How
to Read Your Eyeglass Prescription
When you look at your prescription
for eyeglasses, you will see numbers listed under the headings of OS
and OD. They are Latin abbreviations: OS (oculus sinister) means the left eye and OD (oculus dextrus) means the right eye.
Occasionally, you will see a notation for OU, which means something involving
both eyes. In general, the further away from zero the number on
your prescription, the worse your eyesight and the more vision correction you need. A plus sign in front of the
number means you are farsighted and a minus sign means you are nearsighted. These numbers represent diopters, the unit used to
measure the correction, or focusing power, of the lens your eye requires.
Diopter is often abbreviated "D."
For example, if your prescription says
-1.00, you have one diopter of nearsightedness. This is a fairly mild amount of nearsightedness. If you are -4.25, that means you have 4 and 1/4
diopters of nearsightedness. This is more nearsighted than -1.00, and requires
stronger (thicker) lenses. Similarly, +1.00 would be a small amount of farsightedness and +5 would be more.
For people who have astigmatism, there will be three numbers in your prescription. The
general form for writing these numbers is S x C x Axis
The S refers to the
"spherical" portion of the prescription, which is the degree of
nearsightedness or farsightedness discussed above.
The C refers to the
"cylinder" or astigmatism, and can be a negative or a positive number. It measures
in diopters the degree of astigmatism that you have. The bigger this number, the more
astigmatism you have. Astigmatism most often is caused by a cornea that is shaped more like a football than a
basketball.
The Axis is a number
anywhere between 0 and 180 degrees. It reveals the orientation of the
astigmatism. It is not enough to specify how much astigmatism there is; you
have to know where the difference in curvature is taking place.
-2.00 +1.50 x 180
+3.50 +3.00 x 45
The first prescription means that the
person has 2 diopters of nearsightedness with 1.5 diopters of astigmatism and
an axis of 180 degrees.
The second prescription means that the
person has 3.5 diopters of farsightedness, 3 diopters of astigmatism and an
axis of 45 degrees.
HOW TO READ YOUR EYEGLASS
PRESCRIPTION
You just finished your annual eye exam and with prescription in
hand, you’re thinking about what kind of cool eyeglasses to try. Have you ever
looked at all those numbers and letters and wondered what exactly do they mean?
Whether you’re a longtime glasses wearer or a newbie, knowing how to decipher
your prescription will give you a better understanding of what the eyeglass
prescription could say about your vision and overall health.
YOUR EYEGLASS PRESCRIPTION DECODED
Your eyeglass prescription is typically written in a grid like the
one below, using a combination of terms, abbreviations, and numbers. Usually
you’ll see these abbreviations along the left side:
O.D. stands for oculus dexter, meaning right eye.
O.S. stands for oculus sinister, meaning left eye.
O.U. stands for and oculus uterque, meaning both eyes.
O.S. stands for oculus sinister, meaning left eye.
O.U. stands for and oculus uterque, meaning both eyes.
Then along the top, you may see this lingo. These abbreviations
reflect the measurements of specific aspects of your vision, and can be pretty
confusing at first.
SPH(or
Sphere) indicates the eyeglass prescription power, or how strong your lenses
need to be to correct your vision. For an indicator of how much magnifying
power is needed in a bifocal or progressive lens to correct your vision
problems, look for ADD.
CYL (or Cylinder) indicates astigmatism, a condition where the cornea is irregularly shaped causing blurred or distorted vision. It also tells the lens strength needed to fix it.
AXIS describes the degree and direction of your astigmatism.
ADD is added magnifying power in the lower part of a multi-focal lens. It’s used to correct presbyopia which is the inability to focus on close objects.
CYL (or Cylinder) indicates astigmatism, a condition where the cornea is irregularly shaped causing blurred or distorted vision. It also tells the lens strength needed to fix it.
AXIS describes the degree and direction of your astigmatism.
ADD is added magnifying power in the lower part of a multi-focal lens. It’s used to correct presbyopia which is the inability to focus on close objects.
EYEGLASS PRESCRIPTION CHART
MAKING SENSE OF THE NUMBERS
Your glasses prescription includes a number for every aspect of
your vision that needs correction. When learning how to interpret your
prescription, keep in mind that usually the further from zero the numbers are,
the more correction you need.
A plus or minus sign in front of
your glasses prescription number is shorthand for
near or farsightedness.
near or farsightedness.
+ means you’re farsighted or have trouble seeing things close
up.
