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Understanding your prescription

A prescription is the result of an examination by an optometrist or ophthalmologist. This will specify all parameters necessary to produce a pair ;of glasses appropriate for your needs.


If an examination indicates that corrective lenses are necessary, the optometrist or ophthalmologist will generally provide you with a prescription at the end of the exam.


The parameters will vary from person to person, but a prescription will indicate the power required in order to correct blurred vision due to refractive errors.


Prescription components



The sphere (SPH) specifies the strength of a lens required to correct your focus - a plus sphere would be used to correct long-sight (hypermetropia - difficulty focusing on close objects) and a minus sphere would be used to correct short sight - (myopia - difficulty focusing on distant objects).


Your prescription may have the word Plano or Pl or an infinity sign (∞); this means you are neither long or short sighted, instead this is zero power for the Sphere, however it is still possible to have astigmatism, which is the cylinder (CYL) on your prescription.

Sphere values can range from 0.00 to +/- 20.00, these go up in steps of 0.25.The sphere is measured in a unit called Dioptre's



The cylinder (CYL) and axis on your prescription compensate for astigmatism. Astigmatism is where the front surface of the eye (the cornea) is slightly irregular in shape, rather like a rugby ball, you may have heard this being referred to as toric. This just means the eye or more specifically the cornea has difficulty focusing light onto the light sensitive cells (retina) in your eyes.


It is not unusual for this component to be blank on your prescription, but if your prescription does show a cylinder power this will be accompanied with an axis. Cylinder values range from +/- 0.25 to +/- 6.00 and will go up in steps of 0.25. The cylinder is measured in a unit called Dioptre's



The axis is only present if your prescription contains a cylinder (CYL). The axis identifies where the astigmatism is located on your cornea. The measurement is in Degrees. The values range between 0 and 180 and may go up in steps of ½, 1 degrees.



The Add, short for Reading Addition is the additional correction required for reading, this can be used to make either reading glasses, bifocal glasses or progressive glasses. Some prescriptions may also detail a separate add specifically for intermediate use.


This figure is an indication of how much extra power is required 'on top' of the distance prescription for near or intermediate glasses. This extra power will nearly ALWAYS be the same for each eye and may only appear once on a prescription but generally it is understood to be meant for both eyes unless the prescription specifically states otherwise. The measurement is the Dioptre, and most values range from +0.50 to +3.50 and will go up in steps of 0.25.


Glasses for distance vision will not require a reading ADD. Sometimes opticians use the word 'Add' or 'Near' instead of 'addition'.



A Prism is less common, this is prescribed to correct a lazy eye or muscle imbalance. Prisms are included with prescription to correct some special conditions or some eye disorders (like squints) that require the focussed image to move position. The measurement is Prism Dioptre. The value may be as high as 10 and may go up in steps of ½ or 1 Prism Dioptre’s. We can offer up to 3 Dioptre's prism but in some cases we can offer higher than this.


Prism powers will always be accompanied with a direction which is usually seen as as base direction such as IN, OUT, UP and DOW; for example; Prism 2.00 Base IN. Some people may have prism in two directions for one eye, for example; Prism 2.00 Base IN & 1.00 Base UP.


Pupillary Distance - PD

The Pupillary distance is an essential measurement. It is the distance from the centre of one pupil to the centre of the other pupil measured in mm ( this usually ranges from 50 to 70mm). Each spectacle lens will have an ‘optical centre’, this is a point on the lens where it performs the best and has the least distortion. The optical centre of each lens should be placed directly in front of the centre of each of pupil. This ensures the eyes are looking through the best point on each lens, some prescriptions may have the Pupillary Distance listed for each eye respectively (Mono PD), for example RE: 32 LE: 32, this is the same as 64mm (Binocular PD).


If the Pupillary Distance for the glasses is not set correctly then the eyes may have to strain to look through them, this can lead to headaches, eyestrain, visual distortion and in severe cases, double vision.


The Pupillary Distance is generally measured during a sight test, but is not routinely written on a prescription. This is an important part of the prescription, subsequently it should be accurate to ensure sharp vision.


It is possible to measure your own PD with the help of a friend by using our PD help video, but ideally this should be obtained from your optician, alternatively from a previous pair of glasses. We are able to measure a PD from existing lenses as this measurement will not change after 18 years of age, you can select this option when ordering.


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