Understanding Your Prescription
Discover more about your prescription and what it means with our guide below.
Although you don’t necessarily have to know exactly what your prescription says about your vision, it can be helpful! Find out what each element of your prescription means, and its impact on your eyesight with this helpful guide.
Remember, when you are buying prescription glasses online from us you can always email a copy of your prescription to us so we can then add it to your order.
How to read your prescription
Even though all opticians work to the same parameters when writing an eye prescription, reading and understanding your prescription can be difficult. Here are some terms commonly used that you can look out for:
SPH (Sphere)
The Sphere describes the strength your glasses need to be to correct your focus:
A + value in your SPH box means you are long-sighted (Hyperopic), difficulty in seeing objects close to you. A - value means you are short-sighted (Myopia) and struggle to focus on distant objects.
This is measured in Dioptres (an optical unit of measurement - 1 dioptre = 1m^-1). The values usually range from 0.00 to +/- 20.00 in steps of 0.25. You may also see the word Plano PL or an infinity sign written here, this means that you are neither long nor short-sighted with perfect focus. However, you may still have Astigmatism - A CYL and AXIS value on your prescription.
CYL (Cylinder)
The Cylinder value (along with your AXIS) corrects for astigmatism - Where the eye is shaped more like a rugby ball than a football, the slight change in shape causes a focusing of light at more than one place in the eye. Astigmatism normally occurs alongside short or long sightedness.
If you don't have astigmatism, this box will be empty. If you do, there will be a plus or minus value. Again this is measured in Dioptres and ranges from +/-0.25 to +/-4.00 in steps of 0.25 where the higher the number, the more oval the eye.
It is common to have astigmatism in just one eye, so don't worry if on your prescription you only have this value in one eye. For an empty field please enter 0.00 on our online form when ordering. You could also have "DS" written under your CYL value, this just indicates no astigmatism and is the same as leaving the box empty.
AXIS
You will only see an AXIS value if there is a CYL value present. This value tells us where the astigmatism is on your eye, and the direction of the astigmatism - measured in degrees. Values range from 0 to 180 and usually go up in steps of 1/2, 1, 5 or 10 degrees.
Prism & Base
A Prism in your prescription tries to correct a lazy eye, indicating that your eyes don't work well as a pair (strabismus). This is measured in Prism Dioptres with values from 0.5 to 10 and usually go up in steps of 1/2 or 1 Prism Dioptres.
The Prism value is always accompanied with a Base direction, this is to tell the lab where to put your Prism. This can be IN, OUT, UP and DOWN. An example above shows Prism 0.5 Base UP. On rare occasions, some people can have a prism in two different directions in one eye.
We can provide glasses for customers needing a Prism in their lens, however, we require you to select our Gold Package or above during the order process.
ADD (Addition)
This is short for a reading addition (Add/Near) and describes how much additional correction you need for reading or close-up work, this will always be the same in both eyes. If you have a value in this field you have different prescriptions for distance and reading glasses. This is also measured in Dioptres and usually ranges from +0.50 to +4.00 in steps of 0.25.
If you require reading, bifocal or varifocal glasses please include the Add/Near values, if you require distance/driving glasses then you may omit this.
Pupillary Distance (PD)
Your pupillary distance is the distance (usually measured in millimetres) between the centres of the pupils of the
eyes and is usually between 50 and 70mm. This measurement is different for each person and depends on whether they are looking at near or far objects.
The PD is used as an indicator of where the "optical centre" of the lens should be; in front of the pupils for minimal distortion when looking through your glasses. If not correctly set, the incorrect PD can cause blurred vision and/or headaches.
Some prescriptions have the PD for each eye, for example, R 32 L 31, you would enter this as 63mm on the online form. It is not routinely given on every prescription but you can always ask your Optician to add it to your prescription. For varifocal orders, we send you a PD ruler for us to accurately measure this.
For more help visit our guide on how to measure your PD.
What is “Near Add” on an eye prescription?
The “Near Add" on an eye prescription, sometimes written as "NV" or “Near Addition," refers to the additional lens power prescribed for reading or other close-up tasks. This term is commonly used in prescriptions for bifocal and varifocal lenses to correct presbyopia (difficulty focusing on close-up objects).
It is usually indicated as a positive number (e.g., +1.50) and represents the extra magnifying power needed for near vision, on top of the distance prescription.
Other terms you may see:
BALANCE
Indicates that the eye is not sighted and the same values should be entered in both eyes, so that both the lenses look and feel (weight) the same.
OD (Oculus Dexter)
OD is simply used to refer to the right eye.
OS (Oculus Sinister)
OS is used when referring to the left eye.
The important points to note are:
Make sure you don't confuse + and – values on your sphere and cylinder values.
Make sure you enter the axis values exactly as they appear – do not enter 12.5 if it states 125.
If ordering reading glasses, don’t forget to enter your reading addition.
If you have been prescribed computer use glasses, you will have an intermediate addition written down, and this may also appear as INT ADD and again may be a single figure such as +1.00.
If you have any trouble understanding your prescription or would prefer to email your prescription to us, send us an email and one of our team will add it to your order.
Frequently Asked Questions
Where do I get an eye prescription?
You can get a prescription by visiting an optician, where you will do an eye test. There is more information about how to get a prescription on our dedicated page.
What is considered a strong or weak prescription?
A weak prescription for SPH (Spherical) value is typically less than +/-1.50 while a strong prescription has higher values, typically over +/-5.00. For CYL (Cylindrical) a weak prescription would be less than +/-0.75, and a strong one would be values over +/-2.00.
How often should I update my glasses prescription?
You should update your glasses prescription every one to two years, depending on your eye health and vision needs. Regular eye exams are important because your vision can change over time, even if you don’t notice it right away. If you experience symptoms like frequent headaches, eye strain, or difficulty seeing clearly, it may be a sign that your prescription needs updating sooner.
If you have specific eye conditions or are at risk for certain eye diseases, you may need more frequent check-ups to ensure your prescription remains accurate and your eyes are healthy. Find out more about the importance of eye tests.
What do the symbols, letters, and numbers on my glasses prescription represent?
Your glasses prescription includes various symbols, letters, and numbers that specify your vision correction needs.
A ‘+’ symbol indicates long-sightedness, while a ‘-’ symbol represents short-sightedness.
The SPH (Spherical) number shows the strength of the correction needed.
The CYL (Cylinder) number shows the degree of astigmatism.
AXIS shows the orientation of this correction, ranging between 1 and 180.
PRISM shows if there's a muscle imbalance.
BASE indicates the direction of the prism correction. You can learn more about these terms and how they affect your vision here.
ADD indicates if you need additional correction for reading.
PD (Pupillary Distance) measures the distance between your pupils, essential for proper lens alignment.
