How to Read Your Prescription?

Every time you go for an eye test or a check-up, your optician writes up a prescription for you. This prescription is used for getting prescribed lenses and glasses from online sources; it also helps in saving money. To you, the prescription is just lines, numbers, and some weird symbols that do not make any sense, but there is an easy way to understand all this. So let's get started on How to Read Your Prescription?

OD and OS

The first thing that pops out on the optician's prescription is the two letters, i.e., OD and OS. Here it becomes interesting because the letters are abbreviations of two Latin words for the right eye (Oculus Destrus) and left eye (Oculus Sinister). In some prescriptions, two other letters OU can be observed; it means both eyes (Latin: Oculus Unitas). However, it does not frequently appear on the optician's prescription because not every patient has the same number that applies to both eyes.

Meanwhile, some opticians use English abbreviations for the right eye (RE) and left eye (LE) in their prescriptions as a modern practice.

Sphere (SPH)

It simply represents the lens power (in numbers) required to adjust your sight accurately; it is measured in dioptres (denoted by D). It works in the following way; zero represents perfect vision, more the number diverts away from zero, more powerful lenses are needed to correct your eyesight. 

There are also "+" and "-" signs on the prescription in front of these numbers. 

-- The plus signs mean you have a problem seeing the thing nearby; it means you are a farsighted person.
-- The minus sign on your prescription means you have trouble seeing the thing in the far-off proximity; it means you are a near-sighted person.

In some cases, there is an infinity sign or the word Plano or the letters PI on the prescription. It means the person has no problem associated with distance; maybe it represents astigmatism. The values range between 0.00 to +/- 20.00 in diopters. 

Cylinder (CYL)

The number in this category represents how much astigmatism is present in the eyes. Now, what is this astigmatism? You would like to know. So let me tell you, it is a condition in which light is not focused evenly on the retina of the eye; it makes the vision blurry. 

For near-sighted astigmatism, there will be a plus sign in front of the number in the cylinder column. 

For farsighted astigmatism, there will be a minus sign in front of the number in the cylinder column. 

If there is no number in this category, it means there is no astigmatism or very little to be ignored.

Axis

This is an important category on your prescription; it is represented by the number to show the position of astigmatism in the eye. It is important to know that it is for astigmatism only, not for other eye conditions. The adjustment of the position of the cylindrical power in the lens is determined by this number.

The value of the number ranges from 1 to 180, whereas the vertical and horizontal positions are represented by 90 and 180, respectively. However, these numbers in the prescription are only for the position of astigmatism, not for strength. This Axis number is there in the prescription if there is also a cylinder value for the eye (same one).

Prism

We are moving toward more specific categories in the prescription that deal with specific conditions or an eye disorder. Prism value on the prescription in one of them as it deals with prismatic power that is required for eyes. It is used to correct the conditions such as specific disorders and conditions associated with eyes focusing the image on altering its position.

The eye conditions and disorders associated with vertical and horizontal alignment are prescribed in terms of prismatic power. It is not very common among people, so few optician prescriptions have prism value. 

However, if there is a prism value on the prescription, it is represented in fractional or metric units, i.e., 0.1 or 1/10. Concerning the direction of the prism, the relative position of the thickest edge knows as a base, is used.  

That is why you may find letters such as BU (base up), BO (base out), BD (base down), and BI (base in) on your prescription in the prism category. 

Add

As we age, many of us experienced presbyopia, meaning having difficulty in seeing things nearby. This ADD category deals with this problem as the number indicates the additional power needed by the eye to read or see things up close. 

Think of it as an additional magnifying power you might need to read a book. That is why it is also known as reading addition. So your doctor might prescribe you reading glasses as an addition to your prescription of distance. 

The interesting thing about add value is that it is the same for both eyes and has only plus sign in front of it, so it is mentioned only once in the prescription. It ranges between +0.50 and +3.50.

It is only for reading purposes, so the doctor may not prescribe it to you even if you need glasses for near-sightedness and farsightedness. 

PD

You may also find the letter PD on your prescription for measuring the pupillary distance from the middle of one pupil to the other. It is one of the most important numbers on your prescription as it is necessary for finding a lens that is best suited to your need and provides maximum comfort. The value varies for adult and young ones as it is between 54 to 74 for adults and 43 to 58 for young ones. 

A multi-focal prescription can be converted to single vision use. You just need to add the SPH and half of the Add value. However, it is important to note that there are values that cannot be changed, such as astigmatism, cylinder, and axis, no matter how the focal distance is altered. It affects the PD value as it contracted it by 1 to 2. 

The correction for astigmatism will not change, so the cylinder and axis will remain the same, regardless of the change in focal distance. Narrow the PD by 1-2 mm.

Read More: How to Measure Your PD?

There Is A Prescription, How To Read It For Different Uses?

For progressive or bifocal glasses, the prescription can be read as:

For single vision distance use, the prescription can be read as:

For single vision computer use, the prescription can be read as:

To convert a multi-focal prescription for single vision computer use, simply combine half of the Add number and the Spherical number (If the Add cannot be split evenly, such as ADD +2.25, both +1.25 and +1.00 can be used to convert the prescription). 

The correction for the astigmatism will not change, so the cylinder and axis will remain the same, regardless of the change in focal distance. Narrow the PD by 1-2 mm.

For single vision reading use, the prescription can be read as:

To convert a multi-focal prescription for single vision reading glasses, simply combine the Add number and the Spherical number. 

For example, if the Add were +2.25 and the OD-Sph was -0.50, the new OD-Sph for reading glasses would be +1.75; if the Sph were +0.25, the new Sph would be +2.50. Consequently, the Add will then become zero, the default. 

If you are using Dual PD to calculate near PD, then subtract 1.5mm from each eye’s measurement. For example, if your dual PD is 30/31mm then your near PD would be 28.5/29.5mm.