The image field not only may have curvature but may also be distorted. The image of an off-axis point may be formed
at a location on this surface other than that predicted by the simple paraxial equations. This distortion is different
from coma (where rays from an off-axis point fail to meet perfectly in the image plane). Distortion means that even if
a perfect off-axis point image is formed, its location on the image plane is not correct. Furthermore, the amount of
distortion usually increases with increasing image height. The effect of this can be seen as two different kinds of
distortion: pincushion and barrel (see the figure below). Distortion does not lower system resolution; it simply means
that the image shape does not correspond exactly to the shape of the object. Distortion is a separation of the actual
image point from the paraxially predicted location on the image plane and can be expressed either as an absolute value
or as a percentage of the paraxial image height.
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Pincushion and barrel distortion
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It should be apparent that a lens or lens system has opposite types of distortion depending on whether it is used
forward or backward. This means that if a lens were used to make a photograph, and then used in reverse to project it,
there would be no distortion in the final screen image. Also, perfectly symmetrical optical systems at 1:1
magnification have no distortion or coma.
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