- means you’re nearsighted, or struggle with seeing far away.
- means you’re nearsighted, or struggle with seeing far away.
IS YOUR RX (PRESCRIPTION) SINGLE VISION
OR MULTI FOCAL?
Single and multi-focal eyeglass prescriptions will look different.
If your vision correction is single vision, that means your lens
power corrects for nearsighted or farsighted, but not both. The ADD column
(which stands for additional magnifying in a multi-focal Rx) is blank in the
sample Rx above. If you’ve got an eyeglass prescription for bifocal or
multi-focal/progressives, your lens power corrects for near, far, and sometimes
intermediate range vision. You’ll have a number in the ADD column.
No matter what your eyesight numbers are, or
whatever kind of vision correction you need, it’s good to have your eyes
examined about once a year to make sure your prescription for eyeglasses or contact lenses is
up to date.
How
To Read Your Eyeglass Prescription
See also: The basics of eyeglasses • What's new in eyeglass lenses • Understanding your contact lens prescription
So, you've just had an eye
exam and your optometrist or ophthalmologist has given you an eyeglass prescription. He
or she probably mentioned that you are nearsighted or farsighted, or perhaps that you have astigmatism. (If that's not the case, and you need to see an eye
doctor, click here to find one near you.)
But what do all those
numbers on your eyeglass prescription mean? And what about all those
abbreviated terms, such as OD, OS, SPH and CYL?
This article will help
you decipher all parts of your prescription and discuss it knowledgeably with
an optician when you're buying eyeglasses.
What OD And OS Mean
The first step to
understanding your eyeglass prescription is knowing what "OD" and
OS" mean. They are abbreviations for oculus dexter and oculus
sinister, which are Latin terms for right eye and left eye.
Your eyeglass
prescription also may have a column labeled "OU." This is the
abbreviation for the Latin term oculus uterque, which means
"both eyes."
Though the use of these
abbreviated Latin terms is traditional for prescriptions written for
eyeglasses, contact lenses and eye medicines, some doctors and
clinics have opted to modernize their prescriptions and use RE (right eye) and
LE (left eye) instead of OD and OS.
On your eyeglasses
prescription, the information for your right eye (OD) comes before the
information for your left eye (OS). Eye doctors write prescriptions this way
because when they face you, they see your right eye on their left (first) and
your left eye on their right (second).
Other Terms On Your
Eyeglass Prescription
Your eyeglass
prescription contains other terms and abbreviations as well. These include:
Sphere (SPH). This indicates the amount of
lens power, measured in diopters (D), prescribed to correct nearsightedness or
farsightedness. If the number appearing under this heading has a minus sign
(–), you are nearsighted; if the number has a plus sign (+) or is not preceded
by a plus sign or a minus sign, you are farsighted.
The term
"sphere" means that the correction for nearsightedness or
farsightedness is "spherical," or equal in all meridians of the eye.
Cylinder (CYL). This indicates the amount of
lens power for astigmatism. If nothing appears in this column, either you have
no astigmatism, or your astigmatism is so slight that it is not really
necessary to correct it with your eyeglass lenses.
The term
"cylinder" means that this lens power added to correct astigmatism is
not spherical, but instead is shaped so one meridian has no added curvature,
and the meridian perpendicular to this "no added power" meridian
contains the maximum power and lens curvature to correct astigmatism.
The number in the
cylinder column may be preceded with a minus sign (for the correction of
nearsighted astigmatism) or a plus sign (for farsighted astigmatism). Cylinder
power always follows sphere power in an eyeglass prescription.
Meridians of the eye are determined by superimposing a protractor scale on the eye's front surface. The 90-degree meridian is the vertical meridian of the eye, and the 180-degree meridian is the horizontal meridian. [Enlarge]
Axis. This describes the lens meridian that
contains no cylinder power to correct astigmatism. The axis is defined with a
number from 1 to 180. The number 90 corresponds to the vertical meridian of the
eye, and the number 180 corresponds to the horizontal meridian.
If an eyeglass
prescription includes cylinder power, it also must include an axis value, which
follows the cyl power and is preceded by an "x" when written
freehand.
The axis is the lens
meridian that is 90 degrees away from the meridian that contains the cylinder
power.
Add. This is the added magnifying power applied
to the bottom part of multifocal lenses to correct presbyopia. The number appearing in this section of the prescription is
always a "plus" power, even if it is not preceded by a plus sign.
