| Technical Definitions
Power: The number of times a lens or
lenses multiplies an image with respect to the original size.
Low power is recommended for scanning large surfaces. Higher
powers are generally used for inspecting small areas.
Field of View: The maximum area seen
through a magnifier. The entire field of view may not be in
focus. See Flatness of Field.
Flatness of Field: Due to the physical
laws of optics, the outer part of the image formed by a lens
may be out of focus. The greater the power and curvature of
the lens, the more pronounced this problem becomes. Magnifiers
with multiple lenses can reduce this problem. The visible
area which appears in focus is the flat field.
Working Distance: The distance from
the magnifier lens to the object being viewed. As magnification
power increases, the working distance decreases.
Aberration: This is a distortion caused
by the lens. Spherical aberration is distortion of the image,
as explained in Flatness of Field. Chromatic aberration is
distortion of colors being viewed, as explained in Color Distortion.
Color Distortion: Lenses produce a
prism effect which causes the image to develop false color
fringes. This is due to the fact that different colors focus
at different points. Achromatic lenses correct this problem
by focusing many colors at the same point.
Parallax: Parallax is the apparent
misalignment of two different items when viewed from an angle.
It is the cause of improper registration when stripping multiple
color jobs. Parallax is eliminated when viewing straight down
on registration marks instead of from a slight angle.
Coated Lens: A coated lens helps to
minimize or eliminate color distortion caused by different
colors focusing at different locations. Optimum color viewing
is obtained from coated achromatic lenses.
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