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Understanding Your Spectacles Prescription

When you are ordering your prescription glasses over the internet it is important that the prescription that you provide is correct. Therefore an understanding of your own prescription is needed. If any of the measurements you supply are incorrect you will be unable to see comfortably out of your spectacles.

Sometimes it may be difficult to read or understand what has been written on your prescription. If you are in any doubt please contact the prescriber and ask them for clarification.

An example of a prescription - normally written in this form:

R. -3.00/ -2.75 X 55 
L. -3.00/ -3.75 X 165

The first figure that you will see on a prescription is the SPHERE power (abbreviated Sph).

This is the main power of the prescription the preceding sign is very important, when it is a minus sign it represents a correction for myopia (short sighted)

e.g. -3.00,

and when it is a positive sign it represent a correction for hypermetropia (longsighted)

e.g. +2.00.

Sometimes the prescription may be written with a DS after the Sphere power

e.g.: +2.00DS.

The "DS" signifies Dioptres Sphere, which is the unit of measurement for spectacle power. 
The DS sign is sometimes omitted.

Next comes the CYLINDER power (abbreviated cyl). This represents the astigmatism in your prescription.

Often this figure is shown like this -2.75cyl.

The astigmatic power is always along a certain axis (angle) from 0 to 180 degrees,

e.g. -2.75 x 55.

Sometimes the sphere and cylinder powers can be separated by a slash (/).

The "R". signifies the right eye. (L. for the left). 

The right eye is always given first.


Rt +2.50 / -1.50 x 135 Lt +2.00 / -1.00 x 155

R. +2.50 / -1.50 x 135 
L. +2.00 / -1.00 x 155

This prescription indicates that you are longsighted (hypermetropic), plus have a small amount of astigmatism.

Reading prescription.

If single vision spectacles are required i.e. for distance OR reading only, the prescription will be written as previously mentioned.

Sometimes the prescription will be written with an ADD power after it e.g.:

R +2.50/ -1.50 x 135 
L +2.00/ -1.00 x 155 ADD +2.50

This is common for people aged 45 or over.

This indicates that you have a diminished ability to focus on objects near to you (presbyopic). This means that you need a stronger prescription to help you read.

You may only have presbyopia and perfect distance vision or you can have presbyopia in addition to any of the prescriptions above to correct a distance error.

In the previous example, to obtain the Reading part of the prescription you add +2.50 to the sphere power

i.e.: R +5.00/ -1.50 x 135 L +4.50/-1.00 x 155

When ordering bifocals or varifocals the distance vision power correction is given with an add power e.g.

R -2.00/-0.25 x 45 ADD +2.50 
L -1.50/-1.75 x 180 ADD +2.50

To obtain just the reading power from the above prescription, i.e. if you were ordering just single vision reading glasses again you just add the +2.50 to the Sphere power.

In this example it would be:

R (-2.00 add +2.50 = +0.50) => Rt would read +0.50 / -0.25 x 45 
L (-1.50 add +2.50 = +1.00) => Lt would read +1.00 / -1.75 x 180

This would give you your reading prescription as follows:

R +0.50 / -0.25 x 45 L +1.00 / -1.75 x 180.

It is very important that you enter the correct signs when ordering your prescription sunglasses or spectacles.

i.e. minus(-) OR plus(+), when ordering bifocals you need to include the "ADD" power

To further ensure that your glasses will be supplied with correct fitting lenses the 'ocular centres' need to be provided. 
This is often given as a PD (Pupillary Distance) measurement, which is simply the distance in millimeters between the centre of the two pupils (the black bit in your eye!)


ie +(plus) OR –(minus).

Spectacles Prescription Sunglasses Glasses

Our resident Optician is always happy to give you advice on your requirements.

Please call him on
02392 466436
or email to


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