Generally, it will range from +0.75 to +3.00 D and will be the same power for
both eyes.
Prism. This is the amount of prismatic power,
measured in prism diopters ("p.d." or a superscript triangle when written
freehand), prescribed to compensate for eye alignment problems. Only a small
percentage of eyeglass prescriptions include prism.
When present, the amount
of prism is indicated in either metric or fractional English units (0.5 or ½,
for example), and the direction of the prism is indicated by noting the
relative position of its "base" or thickest edge. Four abbreviations
are used for prism direction: BU = base up; BD = base down; BI = base in
(toward the wearer's nose); BO = base out (toward the wearer's ear).
Sphere power, cylinder
power and add power always appear in diopters. They are in decimal form and
generally are written in quarter-diopter (0.25 D) increments. Axis values are
whole numbers from 1 to 180 and signify only a meridional location, not a
power. When prism diopters are indicated in decimal form, typically only one
digit appears after the period (e.g., 0.5).
Additional Information. Your eye doctor also might write specific
lens recommendations on your eyeglass prescription — such as anti-reflective coating, photochromic lenses and/or progressive lenses — to give you the most comfortable vision
correction possible.
An Example Of An
Eyeglass Prescription
Confused? Let's use an
example to clear things up. (Pun intended.)
Here is a sample
eyeglass prescription:
|
OD
|
-2.00
SPH
|
+2.00
add
|
0.5 p.d. BD
|
|
OS
|
-1.00 -0.50 x
180
|
+2.00
add
|
0.5 p.d. BU
|
In this case, the eye
doctor has prescribed -2.00 D sphere for the correction of myopia in the right
eye (OD). There is no astigmatism correction for this eye, so no cylinder power
or axis is noted. This doctor has elected to add "SPH," to confirm
the right eye is being prescribed only spherical power. (Some doctors will add
"DS" for "diopters sphere;" others will leave this area
blank.)
The left eye (OS) is
being prescribed -1.00 D sphere for myopia plus -0.50 D cylinder for the
correction of astigmatism. The cyl power has its axis at the 180 meridian,
meaning the horizontal (180-degree) meridian of the eye has no added power for
astigmatism and the vertical (90-degree) meridian gets the added -0.50 D.
Both eyes are being
prescribed an "add power" of +2.00 D for the correction of
presbyopia, and this eyeglass prescription includes a prismatic correction of
0.5 prism diopter in each eye. In the right eye, the prism is base down (BD);
in the left eye, it's base up (BU).
An Eyeglass Prescription
Is Not A Contact Lens Prescription
Eyeglass and contact
lens prescriptions aren't the same. An eyeglass prescription is for the
purchase of eyeglasses only. It does not contain certain information that is
crucial to a contact lens prescription and that can be obtained only during a
contact lens consultation and fitting.
In addition to the
information in an eyeglass prescription, a contact lens prescription must
specify the base (central) curve of the back surface of the contact lens, the
lens diameter, and the specific manufacturer and brand name of the lens.
Also, the power of an
eyeglass prescription frequently is modified when determining the best contact
lens power. One reason is that eyeglass lenses are worn some distance (usually
about 12 millimeters) from the surface of the eye, whereas contact lenses rest
directly on the eye's cornea.
An accurate contact lens
prescription can be written only after a contact lens fitting has been
performed and the prescribing doctor has evaluated your eyes' response to the
lenses and to contact lens wear in general.
Your Eyeglass Prescription:
It's Yours To Keep
The Federal Trade
Commission is the U.S. government's consumer protection agency, and in 1980 the
FTC's Prescription Release Rule became law. The rule requires eye doctors (both
optometrists and ophthalmologists) to give patients a copy of their eyeglass
prescription at the end of an eye exam that includes a refraction.
The rule is intended to
protect the "portability" of your eyeglass prescription, allowing you
to use it to buy glasses from the vendor of your choice.
Your eye doctor must
give you a copy of the prescription whether or not you ask for it. Eye doctors
may not condition the release of your prescription on your agreement to
purchase eyeglasses from them, nor may they charge you an extra fee to release
your prescription. They also may not disclaim liability for the accuracy of the
prescription if you purchase eyeglasses elsewhere.
The FTC enforces the
Prescription Release Rule, and eye doctors who violate the rule are subject to
a civil penalty of $10,000.
If you feel your eye
doctor has violated the rule or you want free information on other consumer
issues, you can contact the FTC at 1-877-FTC-HELP (1-877-382-4357). 
