ACADEMY CRANE
[Film Technology]
This large, robust camera
crane has been used in film studio and television studios for many years. Electrically
driven, the camera at the end of its large counter balanced boom has a height range of, for example, 60 cm to 3m
[2 to 10 ft]. The camera can be panned over 1800, the main crane boom slewing round 3600. The camera
comprises the cameraman, a crane driver[tracker] and an crane operator[boom
swinger] to handle the boom. An attached
monitor, intercom
and hand signals helps to co-ordinate its crew. Base area 1x1.8 m [3.5x6 ft] over all length
4.25 m [14.5 ft] maximum. Electronically driven [8 kph/5mph]. The centrally
pivoted boom is counter balanced.[lead weights
in bucket] on its height adjustable support pillar.
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.
