Your Prescription Explained
One important aspect of an eye examination is the measure of your vision, oculomotor status and refractive error. From these examinations our optometrists will produce your spectacle prescription. This contains precise measurements and information to ensure you are given the correct type of prescription lenses to offer you the clearest and most comfortable vision.
But what do the numbers and words on your prescription actually mean?
Vision is a measure of your central vision and your ability to clearly distinguish details, shapes, objects and letters without your spectacles or contact lenses. Distance Vision is measured with chart with progressively smaller letters. 6/6 vision sometimes referred to as 20/20 vision is a good level of relatively normal vision. Having 6/12 vision means you can see an object at six metres that a person with normal vision can see 12 metres away. Vision is measured separately in either eye and then with both eyes together.
The sphere indicates if you are long sighted (hyperopic) or short sighted (myopic) and to what degree. A positive figure e.g. +1.00 indicates that you are long sighted and may struggle to see things close to your clearly. A minus figure e.g. -1.00 tell us you are short sighted and may find distance objects difficult to see clearly. The higher the number the stronger the prescription required to ensure clear vision. The number can range from 0.25 to larger prescriptions like 8.00. Higher prescription can influence the choice of frames most suitable and require special lens materials and design to limit the thickness and weight.
The cylinder indicates the degree of astigmatism present in your eye. Astigmatism is caused by an irregularly shaped cornea or crystalline lens and may mean neither your distance or near vision is clear without spectacles or contact lenses. If the box is empty it means you have no astigmatism. The number tells us how astigmatic you are and therefore how misshapen your cornea is. The number can range from 0.25 to much higher numbers. +5.00 would indicate a significant degree of astigmatism meaning your cornea is shaped more like a rugby ball than a football.
The Axis tells us the direction of your astigmatism and informs the laboratory of which angle to position your lens within your frame to ensure the clearest vision possible. The number can range from 0 to 180 in 1 degree steps.
If you suffer from a muscle imbalance in the eyes due to a variety of reasons such cranial nerve palsy, stroke, strabismusetc a prism lenses may be used to provide correction and help your eyes work better as a pair. Without prism these individuals may suffer from diplopia (double vision) and headaches. Prism will be written in a series of numbers and directions such as up, down, in and out, referred to as the base. This informs the laboratory where to put the prism within your spectacle lenses.
Visual Acuity (VA)
Visual acuity works in exactly the same manner as vision measurement explained above, however VA measures your ability to distinguish small letters whilst wearing your spectacle correction. It is a measure of well you can see at your best when your refractive error has been corrected by the optometrist.
If you are over 40 then it is likely that there will be a number in the Add section. This is a measure of the additional lens power required to ensure clear vision at close distances as a result of presbyopia. This means you may have different prescriptions for distance and reading and may therefore require separate distance spectacles and reading spectacles. Or alternatively bifocals or varifocals may be prescribed.
NV/A (Near Visual Acuity)
Near Visual Acuity is the measure of your ability to read and distinguish small letters and objects at close distances. Is measured in a similar way to distance vision however will be noted in a different way. A NV/A of N5 indicates a good standard of near vision and will mean you will easily be able to read most small prints. A NV/A of N24 means you are limited to reading large letters and numbers and may benefit from low vision adds such as a magnifier.
INT Add (Intermediate Add)
The intermediate add is noted in the same way as the reading Add and is simply the addition lens power required to allow clear vision at intermediate distance such as using a computer screen.