The errors of refraction
(There are a number of images on this page, all taken from Allergan India
Ltd. Patient Education Series on Refraction. Due to which the page may
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The Normal Eye
In the normal eye the refractive components of the eye include the cornea,
the crystalline lens (shown in white) and the vitreous gel (behind the
crystalline lens). The combined refraction of these elements is thus that
rays of light reflected off objects in the front of the eye are focused
on the retina (the fovea centralis). When such a refractive state occurs,
a state of normal vision exists as shown in the diagram above. Unaided
vision will then be declared 6/6 (or 20/20).
Hypermetropia (or hyperopia) and presbyopia
What is it?
Also called far-sightedness, or far-sighted and hyperopia. Rays of light
relected off objects in front of the eye are focused behind the retina
as shown above. Such persons can see well in the distance provided they
are young and the refractive error is not large. This error of refraction
is corrected by a "+" (convex) lens.
Click here for an animation explaining Hypermetropia.
Why does it occur?
It was at one time accepted that in the majority of cases hypermetropia
was axial, that is the length of the eyeball is shorter than it should
be. But this is not always true, the corneal may be flatter than it should
be (curvature hypermetropia). Hypermetropia actually forms a stage in
normal development of the eyes. At birth practically all eyes are hypermetropic
to the extent of 2.5 to 3.0 Diopters and as the growth of the body proceeds,
the lenght of the eye increases until adolescence is passed, the eye should
theoretically be "normal" in size and refraction. Hypermetropia
therefore represents an imperfectly developed eye when it persists in
adult life. Most primitive peoples and many of the lower animals are hypermetropic,
carnivora for example, are almost constantly so. Hypermetropia can also
occur due to change in refractive index of the crystalline lens of the
eye, as in cataract, or due to injury to the eye with dislocation of the
crystalline lens of the eye.
Another form of Hyperopia is Presbyopia, which affects individuals
after or near 40 years of age. This is because age related loss of "accomodation"
due to decreased elastictiy of the crystalline lens surface (capsule).
Persons with presbyopia have difficulty in reading/writing and doing near
work. This refractive error is corrected by adding a "+" lens
to the current refractive correction the person is wearing.
The earliest symptoms of uncorrected hypermetropia are "eye-strain",
"watering", "redness" of the eyes, and often headaches
in the later part of the day. Presbyopic persons complain of blurring
of text specially after some time. Young children with significant hypermetropia
can also develop a convergent squint.
What is it?
Also called short-sightedness or short-sight, because the person affected
is able to perform near work without correction, and has difficulty defining
distance objects. Rays reflected off an object in front of the eye focus
in front of the retina. A "-" or concave lens is needed for
correction of this refractive state.
Click here for an animation explaining myopia.
Why does it occur?
In the great majority of cases the cause of myopia is an increased length
of the eyeball. But it may also be due to an increase in the curvature
of the cornea or the surfaces of the crystalline lens. Cataract can also
produce myopia due to a change in the refractive index of the crystalline
lens of the eye. Myopia is of two types. A simple myopia in which there
is only a myopic state in the eye, the commonest form of myopia, and no
damage to the retina, and pathological myopia in which there is damage
to the retina of the eye. Myopia can also be progressive if the negative
power continues to increase over a time period. Progressive myopia may
require surgical intervention by a retina specialist.
Myopia in childhood may be associated with a divergent squint.
Presbyopes with myopia are able to do their near work without glasses
as the negative correction in myopia cancels the positive correction of
presbyopia (this is of course applicable to lower refractive errors);
as compared to hypermetropes who need presbyopic correction at an earlier
Typically uncorrected myopes do not have "eye-strain", "watering"
of the eyes or headaches as often as hypermetropes do. The myopia is usually
detected in the young when children playing with each other discover that
they can not see distant objects as well as their friends do, or the class-teacher
complains that the child makes too many mistakes copying things from the
What is it?
Rays of light reflected off objects in front of the eye are focused differently
on the retina of the eye, because more than one focal point exists. Astigmatism
is due to distortion of the corneal surface in one or multiple planes.
