A glasses prescription can look more like a code than a clear answer to a simple question: what do I need to see comfortably? Rows of abbreviations, plus and minus signs, and numbers with decimal points can be confusing when you are collecting new glasses or comparing options online. Learning how to understand eye prescription details can make those conversations easier, but it does not replace the advice of your optometrist.
Your prescription is a precise record of the lenses needed to correct your vision at the time of your eye examination. It is individual to each eye, and even a small change in one figure can affect clarity and comfort. Here is what the common terms mean and when they matter.
Start with right and left eye
Most prescriptions have separate columns or rows for each eye. You may see:
- R or OD for the right eye
- L or OS for the left eye
OD and OS are traditional clinical abbreviations, while some prescriptions simply use Right and Left. Do not assume the numbers are interchangeable. It is common for one eye to need a different strength, or a different type of correction, from the other.
You may also see OU, meaning both eyes. This is occasionally used when the same instruction applies to each eye, but your full spectacle prescription will usually show the two eyes separately.
SPH: your main lens strength
SPH means sphere. This is the main lens power required to correct either short-sightedness or long-sightedness. It is measured in dioptres, shown as a number such as -2.00 or +1.25.
A minus sign (-) usually indicates short-sightedness, also called myopia. Close objects may appear clear, while road signs, the television or faces across a room can look blurred. The larger the number after the minus sign, the stronger the correction required. For example, -4.00 is a stronger short-sighted prescription than -1.00.
A plus sign (+) usually indicates long-sightedness, also called hyperopia. This can make close work more difficult, although some people also notice blur in the distance or eye strain when their eyes are working hard to focus. A plus number may also be used as part of a reading prescription, so it is best not to interpret it in isolation.
If there is no plus or minus sign, check carefully before entering a prescription anywhere. A missing sign can completely change the lens required.
CYL and AXIS: correcting astigmatism
If your prescription includes CYL, it means you have astigmatism. This is extremely common and simply means that the front surface of the eye, or sometimes the lens inside it, has a slightly different curve in one direction than another. It is not an eye disease.
Astigmatism can cause blur or distortion at different distances. Some people notice that text appears less crisp, lights have a flare around them at night, or their eyes feel tired after reading. The CYL number describes how much astigmatism correction is needed.
The AXIS is shown as a number between 1 and 180. It tells the lens manufacturer the direction in which to position the CYL correction. AXIS is not a measure of how severe your astigmatism is. A prescription with an AXIS of 180 is not stronger than one with an AXIS of 90 – it is simply aligned differently.
Both figures must be used together. A CYL value without the correct AXIS will not provide the intended correction, which is why copying a prescription accurately matters.
Why prescriptions can be written differently
You may sometimes find that two prescriptions appear different even though they provide the same optical correction. This can happen because astigmatism can be written in either minus-cylinder or plus-cylinder format. The signs, CYL value and AXIS will change as part of the conversion.
This is not something to work out by guesswork when ordering glasses. If a prescription looks different from your last one, ask your optometrist or dispensing optician to explain it. They can confirm whether it represents a genuine change in your vision or a different way of recording the same correction.
ADD: extra help for close vision
ADD, short for addition, is the extra magnifying power needed for close work. It is most often included for people who need varifocal, bifocal or reading lenses.
As we get older, the eye gradually becomes less able to focus up close. This normal change is called presbyopia. You may first notice it when you need to hold your mobile phone further away, find menus harder to read in dim light, or feel tired after close work.
An ADD figure is usually a plus number, such as +1.50 or +2.00. It is added to the distance prescription in the lower part of a varifocal or bifocal lens. For example, a person with a distance SPH of -2.00 and an ADD of +2.00 may have close vision corrected differently from their distance vision.
The best lens option depends on your prescription, working habits and the distances you use most. Someone who spends long periods on a computer may benefit from a different approach than someone who mainly needs help reading in the evening. A clear conversation about your day-to-day vision is as valuable as the figures themselves.
PRISM: a less common but important measurement
Some prescriptions include prism, often marked with a triangle symbol or written as PRISM. Prism can help the eyes work together more comfortably where there is an alignment issue. It may be prescribed to reduce double vision, eyestrain or discomfort in certain situations.
A prism entry will usually include a direction, such as base in, base out, base up or base down. This detail is essential. Prism lenses are more specialised than standard single-vision lenses, so it is particularly important to have them measured and dispensed carefully.
What PD means – and why it may not be on your prescription
PD means pupillary distance. It is the measurement, in millimetres, between the centres of your pupils. It helps position the optical centres of your lenses correctly in the frame.
PD is often needed when making glasses, but it is not always written on the clinical prescription because it is a fitting measurement rather than the lens power itself. For varifocals and other more complex lenses, additional measurements such as fitting height may also be required. These depend on the frame you choose and how it sits on your face.
This is one reason a glasses prescription alone does not guarantee a comfortable result. The frame fit, lens design, measurements and adjustment all play a part.
Your prescription is not the same as visual acuity
You may have been told that your vision is 6/6, 6/9 or another figure. This is your visual acuity – a measure of how well you can see letters on a chart at a set distance. It is not your prescription.
A person can have a relatively low prescription and still not achieve perfect chart vision for other reasons. Equally, someone with a stronger prescription may see very well once it is corrected. Your eye examination considers more than lens power, including the health of your eyes and whether your vision is changing.
Why a contact lens prescription is different
It is tempting to use a glasses prescription to buy contact lenses, but contact lenses sit directly on the eye while glasses sit in front of it. Their power can therefore differ, especially with stronger prescriptions. Contact lenses also need details such as base curve and diameter, and some people require a toric lens for astigmatism or a multifocal design for near vision.
Comfort, eye health and lens fit must be assessed as well as vision. A contact lens check is the right place to confirm the safest and clearest option for your eyes.
When to ask for help with your prescription
A new prescription should make your vision feel clearer and more relaxed, but there can be a short adjustment period with a significant change, astigmatism correction or varifocal lenses. If your new glasses cause persistent blur, headaches, dizziness or difficulty judging steps, contact the practice that supplied them rather than trying to adapt indefinitely.
It is also worth booking an eye examination sooner if you notice a sudden change in vision, flashes, a new shower of floaters, a dark curtain or shadow, pain, or marked redness. These symptoms need prompt professional advice and should not be explained away as a prescription change.
At Eyespy Eye and Dental Care, we believe patients should leave knowing what their prescription means, not simply carrying a printout of numbers. Bring your prescription or your current glasses to your appointment if you would like it explained. A few minutes of clear guidance can help you choose lenses that suit your eyes, your routine and your comfort.
