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Diopter
101: Choosing the strength of your reading glasses
Since
the Middle Ages, lenses set in frames have helped
people improve their vision. Although advances
in optics did not arrive in the Middle Ages,
it became intuitive for people to use lenses
to change the refraction or bending of light
to achieve clearer vision. In later centuries,
optical systems were studied. The most important
optical system of all is the human eye. The
lens within the human eye can bend light, an
attribute referred to as the eye’s optical
power.
The
relationship between your diopter strength and
vision
Lenses,
including those in reading glasses, can also
bend light. These materials also have optical
powers. The strength indicating how much a lens
bends light is measured by its focal length.
The shorter the focal length, the greater optical
power. By the same token, the shorter the focal
length, the less ability the lens has to bend
light. Therefore, depending upon your eyesight,
your focal length is what determines how well
you can see with your glasses.
For
example, if a lens has a focal length of 1 meter
(approximately 40 inches), its optical power
is represented as 1 diopter. The diopter is
the unit of measure for optical power, just
as the MPH (miles per hour) is the unit of measurement
for speed. The optical power of the average
human eye is 40 diopters (approximately 1,574
inches, or 131 feet).
But
the human eye is infinitely more sophisticated
than manufactured lenses and glasses. The human
eye can adjust its focus by 20 diopters, closer
or further. This ability to adjust focus is
called “accommodation.” The bad
news is that accommodation deteriorates as the
eye ages. People over the age of 25 can adjust
their focus only by 10 diopters. And people
over the age of 50 can adjust focus only by
one mere diopter. This severe natual deterioration
is what doctors call presbyopia. As people grow
older, their ability to focus diminishes, is
the reason reading glasses are needed.
Choosing
the right diopter strength for your reading
glasses
Reading
glasses have integral lenses that bend light,
and their optical powers are measured in diopters.
Touching back upon the conversation of focal
length, the diopter indicates how far the lens
can refract light, allowing you to see smaller
and closer objects, such as words on a page.
For example, if you see that the strength of
a pair of reading glasses has a diopter of 2.00,
the lenses can refract light to focus at approximately
½ meter (or 20 inches). Or, if a lens
has a diopter of 3, then the reading glasses
can focus light at approximately 1/3 meter (or
12 inches). Thus, the greater the diopter strength,
the closer and smaller you can see.
The
bending of light magnifies objects seen through
the reading glasses’ lenses. Of course,
the diminishing accommodation of the human eye
is by small degrees. That means the required
lens for the reading glasses must be able to
bend light to shorter than half a meter, but
longer than a third of a meter. For that reason
some reading glasses have diopter strengths
of 2.25, 2.50, and 2.75.
Many
people over the age of 40 begin with 1.25 diopter
reading glasses. And people over the age of
50 commonly use 2.25 diopter reading glasses.
In choosing the best diopter strength for your
reading glasses, you may first begin by utilizing
this eye
test chart. Print out the chart and read
each line until one becomes unclear; that will
be your most likely diopter strength.
The
material provided on this site is for informational
purposes only. It is not meant to replace the
advice of your eye care professional. Have your
eyes examined regularly and always follow your
eye care professional's instructions. If you
experience any pain, discomfort, or visual distortion,
consult your eye care professional immediately.
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