Magnifiers and Other Optical Devices
The magnification effects produced by a piece of convex
glass when held above written text have been known for many centuries.
Indeed the first spectacles were basically a pair of such magnifiers
mounted into a frame. Consequently magnifiers, and magnifying spectacles,
have been used as vision enhancing devices for countless years.
The basic principal underlying the use of any magnifying device
is to provide an increased image size that will cover a larger area
of the retina. This increased image coverage of the light sensitive
retina, compared to the unmagnified image, allows the brain to interpret
the image more easily.
Optical magnifying devices fall into two distinct
categories, simple magnifiers and telescopic lens systems. Of these
the former are probably the easiest to acquire and use. Telescopic
devices tend to be a more specialised unit, normally available through
an eye care professional, and require more skill in their use. Magnifying
devices are available in a large range of powers and forms and are
a very effective aid to vision.
However, due to physical limitations of optics, there
are finite values for magnification, area and field of view. As
a general rule of thumb, when considering any magnifier, it should
be remembered that the larger a magnifier the weaker its magnification.
Aberrations arising from the optics of the lenses will cause effects
such as distortion and colour fringes. Consequently magnifier designers
have to balance aberrations against benefits when producing their
magnifiers.
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