Your Eye Prescription Explained
An optical prescription can have several components to it and therefore, you may find that it is often difficult to understand the optical terminology present on it. Generally, an optical prescription has the following components:
Sphere (SPH), Cylinder (CYL), Axis, Prism, Base and Add.
Sphere – The term ‘Sphere’ means that the optical correction required is equal in all meridians (directions). It’s an indication of the lens power required, measured in an optical unit called dioptres (D), prescribed to correct our eyes. If the number on the prescription is preceded by a (+) sign, then you are generally ‘long-sighted’. However if it’s preceded by a (-) sign, then it means that you are generally ‘short-sighted’.
Cylinder (Cyl) – This term refers to any ‘astigmatism’ that may be required to correct your eyes fully. Again, measured in Dioptres. The optical surface of our eyes may not be fully spherical and irregularly shaped therefore an extra lens power is necessary to correct the aspherical nature of our cornea. This is commonly referred to as ‘a rugby ball shaped eye.’ Not all patients need an extra correcting power so it’s presence maybe limited.
Axis – The axis pertains to the direction of the astigmatism. The axis is defined from 0 - 180, just like a mathematical protractor for measuring angles. For e.g. if 90 appears on the prescription, then this corresponds to the vertical meridian of the eye, and if 180 appears, then it corresponds to the horizontal meridian of the eyes. If no astigmatism is required in the prescription, then an axis will not be present. AN AXIS MUST BE ACCOMPANIED BY ASTIGMATISM.
Add – This corresponds to the added power of magnification required to correct our reading. Patients over the age of 40 are most likely to have this component present in their prescription, known as Presbyopia. Typically, the power varies from +0.50 to +4.50 (in extreme cases). This power is then added to the main prescription to formulate the reading prescription.
Prism – This refers to the ‘prismatic power’ required to correct muscle imbalance, measured in prism dioptres. This is generally written as a superscript triangle when written manually. Statistically, only a small percentage of people require this component of the prescription. When written, the prism power is denoted by a direction as well to indicate the position of the prism when incorporated in to the Lens. Abbreviations include: Base-Up, Base-Down, Base-In and Base-Out.
On high powered prescriptions, you may find the abbreviation BVD, usually written on the side of the prescription. This is short for ‘Back Vertex Distance’ measured in millimetres, between the patient's cornea and the back of the ophthalmic lens. It is imperative for your optometrist to stipulate this distance as it most definitely affects the clarity of the prescription when glazed into spectacles.