How to Read an Eye Prescription: What do the Numbers Mean?

How to Read an Eye Prescription: What do the Numbers Mean? Many people in the UK and Europe attend an optician who tests your eyes and produces your glasses for you. Unlike our American counterparts, we do not necessarily understand each element of our prescription, why would we? Someone else takes care of it. In this article we will look at your prescription and the different readings that appear on it.

What Does Sphere Cylinder and Axis Mean?

It does not matter whether you have been wearing glasses for years, even decades or if you have only just started wearing them recently. Either way, your prescription will probably look like a confusing collection of meaningless numbers. You are not the only one to think that. One part of the prescription that puzzles many people is the sphere cylinder and axis. In the following post, though, we hope to give a full explanation of what both these terms and measurements mean. We are also going to discuss some of the other crucial parts of the prescription.

If you tend to get your glasses from the same commercial opticians you were tested, then this is not going to be too much of a problem. However, if you want to use your prescription to buy glasses elsewhere or need a replacement or additional pair of prescription glasses or sunglasses, then understanding your prescription and its different terminology will be helpful. You will need to know how to read an eye prescription.

So, What Are Sphere Cylinder and Axis on a Prescription?

Let’s look at each part individually.

Sphere 

Often written as SPH, this is the lens power required and prescribed to correct either long-sightedness or near-sightedness. It is measured in the unit known as dioptres (D). You know you are near-sighted if the number that is listed for the Sphere heading in the prescription has a (-) minus sign next to it. Whereas, you are farsighted if there is a (+) plus sign next to it.

The term sphere is used because the correction of either the far-sightedness or near-sightedness is equal across all the meridians of your eye, or spherical.

Cylinder 

Cylinder or CYL is used to identify how much lens power in your prescription is for astigmatism if you suffer from it. It may be that nothing is filled in for that part of the prescription at all. That basically means you either have no or very minor astigmatism that needs to be corrected.

The word cylinder is used to show that this particular lens power is shaped to give one meridian of the eye no extra curvature and is not spherical and that the perpendicular meridian to the one with no extra power has the lens curvature and maximum amount of power to correct your astigmatism.

You will find that the number for this column in your prescription again either has a (-) minus sign or (+) plus sign as a reference to whether it is near-sighted astigmatism or far-sighted astigmatism correction respectively. Cylinder power will always come after sphere power in a prescription.

The meridians of your eye are identified using a protractor scale against the front surface of the eye. The vertical meridian of your eye is the 90-degree meridian, while the horizontal meridian is the 180-degree meridian.

Axis 

Now, the axis identifies the lens meridian that has no cylinder power for correcting astigmatism. The axis in your prescription will be measured in numerical form from 1 to 180. Where 180 is your eyes horizontal meridian, 90 is the vertical meridian.

If a cylinder power value is included in your prescription, there will always be a value in the axis column. This is written after cylinder power and when it is written in freehand form it is written with an x before it.

The axis is your prescriptions lens meridian that is set at 90-degrees from the cylinder power-containing meridian that is used for correcting astigmatism.

To know how to read an eye prescription you will definitely need to know all of the above terms, however there are some other words, abbreviations and measurements that can also appear.

What Are OS and OD on a prescription?

Now that we have discussed sphere, cylinder and axis, we will look at the other main parts of your prescription to round out your understanding. You will see the initials OS and OD written on your prescription.

These are abbreviations that denote to oculus sinister and oculus dexter in Latin and mean left eye and right eye when translated.

You may also find your prescription has another column with the abbreviation OU. This means oculus uterque in Latin or both eyes in English.

While Latin terms being used in glasses, eye medicine, and contact lenses prescriptions, there are some clinics and doctors that have modernised the terminology they use and have LE and RE instead, meaning left eye and right eye respectively.

On the prescription for glasses you are given, it will always feature the right eye information before the left eye details. A prescription is written in that order because when the doctor looks at you, they will obviously see first your right eye and then the left eye second, from left to right.

What are Add and Prism on a prescription?

That leaves just two other measurements on your prescription.

Add

This refers to the magnifying power added to the bottom of multifocal lenses for presbyopia correction. This will always be written as a + power, even if the doctor has not filled it out with a plus sign. It normally ranges from between +0.75 and +3.00 D and is the same level for both your eyes.

Prism

This term refers to the prismatic power, which has a value in prism dioptres, which is either written as p.d. or when it is written freehand, a superscript triangle. This part of the prescription relates to compensation made for eye alignment issues. Relatively few glasses prescriptions have a prism reading.

When it is featured on a prescription, the prism is normally identified in either fractional or metric units, ½ or 0.5, as an example. The prism direction is identified by the position of its thickest edge, or base. There are four different abbreviations used in describing prism direction. BO is short for base out and means towards your ear, BI is short for base in and means towards your nose, BD is short for base down, while BU is short for base up.

As you can no doubt see, therefore, the sphere, cylinder and, where appropriate, axis values on your prescription are three of the most important numbers. When you understand those and understand what part describes the lens power needed for each eye, you have a better understanding of what your glasses do to correct your vision issues.

Did you find out How to Read an Eye Prescription? Find out more information in our resource centre.