ABERRATION
[Film Technology]
Light inter-reflecting between the various
individual lenses in a camera
lens system, resulting in reduced image contrast [image contrast]and spurious light
patches, veiling etc. considerably
reduced by surface coating of lenses ,and by electronic
flare correction circuits. In video. If
open up a lens fully [maximum aperture] for
minimum depth of field or when shooting under low light conditions, picture definition and tonal clarity may deteriorate as various defects
appear [flare/aberrations]. Another name is flare.
Geometrical optics predicts
that rays of light emanating from a point are imaged by spherical optical elements
as a small blur.
The outer parts of a spherical surface have a focal
length different from that of the central
area, and this defect causes a point to be imaged as a small circle. The
difference in focal
length for the various parts of the spherical
section is called spherical aberration. If, instead of being a portion of a sphere, a concave mirror is a section
of a paraboloid (see Parabola) of revolution, parallel rays incident on all areas of the
surface are reflected to a point without spherical aberration. Combinations of convex lenses and concave lenses can help to
correct spherical aberration, but this defect cannot be eliminated from a single spherical lens for a
real object
and image. The
result of differences in lateral magnification for rays coming from an object
point not on the optic axis is an effect
called coma. [Coma is a lens aberration where
by rays from different sections of the lens
meet at different points. This can be corrected by using a compound lens with different elements of different
types of glass. ]If coma is present, light from a
point is spread out into a family of circles that fit into a cone, and in a plane perpendicular to the optic axis the image pattern is
comet-shaped. Coma may be eliminated for a single object-image point pair,
but not for all such points, by a suitable choice of surfaces. Corresponding, or
conjugate, object points and image points, free from both spherical aberration and coma, are known
as aplanatic points, and a lens having such a pair of points is called an aplanatic lens. Astigmatism is the
defect in which the light coming from an off-axis object point is spread along
the direction of the optic axis. If the object is a vertical line, the cross
section of the refracted beam at successively greater distances from the lens is an ellipse that collapses
first into a horizontal line, spreads out again, and later becomes a vertical
line. If, for a flat object, the surface of best focus is curved, the situation is described as curvature of field. Distortion arises from a variation of magnification with axial distance and is not caused by a lack of sharpness in the image. Because the index of refraction varies
with wavelength,
the focal length
of a lens also varies and causes longitudinal or axial chromatic aberration. Each wavelength forms an image of a slightly different size, giving rise
to what is known as lateral chromatic aberration. Combinations of converging and diverging lenses, and of components made of glasses with different dispersions, help to
minimize chromatic aberration. Mirrors are free of this defect. In general, achromatic lens combinations
are corrected for chromatic aberration for two or three colours.
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