YOUR PRESCRIPTION - and what it means
We see by forming an image on the retina - a carpet of light sensitive cells, each
acting like a pixel on a computer screen, that lines the inside of the back of the
eye. Images are focussed on the retina by the cornea (the clear part of the front
of the eye) and the natural lens (suspended within the eye just behind the pupil).
About half the UK population (50%) requires spectacles or contact lenses to focus
a clear image of a distant object on the retina.
Spectacles and contact lenses either add or subtract focussing power to help form
a clear image on the retina where the natural focussing power of the eye is incorrect.
Defects in natural focussing power are called refractive errors. Refractive errors
are measured in units of lens power ("dioptres" or D) and represented for each eye
in your spectacle or contact lens prescription by a number prefixed by a sign (e.g.
+1.00D or –12.50D). The sign indicates whether the spectacle correction required
is for long (+) or short (-) sight.
Short sight (myopia) and long sight (hypermetropia) are often accompanied by an
element of uneven focussing power (astigmatism). Imagine that the cornea is more
rugby ball shaped than football shaped. The extent of the difference in focussing
power between the smallest and largest radius of curvature for this uneven (toric)
surface would be the amount of astigmatism. This is represented in your spectacle
prescription by a second number and an angle (e.g. –4.00D at 80°) indicating
the focussing power and orientation of the lens required to correct the astigmatic
component of your refractive error.
The younger eye is able to increase focussing power, or accommodate, to see near
objects clearly. This flexibility of focus is provided by flexibility in the shape
of the natural lens. As we get older, the natural lens becomes less flexible (presbyopia),
and the ability to accommodate diminishes. This is why normally sighted people need
reading glasses from their mid forties on. The final component of your spectacle
prescription describes the reading addition, or the difference between your prescriptions
for distance and reading glasses. Typically, this varies from +1.00D in your mid
forties to a maximum level of +3.00D by your late fifties.