What Is A Prescription?

Mr Glenn Carp provides an explanation of what prescriptions are and what the numbers mean in this 35 second video.

Mr Glenn Carp – “A prescription is essentially the patient’s refractive error.  When the light enters the eye through the cornea and through the lens to reach the retina it needs to be focused on the retina.

“So any little refractive error in the pathway, whether it be in the lens or in the cornea in the front of the eye, will result in some de-focus in terms of the light reaching the retina.

“So your prescription essentially is just the correction factor to sharpen the light rays so that they do reach the target.”

Prescriptions Explained Further

To gain a prescription value, optometrists and ophthalmologists perform tests to identify any disorders of the eye, including myopia, hyperopia, astigmatism, and presbyopia. The extent of these disorders are measures in units called dioptres.

These measurements show the amount of correction required to achieve standard vision. For example, one dioptre is the equivalent of a lens that can focus on an object one metre away. Either a plus (+) or minus (-) sign before the value determines whether the patient is long- or short-sighted, respectively.

A typical prescription contains three values: e.g. -5.00 / -1.50 x 180.

The first value (-5.00) of a prescription identifies your degree of long- or short-sightedness. In this example, the patient is short-sighted, as indicated by the minus value.

The second value (-1.50) of a prescription identifies the degree of astigmatism. Again, this is written as either a plus or minus value and will vary depending on the severity of the disorder.

Finally, the last value (180) of a prescription indicates the axis of astigmatism. This value is a measurement of degrees as opposed to dioptres. An axis of 180 degrees, as in the example prescription, means the astigmatism is horizontal.

Altogether, the example prescription indicates that the patient is moderately short-sighted, with a moderate degree of astigmatism in a horizontal direction.

The table below illustrates the ranges from mild to severe short-sightedness and mild to severe long-sightedness:
Mild short-sightednessMyopia up to -3.00 dioptres
Low short-sightednessMyopia up to -3.25 to 6.00 dioptres
Moderate short-sightednessMyopia from -6.25 to -11.00 dioptres
Severe short-sightednessMyopia from -11.25 to -23.50 dioptres
Low long-sightednessHyperopia from +0.75 to +2.50 dioptres
Moderate long-sightednessHyperopia from +2.75 to +6.00 dioptres
Severe long-sightednessHyperopia from +6.25 to +12.00 dioptres


If you have any questions regarding your prescription, give us a call today on 020 7224 1005. One of our Clinic Coordinators will be happy to help.