Understanding Your Glasses Prescription - A Comprehensive Guide

Whether you’ve just received your glasses prescription, or have just had it change, it can look quite confusing at first glance. The trick of understanding your eye prescription is all down to recognising abbreviations, numbers, and a couple of symbols. Once you get to grips with these, it’ll all make sense.

How To Read A Prescription

To understand a prescription, you’ll need to get familiar with the different numbers, symbols, and words used. These are usually placed inside a grid, with two rows or columns - one for the left eye, and one for right. Depending who is issuing the prescription, the appearance and the placement of these grids will vary. When it comes to understanding your eye prescription, it’s all a recognition game of seeing particular combinations together. Once you know these, understanding your prescription and what it means will become a whole lot clearer. Let’s take a closer look at what you might find.

What Does a + (Plus) or - (Minus) Symbols Mean?

One of the things that is most noticeable about an eye prescription is the + and - symbols, which are found in front of numbers. This is what they mean:

• + (Plus) means long sighted. This means you may have difficulty seeing things up close. You may see this be referred to as hyperopia. Long sightedness can be caused by having a short eyeball, which results in light falling behind your retina as opposed to it. This results in nearby objects looking fuzzy. Long sightedness is also caused by flat corneas.
• - (Minus) means short sighted. This means you may have difficulty seeing things far away. You may see this be called myopia. Short sightedness is often the result of having long eyeballs, which creates a great distance between your cornea and your retinas. This means that things further away end up looking blurry, as less light is actually falling onto your retinas. Another cause of short sightedness is a cornea that is too curved.

You may see these symbols across your prescription. If you do, they always represent the same thing.

Explanation of the Words and Numbers

As we have mentioned, a glasses prescription includes a series of words and numbers. If you’re not already aware of their meaning, it can be a little confusing. Here’s a breakdown of what they are and how they relate to your prescription:

OD and OS

When you see either of these abbreviations on an eye prescription, they’re referring to your individual eyes. OS if your left eye, and OD is your right eye. These are abbreviations for latin terminology for each eye - oculus dexter and oculus sinister. These aren’t found in all eye prescriptions. Sometimes a simple ‘left’ and ‘right’ are used.

SPH - (Sphere)

SPH in an eye prescription relates to how strong the correction in the lenses needs to be. Simply put, this is how powerful they need to be to help a person’s sight improve. This is a number, which may have a + or - symbol before it. As we previously discussed, the + and - symbols note whether it’s long or short sighted. This number included here is in dioptre units - this is what is used to measure the lens power you need. The higher the number, the stronger your lenses will be. Sometimes you might see a number without a + or -. This is usually an indicator of longsightedness. It represents the same as if there was a plus symbol in front of it.

In some prescriptions, you may see the infinity symbol (which looks like an ‘8’ on its side) or Piano (PL). When these are used, it shows that no correction is needed. In some cases where no correction is needed, it is simply left blank.

CYL - (Cylinder)

The cylinder number relates to astigmatism. This is all to do with the shape of your corneas - this is the front, clear layer that covers your iris and pupil. If these are not spherical/round, they are astigmatic - think rugby ball shapes. This can create difficulty with vision, as oval curvature can cause things to appear blurred and out of focus. Cylinder power helps correct this. On a prescription, the higher the CYL number, the greater your astigmatism. These go up and down by increments of 0.25. If you don’t have an astigmatism, nothing will be entered in this section.

AXIS

If you have an astigmatism, this number indicates the curvature direction of where the cylinder power needs to be. This shows the position of the cylinder, and will be between 1 and 180. This doesn’t relate to power in this case - it’s just to do with positioning.

BVD

BVD in an eye prescription is the measurement from the front of your eye to the back of your glasses lens. It’s an abbreviation of the term ‘back vertex distance’, and is measured in millimetres. This is more commonly to be featured in stronger prescriptions, as it is something that can help make lenses more powerful, magnified, and effective. It’s also used in eye glasses prescriptions, as opposed to contact lenses. This is because there’s very little distance between contact lenses and the eye.

ADD

ADD here stands for addition (or reading addition). This abbreviation is included in an eye prescription if you require reading glasses, bifocal, or varifocal lenses. These are often required for helping correct vision for older individuals with presbyopia, particularly when it comes to reading. ADD here notes that additional power is needed on top of any distance correcting requirements.

PD

This is also known as pupillary distance, which is the measurement between the centre of your pupils. This is taken to ensure that your lenses have the correct centre point for your eyes, which will give you clearer vision.

Prism

If you have difficulty with your eyes working as a pair or experience double vision, it may be due to a muscle imbalance in your eye. The prism number in a prescription indicates prismatic correction, which can help with this. This number is usually written in fractions, such as ¼ and ½. 

Base

If you require prismatic correction in your glasses, this notes the direction the prism will go in. These are usually instructional words, like UP, DOWN, IN, or OUT.

V/A or VA

V/A (used interchangeably with VA) stands for visual acuity, and is all about how well your eyes can make out different shapes and their details at a distance. An eye chart test with letters is often used to test this. When featured on an eye prescription, VA is a number out of six. In the UK, the standardised distance for testing an individual’s V/A is six metres. So, if you have a result of 6/6, this means you can see objects at a distance clearly. The lower the number, the more difficult you see objects at this distance.

Example Prescription Chart

Find below an example of what you can expect your eye prescription to look like.

	SPH	CYL	AXIS	ADD	PRISM	BASE
OD	-4.25	-3.25	146	+1.50
OS	-1.50	-0.50	15	+1.25

It’s worth remembering that not all prescriptions will have all of the abbreviations and words that we have covered in this guide.

You might also see ‘near’, ‘intermediate’, or ‘distance’ featured on an eye prescription. These all relate to your vision at different distances - nearby, mid-range, and at a distance.

Are there differences in Contact Lens and Glasses Prescriptions?

In a word, yes. Because of the differences between contact lenses and glasses, prescriptions are different. This is all to do with the distance between either contacts and glasses, and your eyes. They look similar, but the numbers in them will vary depending on what you opt for. Basically, how they correct eyesight changes based on how close they are to your eye.

Contact lens prescriptions will note the power/ SPH the lens needs to be, along with some other numbers. These are measurements of the shape and size of your eye, called ‘base curve’ (BC) and ‘diameter’. These are taken to make sure your lenses fit comfortably. BC is based on the shape and curve of your cornea, and diameter (measured in millimetres) defines the size of the lenses. If you have an astigmatism, you’ll also need a special toric lens to correct your vision. We’ve included an example of what this might look like below:

	SPH	BC	Diameter
Right	-2.25	8.8	14
Left	-4.75	8.6	14.5

Your contact lens prescription may also include CYL and AXIS columns, along with other terms that we have covered in this guide. If you are ever unsure about what these mean, feel free to send us a message and we’ll talk you through your prescription with us.