Understanding Your Prescription

A glasses or lenses prescription is usually shown as a series of numbers in boxes. For Example:

OD -4.50D -0.25D 180
OS -3.75D -0.50D 90

OD (oculus dextrum) indicates that the readings are for your right eye. OS (oculus sinistrum) refers to your left eye.

The SPH (Sphere) box shows a measurement of how long sighted (hyperopic) or short sighted (myopic) you are.

If you are short sighted, the SPH value will have a minus (-) sign before it.

If you are long sighted, the SPH value will have a plus (+) sign before it.

The (D) stands for dioptres which is the standard measurement that is used. If the SPH value is between 0 and 3.0D then you have a mild prescription. 3.0 to 6.0D is moderate and 6.0D and above is a high prescription.

The CYL (Cylinder) box shows the strength of any astigmatism. An eye with astigmatism will not be perfectly spherical and will be like a rugby ball in shape. Astigmatism can be plus (+) or minus (-) and is also measured in dioptres.

The AXIS box shows the orientation of the cylinder (from 0-180 degrees).

At Optimax we add the Sphere and Cylinder together and use this figure to determine suitability for treatment. For example a prescription of -3.00 Sph and -1.50 Cyl becomes -4.50.

Some prescriptions also have extra information such as "VA", "Rdg add" and "Near VA".

"VA" stands for visual acuity. This indicates the standard of vision (for each eye) when corrected and is often presented as a fraction. This is where the term "20/20 vision" comes from, which means a person can see detail from 20 feet away the same as a person with normal eyesight would see from 20 feet. If you had 20/40 vision you could see detail from 20 feet away the same as a person with normal eyesight would see it from 40 feet away. VA can also be converted to a decimal so that 20/20 is 1.0 and 20/40 is 0.5. VA is usually measured by a Snellen chart which shows different letters that get smaller as you read down the chart. The further down the chart you can read, the better your visual acuity.

"Rdg add" (reading addition) shows the additional positive power that is needed to enable the eyes to focus for close work. This is usually only needed for patients over 40-45, since focusing ability declines as we get older (presbyopia). If a reading addition is stated, this means you need different spectacle prescriptions for reading and for distance.

"Near VA" represents the smallest sized print that can be read with the prescription. This is usually written as Nx where x is a number representing the print size. N5 is the smallest sized type that you will normally find and N8 is approximately the size of normal newsprint.