Your Optical Prescription Explained

Your optical prescription explained

An optical prescription can have several components, and optical terminology over the years may have been a bit of a mystery to you …

Generally, an optical prescription can have the following components:

  • Sphere (SPH),
  • Cylinder (CYL), and cylinder Axis,
  • Addition (Add)
  • Prism, and Base direction

Sphere (SPH)

Spherical correction is optically equal in all directions, a round globe like a football. Spherical lens power is measured in an optical unit called dioptres (D).

If the power number of the prescription is preceded by a plus (+) sign, then you are ‘long-sighted’. Whilst a negative (-) sign means that you are ‘short-sighted’.

Cylinder (CYL), and cylinder Axis

The optical surface of our eyes may not be fully spherical, this irregularity is called astigmatism.

This is corrected using a cylinder which is similar in shape to a can of pop. It’s diameter being described as a power (CYL) and the cylinder direction expressed as an angle between 1 and 180 just like a mathematical protractor used for measuring angles.

An optical prescription with both SPH and CYL components forms a complex surface shape as found on the surface of a rugby ball!

Additional (Add) Inter and/or Near

This is the additional power required to bring closer visual tasks like reading or VDU work into focus. The prescribed power depends on a patients age and required working distance. Patients over the age of forty are most likely to have this component present in their prescription.

This additional power added to the main prescription provides your reading prescription. Typically, this power varies from +0.50D to +3.00D.

Prism, and Base direction

This is used to correct eye-muscle imbalance. A unit of measure called ‘prism dioptres’ is used to record required power. Prism power like the CYL component requires a directional axis, this direction is normally recorded as Base Up or Down, and/or Base In or Out.

High Powered Prescriptions

On higher powered prescriptions, you may also find the abbreviation BVD. This is short for ‘Back Vertex Distance’. This gap between the cornea and the back of the ophthalmic lens is measured in mm.

It is imperative for your optometrist to stipulate this distance as it most definitely affects the clarity of your prescription when glazed into spectacles.

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