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SPECTACLES
Spectacles or eyeglasses
form the cornerstone of optical low vision prescriptions. Any lens
type or power that can be found in any of the other devices, can be mounted
in an eyeglass frame. These glasses range from stronger than usual
reading glasses and bifocals (for patients with relatively good vision),
to extremely strong lenses or lens systems that require very close working
distances.
The
principle patient objection to strong prescriptions is that a close working
distance is required. This is often more of a psychological barrier
than a physical one, although there is some initial arm discomfort when
working at distances closer than about four inches.
Research
has shown that a well adapted patient will read faster with spectacles
than with any other optical magnification device. In fact, the only
devices that can yield faster reading speeds are video and computer based
devices costing at least ten to twenty times as much.
Hand
Magnifiers
Hand magnifiers are the
most familiar of Low Vision Optical devices. They are available in
powers ranging from 1X to 20X. These are NOT the lenses you will
find in your local hobby shop or dime store. The stronger powers
should be made in aspheric curves to reduce distortion. The biggest
patient complaint about hand magnifiers is that they become smaller as
they become stronger. This is a limitation based upon optics.
While in some cases it may be possible to create a larger lens if price
is no object, there are physical limitations to the maximum size possible.
Why
not buy a magnifier from a store?
Several reasons:
First, patients have
unrealistic expectations of how a magnifier should work. For this
reason they often choose a "large" magnifier because it covers more area,
only to find it doesn't magnify sufficiently for their eyes.
Conversely, some patients
are "magnification junkies" , selecting a lens that gives an adequate amount
of magnification, but that has a far smaller field of view than a lens
of appropriate power for that patient's eyes. The proper lens power
will allow for faster, longer, and more comfortable reading.
Second, magnifiers, particularly
stand magnifiers, must be chosen with the patient's eyeglass correction
in mind. Some stand magnifiers are designed to be used with distance
glasses, some with standard bifocal strengths, and some with entirely different
lens powers. Without a full understanding of the optics involved,
it is easy to select a magnifier that will provide a blurry image, or one
that will create eye strain, fatigue and headaches.
Third, many manufacturers
mislabel the strength of their magnifiers. The convention used by
doctors and the international standard is to label the magnification by
dividing the actual Dioptric lens power by the number 4. By this
system a +8.00 diopter lens would be labelled 2X, and a +20 lens
labelled 5X, and so on.
Some manufacturers apparently
feel that consumers will not buy a 1X lens, the most common hobby type
sold, (+4) so they arbitrarily add 1X to all their magnification labels.
Some manufacturers do this across their product line, others do it haphazardly,
and some use the international standard. In addition, some low quality
magnifiers use any power at all thrown on the label, with some going as
far as to call a +8.00 lens and 8X lens!!!
In short, the consumer
has no way of knowing the real power.
Limitations of Magnifiers:
The most common patient
disappointment is when the size of the higher powered magnifiers shrinks.
I wish I had a dollar
for every time someone said that the 5X magnifier I just prescribed would
be perfect if only it covered the area of an entire page!
Unfortunately, the simple
laws of optical design preclude the manufacture of very wide field magnifiers
in high powers, at least at any reasonable cost.
One manufacturer once
told me that they could produce what the patients want, but no-one could
afford to buy it.
Indeed, manufacturers
HAVE made improvements in the sizes of available magnifiers over the years,
and I almost daily find myself explaining to patients why a particular
magnifier costs $50 to $100 dollars when the ones their cousin Myrtle purchased
in the 5&10 was only $4.00!
Quality optics are expensive
(although most are still under $100.00). The higher cost magnifiers
we prescribe are made up of computer designed ASPHERIC curves to minimize
distortion. The process of design and manufacture of these high quality
lenses requires an investment of tens of thousands of dollars to design
every new lens. Compounding this is the fact that most of the devices
we prescribe are made in Europe, inflating the price even more. The
advantage however, is that patients have a better selection of magnifiers
available today , than ever before.
Stand
Magnifiers
Stand magnifiers are the
easiest to use, of Low Vision Optical devices. They are available
in powers ranging from 1X to 20X. These are NOT lenses you will find
in your local hobby shop or dime store.
The magnifiers we use
are manufactured with distortion reducing aspheric curves. They are
available with and without built in illumination systems, both battery
and plug in types. The most expensive models have electric halogen
illumination with a built in Rheostat , or Dimmer.
While
these devices are often the easiest to use, they are also the most optically
complicated in use and in prescribing. Because the height the lens
is mounted varies widely, relative to the lens' focal length, some of these
may produce unclear images when used with distance or reading prescriptions
that do not match the design of the lens. Proper prescribing or Stand
magnifiers requires not only knowledge of each particular lens's idiosyncrasies,
but a thorough knowledge of the patients prescription glasses as well.
Just
as with Hand Magnifiers, the most common patient disappointment is when
the size of the higher powered magnifiers shrinks.
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