Abbreviations and terms
Similar to medical prescriptions, eyeglass prescriptions
are written on paper pads that frequently contain a number of different
abbreviations and terms:
- D.V. is an abbreviation for "distance vision". This specifies the
part of the prescription designed primarily to improve far vision. In a bifocal
lens, this generally indicates what is to be placed in the top segment.
- N.V. is an abbreviation for "near vision". This may represent a
single-vision lens prescription to improve near work, or the reading portion of
a bifocal lens.
- O.D. is an abbreviation for oculus dexter, Latin for "right eye." (Some eyeglass
prescriptions simply say "left" and "right" instead of "O.S." and "O.D.")
Oculus means "eye" (as in "oculist," an old term for an eye doctor).
- O.S. is an abbreviation for oculus sinister, Latin for "left eye".
- O.U. is an abbreviation for oculi uterque, Latin for "both eyes".
- A spherical correction corrects the refractive error of the eye by adding or subtracting
refractive strength symmetrically, by the same amount, along the horizontal and vertical axis.
- A cylindrical correction corrects the refractive error of the eye by adding or subtracting
strength along the horizontal axis, the vertical axis, or a diagonal axis.
- The Axis is the horizontal, vertical or diagonal angle of the cylindrical correction. The axis
is measured in a clock face that starts with zero degrees in the 9 o'clock (or east) direction, and
increases through 90 and 180 degrees in a counter-clockwise direction.
Most eyeglass prescriptions will contain values here. The "spherical" and
"cylindrical" columns contain lens strengths in diopters (see below); "axis"
contains the direction of the cylinder axis in degrees.
- Prism and Base are usually left empty; they refer to prescription features that are
used to treat muscular imbalance or other conditions that cause errors in eye orientation, and are not
seen in most prescriptions.
Blur is the subjective experience or perception of a defocus aberration within the
eye. Blur may appear differently depending on the amount and type of refractive error. The following are some examples of
blurred images that may result from refractive errors:
Blur is corrected by focusing light on the retina. This may be done with eyeglasses or contact lenses,
or by altering the shape of various eye structure via refractive surgery or special contact lenses.
Eyeglasses sometimes have unwanted effects including magnification or minification, distortion, color
fringes, altered depth perception, etc. Although many people think of lenses as magnifiers, the lenses
within eyeglasses improve vision primarily by reducing blur. Depending on the optical setup, they may also
produce magnfication or minification of images which may or may not be intentional or desirable.
The values indicated in the 'sphere' and 'cylinder' columns of an eyeglass prescription specify the
optical power of the lenses in diopters, abbreviated D. The higher the number of diopters, the more the
lens refracts or bends light. A diopter is the reciprocal of the focal length in meters. If a lens has a
focal length of 1/3 meters, it is a 3 diopter lens.
A +10 diopter lens, which has a focal length of 10 centimeters, would make a good magnifying glass. Eyeglass
lenses are usually much weaker, because eyeglasses do not work by magnifying; they work by correcting focus. The eye
itself has a refractive power of 60 diopters.
Stacking lenses combines their strength. A +1 diopter lens combined with a +2 diopter lens forms a +3 diopter system.
Lenses come in positive (plus) and negative (minus) powers. Given that a positive power lens will magnify
an object and a negative power lens will minify it, it is often possible to tell whether a lens is positive
or negative by looking through it.
Positive eyeglass lenses can concentrate sunlight.
This series of pictures shows the shadow cast by a pair of 1 diopter drugstore "reading glasses" outdoors
in sunlight as we hold it farther and farther away from a wall. As the distance from the wall increases, the shadow of
the frame seems to thicken and the bright area in the center gets smaller and brighter. It slowly changes
from being "eyeglass-shaped" to circular.
Negative lenses spread sunlight instead of concentrating it.
A negative lens combined with a positive lens removes some of its strength. A -2 lens combined with a +5 lens forms a +3 diopter system.
A -3 lens stacked on top of a +3 lens looks almost like flat glass, because the combined strength is 0.
In science textbooks, positive lenses are usually diagrammed as convex on both sides; negative lenses are usually diagrammed as concave on both sides. In
a real optical system, you usually get the best optical quality when most rays of light are roughly normal to the lens surface. In the case of an eyeglass
lens, this means that the lens should be roughly shaped like a cup with the hollow side toward the eye. So most eyeglass lenses are meniscus in shape.
Spherical lenses and spherical correction
- the "spherical" component is the main correction
- the "cylindrical" component is "fine tuning."
Depending on the optical setup, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur.
Whatever the setup, spherical lenses act equally in all meridians: they magnify, introduce blur, or correct blur the same amount in every direction.
An ordinary magnifying glass is a kind of spherical lens. When a spherical lens acts as a magnifier, it magnifies equally in all meridians. Here, note that
the magnified letters are magnified both in height and in width.
Similarly, when a spherical lens puts an optical system out of focus and introduces blur, it blurs equally in all meridians:
Here is how this kind of blur looks when viewing an eye chart. This kind of blur involves no astigmatism at all; it is equally
blurred in all meridians.
Amount of refractive error and degree of blur
The leftmost image above shows a Snellen eye chart as it might be seen by a person who needs no correction, or by a person who is wearing
eyeglasses or contacts that properly correct any refractive errors he or she may have.
The images labelled 1D, 2D, and 3D give a very rough impression of the degree of blur that might be seen
by someone who has one, two, or three diopters of refractive error. For example, a nearsighted person who needs a -2.0 diopter
corrective lens will see something like the 2D image when viewing a standard eye chart at the standard 20-foot distance without glasses.
