Eye Prescription Explained: SPH, CYL, Axis and PD

An eye prescription is a small grid. Each row belongs to one eye, labelled OD or R for the right and OS or L for the left. The columns describe the lenses you need: SPH for short- or long-sightedness, CYL and Axis for astigmatism, ADD for extra reading power, and occasionally Prism. PD, your pupillary distance, tells whoever makes the glasses where to centre each lens. Once you know what each box means, the numbers become much less mysterious.

Right eye, left eye: OD, OS and OU

Many prescriptions use Latin abbreviations. OD stands for oculus dexter, the right eye, and OS for oculus sinister, the left eye. OU, oculus uterque, means both eyes. Plenty of practices simply write R and L instead. It is worth checking that the rows are the right way round on any order form, as swapping them is an easy mistake to make.

Column by column

Box	Short for	What it describes
SPH	Sphere	The main lens power. A minus sign means short-sightedness (distant things blur); a plus sign means long-sightedness (close work is harder).
CYL	Cylinder	The amount of astigmatism, where the front of the eye is curved more in one direction than another. Blank, or marked DS or SPH only, means none to correct.
Axis	Axis	The angle, from 0 to 180 degrees, at which the cylinder power is placed. It only appears alongside a CYL value.
ADD	Addition	Extra magnifying power for near work, used in reading glasses, bifocals and varifocals. Often the same for both eyes.
Prism / Base	Prism	A correction that shifts the image to help the eyes work together. The base shows its direction: up, down, in or out.
PD	Pupillary distance	The distance between the centres of your pupils in millimetres, used to position the lenses.

Lens powers are measured in dioptres and usually move in quarter steps, so you will see figures such as 0.25, 1.50 or 2.75. The bigger the number, regardless of sign, the stronger the lens.

Reading an example

Here is an illustrative prescription, not a typical or target one:

• R: SPH -2.00, CYL -0.75, Axis 180
• L: SPH -1.50, CYL -0.50, Axis 170
• ADD: +1.50 for both eyes

Read across, this person is short-sighted in both eyes, slightly more so on the right. Both eyes have a small amount of astigmatism, set at nearly horizontal angles. The ADD shows they also need extra power for close work, so varifocals or separate reading glasses might be discussed.

What does the axis number actually tell you?

The axis is a direction, not a strength. An axis of 180 is not "worse" than an axis of 90; it simply tells the lab how to rotate the cylinder within the frame. That is why an axis that is off by even a few degrees can make new glasses feel strange.

Why are my two eyes different?

It is very common for each eye to need a slightly different lens. Eyes grow and age individually, and a small difference between them is usually nothing to worry about.

Plus and minus cylinder

Some practitioners write cylinder as a minus number and others as a plus number. The two forms describe the same lens when converted correctly, so a prescription that looks different from your last one is not necessarily a change. If in doubt, ask the practice to explain.

Glasses and contact lenses need different prescriptions

A contact lens sits directly on the eye rather than a short distance in front of it, so the power can differ, especially in stronger prescriptions. A contact lens prescription also includes details such as the base curve (BC), the lens diameter (DIA) and often the specific lens type. These come from a fitting, where the lens is checked on your eye. A glasses prescription cannot simply be used to order contacts.

About PD

Pupillary distance lines up the optical centre of each lens with your pupil. It may be written as one figure, or as two monocular figures measured from the bridge of the nose to each pupil, which can be more accurate for stronger lenses and varifocals. Distance PD and near PD differ slightly because the eyes turn inwards to read. PD is not always printed on the prescription; if you need it, ask the practice that measured it.

Common misreadings

• Treating the axis as a measure of how bad the astigmatism is
• Mixing up right and left when copying numbers onto an order
• Assuming a plus sign is "good" and a minus sign is "bad"
• Using an old prescription after your vision has changed
• Ordering contacts with a glasses prescription

Keeping it current

Prescriptions change over time, and your eyes can change without you noticing. If text has started to blur, you are getting more headaches, or screens feel harder work, book a test; our guides on what happens at a routine eye exam and reducing eye strain from screens cover both sides of that. And once the new pair arrives, cleaning the lenses carefully keeps the prescription you paid for working at its best.