Have you ever wondered what the mysterious and intriguing symbols and figures mean in your optical prescription?

On this page we will try to explain in non-medical terms what it all means. This is not meant to be optical advice, merely to satisfy curiosity about your prescription form.

Here is an example of a prescription from an optician. The optical technician who produces the spectacles will use this information to create the lenses.

	Sph	Cyl	Axis	Add	Prism	Base
Right eye	-2.75	-1.00	30	+1.25
Left eye	-3.50	-1.00	180	+1.25

The measurement of the focussing power of a lens is expressed in dioptres. A lens with the power of one dioptre has a focal distance of one metre. As a stronger lens has a greater refractive power, this means the focal distance will be shorter. Therefore the strength in dioptres is the reciprocal of the focal length expressed in metres. The figures shown under Sphere and Cylinder in the example above are dioptres.

The higher the number, the stronger the prescription in the lens.

Because eyes are both short sighted and long sighted, spectacle lenses come in positive (plus) and negative (minus) strengths. Positive lenses magnify the image and negative lenses diminish the image. Positive lenses can concentrate sunlight in the same way as a magnifying glass. Negative lenses actually disperse or spread sunlight, not concentrate it.

However, spectacle lenses are designed to correct the focus of the eye, not to act as magnifying glasses.

Sph (also expressed as 'sphere' or 'spherical')

This is the main component in the correction of vision. Spherical lenses act in all directions so that the image is magnified, blurred or has blur corrected equally in height and width. Examples of the use of spherical lenses are in a normal magnifying glass, or telescope.

Cyl (also expressed as cylinder or cylindrical)

This is for 'fine tuning' of the lens. An optician will begin by finding the best spherical correction and then, if there is astigmatism (see under Eye Diseases) add the right amount of cylindrical correction.

Cylindrical lenses act only in one direction, so that the image is either magnified in height or width but not both. An example of a cylindrical lens is that used in certain types of reading magnifiers which, when held over the page, will magnify in one direction only.

Axis

Cylindrical lenses have an axis. Turning the lens round so the axis points in different directions changes the way it magnifies.

The figure under Axis is the number of degrees from horizontal that the axis of the lens is to be produced, to correct the vision of the patient.

There will not be a figure under Axis if the eye does not require this correction.

Pupil distance

This is the distance between the centre of your pupils. The pupil is the black spot in the middle of your eye.

Not all prescriptions include a pupil distance measurement.

Your optometrist will include this on your form if necessary and you can enter this where shown on the Specs2go registration form.

Want to know more about lenses?

Origins of the lens
A brief history of spectacles
What your spectacles prescription means
Myths about spectacles
About sunglasses
About lens tints