Optical Prescription

What does my optical prescription mean?

When your eye examination is complete you’ll be given a prescription, which, like all prescriptions, contains medical jargon.

Don’t be put off by the strange coding.  Here’s a simple guide to what the categories mean.

  • The “sph” box for “sphere” represents the amount of long or short sight that is present.  The larger the number the stronger, and therefore the thicker the spectacle lens will be.  ‘Plus’ lenses are used to correct long sight and ‘minus’ lenses are used to correct short sight.
  • The “cyl” box for “cylinder” represents the amount of astigmatism that is present.  Astigmatism is caused when the eye is not completely spherical, like a football, but is shaped more like a rugby ball.  This causes the vision to be distorted for both distance and near objects.  The cylinder may be ‘plus’ or ‘minus’ regardless of whether the sphere is positive or negative.
  • The “axis” box represents the orientation of the cylinder (from 0-180 degrees) and is the angle at which the lens is set into the frame.
  • The “prism” is the correction needed (if any) to align the eyes, so that they are looking straight and working well together.  A prism is a lens that bends the path of light without altering its focus.
  • “VA” stands for visual acuity. This indicates the standard of vision (for each eye) when corrected. It may or may not be included in the prescription and is usually presented as a fraction. In Britain, VA is conventionally measured at six metres, so the numerator is 6 (e.g. 6/6). In the US, VA is measured at 20 feet (e.g. 20/20). The larger the denominator, the worse the eye sees, so a VA of 6/12 is half as good as 6/6.
  • “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 as focusing ability declines as we get older (presbyopia). If a reading addition is stated, this means you need different glasses 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.

A prescription is usually valid for two years, but your optometrist may recommend that you have your eyes examined more frequently, depending on your particular circumstances.