A very rough rule of thumb is that there is a loss of about one line on an eye chart for each additional 0.25 to 0.5 diopters of refractive error.
The top letter on many eye charts represents 20/200 vision. Many people cannot read even this top letter
without glasses. The definition of legal blindness is not based on vision without glasses. The U. S. Social
Security administration, for example, states that 'we consider you to be legally blind if your vision cannot be
corrected to better than 20/200 in your better eye.' In other words, what matters in evaluating legal blindness,
and for many other purposes, is corrected acuity—which line of the chart can be read with proper lenses. One
purpose in which uncorrected vision makes a difference is for receiving a driver's licence that is good without lenses; a person who needs
lenses may receive one that requires them to wear the said lenses.
Cylindrical lenses and cylindrical correction
Some kinds of magnifying glasses, made specifically for reading wide columns of print, are cylindrical lenses.
When a cylindrical lens acts as a magnifier, it magnifies only in one direction. For example, the magnifier shown magnifies
letters only in height, not in width.
Similarly when a cylindrical lens puts an optical system out of focus and introduces blur, it blurs only in one direction.
This is the kind of blur that results from uncorrected astigmatism. The letters are smeared out
directionally, as if an artist had rubbed his or her thumb across a charcoal drawing. A cylindrical lens of the right strength can
correct this kind of blur. When viewing an eye chart, this is how this kind of blur might appear:
Compare it to the kind of blur that is equally blurred in all directions.
When an eye doctor measures an eye—a procedure known as refraction—usually he or she begins by
finding the best spherical correction. If there is astigmatism, the next step is to remove it by adding the
right amount of cylindrical correction.
Spherical lenses just have a strength, such as +1.0D, or -2.5D.
Astigmatism, however, causes a directional blur. Here are two examples of the kind of blur you
get from astigmatism. The letters are smeared out directionally, as if an artist had rubbed his or her
thumb across a charcoal drawing.
A cylindrical lens of the right strength can correct this kind of blur. The second example is a
little bit more blurred, and needs a stronger cylindrical lens.
But notice that in addition to being smeared more, the second example is smeared out in a different direction.
A spherical lens is the same in all directions; you can turn it around, and it doesn't change the way it magnifies, or the way it blurs:
A cylindrical lens has both a strength and an axis. Turning it around
so that the axis points in different directions changes the way it magnifies, and the way it blurs.
The axis specification on a prescription gives the orientation of
the axis of the cylindrical correction, and it can vary from 1 to 180 degrees:
The total power of a cylindrical lens varies from zero along its main axis to its maximal value along the
axis 90 degrees away. The total power of a lens with a spherical and cylindrical correction changes accordingly:
along the axis specified on the prescription it is equal to the value listed under "spherical", and it reaches
the sum of "spherical" and "cylindrical" along the axis perpendicular to the one listed on the prescription.
Distant vision and near vision
The DV portion of the prescription describes the corrections for distant vision.
For most people under forty years of age, this is the only part of the prescription that is filled in.
The NV or near-vision portion of the prescription is blank because a separate correction for near
vision is not needed.
The NV portion is used in prescriptions for bifocals.
In younger people, the lens of the eye is still flexible enough to accommodate over a wide range of
distances. With age, the lens hardens and becomes less and less able to accommodate.
This is called 'presbyopia;' the 'presby-' root means 'old' or 'elder.' (It is the same root as in the words 'priest' and 'presbyterian'.)
The hardening of the lens is a continuous process, not something that suddenly happens in middle age.
It is occurring all along. All that happens around middle age is that the process progresses to the point
where it starts to interfere with reading. Therefore almost everybody needs glasses for reading from the age
Because young children have a wider range of accommodation than adults, they sometimes examine objects by
holding them much closer to the eye than an adult would.
This chart (which is approximate) shows that a schoolchild has over ten diopters of accommodation,
while a fifty-year-old has only two. This means that a schoolchild is able to focus on an object
about 10 cm. (4') from the eye, a task for which an adult needs a magnifying glass with a rated power of about 3.5X.
The NV correction due to presbyopia can be predicted using the parameter age only. The accuracy of such
a prediction is sufficient in many practical cases, especially when the total correction is less than 3
diopters. See also the following calculator for computing this correction.
Variations in prescription writing
There is a surprising amount of variation in the way prescriptions are written; the layout and
terminology used is not uniform.
When no correction is needed, the spherical power will sometimes be written as "0.00" and sometimes as
"Plano" or "Pl" because the lens, although not flat, is optically equivalent to a flat piece of glass.
When cylindrical correction is needed, the mathematics and optics of the way lenses combine mean that
there are two different ways to write the same correction. One is called the plus-cylinder form and the
other the minus-cylinder form. These two prescriptions are equivalent:
Both of them specify a power of 2.00 diopters at the 90 degree axis and 3.00
diopters at the 180 degree axis.
The first one specifies a 2.00 spherical component, which, by itself, would
give a power of 2.00 diopters along both the 180 and 90 degree axis, and adds a
1.00 cylindrical component at 180 degrees (perpendicular to the axis listed on
the prescription, as explained under Axis). The result is 2.00
diopters at 90 degrees and 2.00 + 1.00 = 3.00 diopters at 180 degrees.
The second specifies a 3.00 spherical component, which by itself would give a
power of 3.00 diopters along both the 180 and 90 degree axis, and subtracts a
1.00 cylindrical component at 90 degrees. The result is 3.00 - 1.00 = 2.00
diopters at 90 degrees and 3.00 diopters at 180 degrees.