A cylindrical lens with either "+" or "-" power is
required to correct the refractive error.
Click here for an animation explaining astigmatism.
Click here for an animation explaining toric lenses
used for the correction of astigmatism.
Why does it occur?
Astigmatism may either be an error of curvature, of centring, or of refractive
index. Curvature astigmatism of any degree occurs in the cornea. The commonest
error is one wherein the vertical curvature is greater than the horizontal.
Injuries to the eyes can also cause astigmatism, so do surgeries to the
Astigmatic persons often complain of headaches, low vision or reduced
vision for distance and near, "watering", "redness",
"foreign body sensation" in the eyes as the day progresses.
Such persons may also complain of "running together of text"
while reading. An astigmatic person can see well neither a distant nor
a near object, more so the difficulty is with distant objects. In uncorrected
astigmatism with oblique axes there may be a compensatory tilt in the
Correcting refractive errors of the eye
By far the commonest used correction for refractive errors are spectacles.
An optometrist performs either a subjective or an objective assessment
of your vision and prescribes the lenses (and even dispenses) you will
need for your refractive error.
The advantages of using spectacles:
- Cost effective
- Freely available even in remote places
- Can be worn and removed easily
- Can be cleaned easily
- Difficult to handle for children and sports persons
- Misplaced or lost easily
- Accidental breakage and "scratching" more common
- Provided limited field of peripheral vision
- Peripheral distortion specially in high powered lenses
- "Heavy" to wear specially in high powered lenses
- Some persons are prone to dermatitis over the bridge of the nose (contact
- Some persons find it difficult to adjust to bifocals, trifocals and
- Social stigma specially in children
Contact lenses rest on a thin layer of tearfilm on the front surface
of the eye (the cornea). Various derivatives of Acrylic material are used
in the manufacture of contact lenses. Such lenses can be of 3 types;
- Hard lenses
- Semisoft or Gas Permeable lenses
- Soft lenses
The most commonly used contact lenses now a days are the soft lenses,
though semisoft (gas permeable lenses) are indicated occasionally. Both
soft lenses and semisoft lenses are gas permeable, that is, they allow
the cornea to "breathe" through the lenses, making them more
comfortable to wear.
- Better field of vision
- Overcome peripheral distortions in spectacles specially if the power
requirement is high
- More suitable for sports persons and similar professions
- Cosmetically more acceptable
- Freely available
- The only form of correction for irregular corneal surfaces (irregular
astigmatism) and mild corneal scars
- Require more maintenance (or meticulous maintenance)
- Chances of infection in the eye are more common if cleanliness is
- Require better hand-eye co-ordination while wearing
- Cannot be used without supervision by children and mentally handicapped
- More chance of injury to the eye in case of an accident specially
with hard and semisoft lenses
- Cannot be "over-worn" (most lenses have a wearing time of
- Uncomfortable to use in dusty, humid, or polluted environments, or
in the presence of chemical fumes, etc.
- Costlier than spectacles
The Surgical Correction of Refractive Errors
There are numerous surgical procedures for the correction of all types
of refractive errors. Most of the procedures are aimed at altering the
front surface of the eye (the cornea) so that a state of normal refraction
can be acheived. Surgical procedures such as Radial Keratotomy are performed
using a special knife, and LASIK (Laser In Situ Keratomileusis), PRK (Photorefractive
keratectomy) using a special LASER. The latest in the series of surgeries
is in the form of implants within the corneal stroma "INTACS (link)"
or lens implants in the eye "phakic intraocular lens implants"
Apart from changing the surface, in case of corneal opacities and irregularities
the cornea itself may be changed (corneal transplant surgeries, which
may be partial thickness, or full thickness).
- In case of high refractive errors, the error may be brought down to
a more manageable level so that it can be corrected by other means
- No need for glasses, contact lenses, no maximum wearing time
- All surgical procedures have a statistical failure rate which may
range from improper correction to loss of the eye
- Costly instrumentation and surgery
- Not freely available
- Cannot be undone in most cases, sometimes residual refractive errors
cannot be corrected using contact lenses or glasses
- Stringent case selection