Understanding Your Prescription

When you have a sight test, your optician is obliged to give you a copy of your prescription unless the sight test cannot be completed, for instance, if you were referred in an emergency.

Please note that your contact lens prescription is not the same as your spectacle prescription – it is your spectacle prescription that we require.

Your prescription is normally valid for two years from the date of the sight test unless it states otherwise.

Your prescription will usually be written in one of two ways.

  Sph Cyl Axis Prism Add
R -2.00 -1.00 90 +1.50
L -2.50 -0.75 100 +1.50


  Sph Cyl Axis Prism Add Sph Cyl Axis Prism Add  
R +4.50 DS +4.25 DS L

Sph = Sphere
Cyl = Cylinder
Axis = Axis
Prism = Prism
Add = Reading (NV) Add

Sphere and Cylinder are measured in Dioptres. Hence DS = Dioptre Sphere in the second prescription shown. DC = Dioptre Cylinder, but you are unlikely to see this written.

Prescriptions change in quarter dioptre steps, i.e. +1.00, +1.25, +1.50, +1.75, +2.00. They should have two decimal places but are not always written like that. If the value for the sphere is +0.00, it may be written as Plano, or ∞.

A sphere’s power is the same in all directions, so if you rotate the lens, the power will be the same. A positive value indicates a + lens which makes the image larger, and a negative value indicates a – lens which makes the image smaller. You must enter the sign correctly, as +2.00 and -2.00 are very different lenses, and you will not be able to see.

Both eyes do not have to have the same sign, especially on small prescriptions, but +5.00 in the right and -5.00 in the left is probably incorrect.

If there is a value in the cylinder box, there must also be a value in the axis box. A cylinder has a power in one direction only, and this direction is defined by the axis, which must have a value of 1-180. A cylinder must also have a + or – sign, which will vary according to the prescribing optician.

Some prefer plus cyl, and some prefer minus cyl. It just depends on how they have arrived at your final prescription. Be sure to enter the correct sign for the cylinder, and be aware that if there is a cylinder in both eyes, the sign for the cylinder will be the same.

Every prescription will have a sphere value in both eyes, but a cylinder may be in both eyes or just one.

If your prescription has Balance or Bal written in one eye, this indicates that the vision in that eye is not great and is essentially not worth trying to correct.

For instance, the optician may arrive a prescription of -12.00D but issue a Balance of -2.00 to match the other eye as the -12.00 will be thick and heavy, and it will also not improve the vision sufficiently to make it worth prescribing. If the prescription says Bal, enter +0.00 for that eye, and we will use the prescription in your other eye to work out the power of the balance lens.

Prism is uncommon and may be different in each eye or even different for distance and reading. It is there to help your eyes work together, and it is important to get this right. We recommend sending us a copy of your prescription by email to sales@favershamoptical.com or uploading it as instructed in the order screen. A clear photograph from your phone will suffice.

Prisms are triangular additions to the lenses, so they will add to the lens thickness, sometimes significantly. The lens will be thickest at the base of the prism, so “base in” will make the lenses thicker at the nasal side. You may want to consider a thinner lens to improve the appearance.

Reading Add – This will only be present when you need a separate prescription to help you see things up close. It could be for needlework or close-up electronics, not just for reading. It is unusual to have a reading add before the age of 40-45, but not impossible.

Please enter your prescription as written and select distance, reading, bifocal or varifocal as required on the product page. There is more information in the Choosing your lenses section. Your Reading Add will always be a positive (+) number, measured in Dioptres like the sphere, and it will most likely be in the range of +0.75 to +4.00.

As we age, the lens inside your eye continues to grow and become less flexible, preventing it from changing shape as easily, which is how we bring close objects into focus. Eventually, it reaches a point where everything, perhaps up to arm’s length, is impossible to bring into a sharp focus, making reading difficult. It is usually when people start to notice they have to hold things further away that they approach their optician for help. A reading add helps to counteract this.

Occasionally your optician may write your prescription in a different format that doesn’t show the reading add directly. Like so.

R Sph Cyl Axis L Sph Cyl Axis
-4.25 +0.25 20 -4.50 +0.25 180
-2.00 +0.25 20 -2.25 +0.25 180

The Reading Add is the difference between the two Sphere values. Distance is shown at the top and Reading at the bottom, so to get from -4.25 to -2.00, we have to add +2.25. It should be the same in the left eye -4.50 +2.25 = -2.25, so the Reading add is +2.25. As the name implies, this will always be a positive number in 0.25D steps.

The Cyl and the Axis will always be the same; the add only affects the Sphere value.

In another example

R Sph Cyl Axis L Sph Cyl Axis
+1.00 -2.00 90 +1.75 -1.50 95
+2.50 -2.00 90 +3.25 -1.50 95

The reading add is +1.50.

If you are unsure, send us the prescription by post or email, and we will ensure you get the correct lenses. If there is any doubt, give us a call, and we will explain.

Intermediate add – this is a separate add to help with computer screens, music stands, etc. It is designed for a longer working distance than a reading add and will always be a positive value, and a lower number than your reading add. Most people will not have a separate intermediate add.

There is potentially lots of other information on your prescription, such as your visual acuities, normally written as 6/6, 6/5, etc. and your near vision acuity, usually expressed as N5, N6, etc. These measure how well you can see with the prescription given, and we do not need this information to process your order.

There may be other things written, such as drugs prescribed, e.g. hypromellose; again, these are not usually relevant to the manufacture of your spectacles. All we need is the actual prescription.

Some common optical terms

Myopia – commonly known as short-sighted, you will require a negative prescription to correct this, e.g. -1.00.

Hypermetropia (hyperopia in the USA) – commonly known as long-sighted, you will require a positive prescription to correct this, e.g. +1.00.

Astigmatism – your eyes require a different prescription along one axis than another. This is very common, and the majority of prescriptions contain some astigmatism. You will need a cylinder and an axis to correct this, e.g., 1.00/-0.50×180.

It is commonly, although not exclusively, caused by your cornea not being quite spherical. Your optician may have referred to this as having rugby ball-shaped eyes, i.e. the curvature along one axis is steeper than on the other. The cylinder corrects the power, and the axis tells us where to place the cylinder.

Presbyopia – the Latin for old eye. As we get older, we lose the ability to accommodate (see things close up), so your optician has prescribed a different prescription for close work, usually reading. This is a natural process caused by the gradual inability of the lens within your eye to change shape.

If you are unsure, send us a copy of your prescription by email to sales@favershamoptical.com or upload it as instructed in the order screen.