlin Heidelberg 2012, ISBN 978-3-642-23965-6 .

See also

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