or spectacles are indispensable for most of us.
They are the most common means to correct disorders
related to the focus of the eye, such as presbyopia,
myopia, hypermetropia, astigmatism etc.
Type and strength of spectacle lenses
Spectacles are made of lenses, the type and
power of which depends on the degree of focusing
error of the eye. Generally, there are two
broad categories of lenses:
- A 'plus' lens is convex and focuses light
- A 'minus' lens is concave and focuses light
The power of a lens is measured in dioptres
(D). The higher the dioptre count the stronger
the lens, in either a plus or a minus direction.
Lenses can also be also categorized according
to the errors they correct:
These lenses correct myopia, hypermetropia,
presbyopia and astigmatism. Myopic patients need
single concave lenses that are thicker at the
edges and make objects look smaller. A single
lens to correct hypermetropia or presbyopia is
just the opposite; being thicker in the centre
and making things look larger. The single lens
spectacle prescribed for astigmatism has a straight
line through the lens, which rotates if the lens
is moved both clockwise and anti-clockwise.
These are commonly known as bifocals and half-glasses
and are used to correct presbyopia. These are
a combination of lenses of two or more powers
in two parts, in which the larger upper part
is for far vision (minus lens) and a smaller,
base part (plus lens) for close viewing.
The presbyope can have a half-glass, the lower
half of which can correct vision for near objects
and the upper half for far objects. Presbyopes
are spared the trouble of keeping two separate
sets of eyeglasses for near and far vision if
they use bifocals. However, bifocals are not
tolerated by everybody. Often there is confusion
of focus, say for instance, when you look down
at the stairs with the bifocals on.
Thus, evolved the concept of a trifocal. A trifocal
consists of a third, intermediate part added
to a lens (with a power between the far and near
viewing sections). Such trifocals are suited
to patients who need a middle distance vision.
To prevent the image from 'jumping' between the
various portions of the lens, it needs to be
combined gradually for a range of viewing distances.
Spectacles also often include prisms that bend
all light in a single direction. They help to
correct double vision. Prisms are fitted into
eyeglasses either permanently or temporarily
by simply sticking an adaptable plastic Fresnel
prism onto a spectacle lens.
How to get the correct eyeglasses?
Professionals trained in eye care such as the
optometrist, optician and ophthalmic medical
practitioner can accurately determine your need
for glasses and other optical requirements. They
can even approximately determine the type and
measure the strength of the spectacle lenses
by just looking at and through the lens.
Patients can also clinically test for focusing
errors by a mechanized process, technically called
'refraction.' The process involves a retinoscope
that measures the power and determines the type
of lens required by a patient. This is followed
by trails with lens of various strengths to reach
the optimal power suited to a patient.
Once the power of your lens has been determined
and a prescription issued, you need to get hold
of a pair of eyeglasses, unless you opt for contact
lenses. Remember the following when you get your
- Those under 16 must get their glasses
from a registered optician
- Those over 16
can get their glasses from a registered optician
or any other supplier
- Unregistered suppliers
cannot sell prescription spectacles to kids
or to adults with their
names in the blind or partially sighted records
- Adults can get reading glasses
without a prescription at retail outlets, chemist
shops and even
- There are many concessions available as well.
But do not compromise on an examination
by a qualified practitioner, as this could
What material to choose for your glasses?
Glass, as the name goes, is the conventional
material for spectacle lenses. It is the still
the most common choice since it is cost effective,
has sound optical properties and is more resistant
to pressure or heat and scratches. Its main disadvantages
are its weight and breakability. It becomes a
very heavy option when the lens has to be solid
to correct large errors, especially if the lens
of the eye has been clinically removed in a condition
called aphakia. For these special cases, the
glass may be made more durable.
Plastic lenses are the other option. They are
lighter and hardy, but develop scratches easily.
Modern resins have addressed most of these problems,
but they are cost prohibitive for many. There
is also the option of tinted lens, which darken
in response to light. They generally act as sunshades
but are also prescribed for clinical conditions
like dilated pupils or people with albinism.