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.

What Is the Difference Between an Eye Test and a Contact Lens Test?

A standard, annual eye check-up is not the same as a contact lens test and the results on your prescription are very different. If you need to wear contact lens, an additional contact lens assessment is necessary to ensure that the lens are fitting both eyes correctly and that the health of the eyes is not harmed by the contact lenses.

Standard Eye Test and Check Ups

A comprehensive eye test is an important part of caring for your overall health whether you need vision correction or not. By looking into your eyes, your doctor can check for signs of serious health conditions like diabetes, high blood pressure and glaucoma.

During these tests, your doctor will perform a series of assessments to check your vision sharpness, determine your prescription strength, examine how both of your eyes work together, and check the fluid pressure in your eyes.

Contact Lens Assessments

If you wear or want to begin using contacts lens, you will need to undertake an assessment in addition to a standard eye test. The first test will measure your eye surface to determine what size and type of contacts are best for you. Your doctor may also do a ‘tear film evaluation’ which confirms you have enough tears to wear contact lens. This reduces the chance of you catching an infection or finding your eye constantly irritated due to the lack of moisture (known as ‘Dry Eye’).

The main difference between a glasses and a contact lens prescription is the distance of the lens from your eye. In the test for glasses, the distance of the glass lens is approximately 10 mm from the eye while a contact lens measures for the lens directly on the surface for the eye. If the measurement is incorrect, it is similar to wearing someone else’s glasses, where your vision is blurred and it can damage the health of your eyes.

Whether you decide to wear glasses or opt for contact lens, it's a good idea to get an annual eye test to see if you have new or existing vision or health problems, and if you need vision corrective treatment.

Laser Eye Surgery Tests and Prescriptions

Laser eye surgery consultations are quite different from routine eye check-ups and contact lens assessments. As the different types of laser eye surgery are medical procedures, there are many more checks and evaluations in place to ensure you get a safe and comfortable treatment with the best possible results. This also means that many initial consultations are offered for free with no obligation to progress to surgery.


Typically, the consultation will begin with a series of standard assessments, the surgeon will look at your general health and discuss what you’d like to achieve from the treatment. For example, many believe laser eye treatment will mean they will never require glasses again, however some adults require reading glasses as they reach an older age. Good laser eye surgery candidates have realistic expectations, such as reducing the need for glasses and contact lens or enjoying an active lifestyle.

Following the discussion, the surgeon will continue with a comprehensive examination of your eyes. Several of the tests are similar to those at routine check ups and contact lens examinations such as pupil dilation and tear film evaluations, but there are many others that are specific to laser eye surgery. The surgeon will measure the shape of your eye and your cornea, as well as the lens’ thickness and pupil dilation. These tests are painless and tell the surgeon what laser eye treatment is best for you. If surgery is not a good idea, there are many different alternatives the surgeon will be able to discuss with you.

<<<previous page
next page>>>