Eye Prescriptions

A typical eye test will involve assessing the degree of focussing error in your eyes.  The results from this will determine your prescription and the right type of lens for you.  A prescription form will be completed.  This describes precisely the specification of the lenses needed to correct your eyes' focussing errors.  Under British law you are under no obligation to purchase glasses from your optician and you should be able to pay for your prescription form, unless you meet certain conditions, which entitle you to free eye tests.

Your prescription will typically include the following information:

  Sph Cyl Axis Prism VA Rdg Add Near VA
Right Eye              
Left Eye              

Note that a prescription is usually valid for two years.

The refractive error or your optical prescription, is based on two factors:

1. The sphere (sph) - This is the strength of the corrective lens required to correct myopia (-) or hypermetropia (+).  It is measured in dioptres (D).

2. The cylinder (cyl) - This is the strength of a lens to correct for astigmatism.  It is measured in dioptres (D).

  • Sph - The sphere represents the amount of long/short light that is present.  A larger number donates a stronger lens.  A plus sign indicates long sight and minus sign indicates short sight.
  • Cyl - The cylinder represents the amount of astigmatism that is present and causes 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.
  • Axis - This represents the orientation of the cylinder (from 0-180 degrees) and is the angle at which the lens is set into the frame.
  • Prism - This is the correction needed to align the eyes, so that they are looking straight and work together.  A prism is a lens that bends the path of light without altering its focus.
  • VA - Visual acuity indicates the standard of vision when it is corrected.  It is conventionally measured at 6 metres.  The larger the denominator, the worse the vision, 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-up work.  This section is usually only completed if you are over 40 years old, due to presbyopia.  If you are given a reading addition, you may require different prescriptions for reading and for distance vision.
  • Near VA - This represents the smallest sized print that can be read with the prescription.  N5 is typically the smallest sized type and N8 is approximately the size of newsprint.

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