Understanding Prescriptions

Understanding Your Prescription

What do the numbers and symbols all mean?

Optometrists measure refractive errors – such as myopia (short-sightedness), hyperopia (long-sightedness), astigmatism, and presbyopia – in units called dioptres.

These measurements refer to the amount of refractive correction you require to see normally. One dioptre is the equivalent of a lens that can focus your vision on an object one metre away.

Therefore, the more severe a refractive error is, the higher the prescription will be in dioptres. A typical prescription is made up of three numbers; for example: -5.00 / -1.50 x 180.

  • The first number (-5.00) identifies the degree of short-sightedness (indicated by a minus sign) or long-sightedness (indicated by a plus sign);
  • The second number (-1.50) identifies the amount of astigmatism you have. This may be written with either a plus sign or a minus sign (usually minus in the UK);
  • The third number (180) indicates the axis in degrees, communicating the orientation of your astigmatism. In this example, an axis of 180 degrees indicates that the astigmatism is horizontal.

Therefore, the example prescription above tells us that the patient is moderately short-sighted, with a moderate degree of astigmatism in a horizontal direction.