How To Read A Spectacle Prescription

A spectacle prescription is a short table of numbers that most people never have explained. Each column describes a different aspect of how a particular eye fails to focus.

Sphere describes the basic focusing error

The sphere value states the lens power, in dioptres, needed to bring light to focus on the retina rather than in front of or behind it.

A negative value indicates short sight, where the eye focuses light before the retina, usually because the eyeball is slightly too long for its optical power.

A positive value indicates long sight, where the focal point falls behind the retina. The magnitude of the number reflects how far the correction has to move that point.

Cylinder and axis describe astigmatism

A cylinder value appears when the eye's surfaces are not equally curved in all directions, so light focuses at different distances depending on orientation.

The cylinder gives the additional power needed, and the axis, expressed in degrees, states the orientation at which that power must be applied.

Axis is a direction rather than a strength, which is why a small error in it degrades vision noticeably even when the powers are correct.

Addition covers near work

The addition figure is extra positive power for close focus, added to the distance prescription for reading or for the lower part of a varifocal lens.

It becomes necessary as the lens inside the eye loses flexibility with age, a change that affects essentially everyone from middle age onward.

The same addition value normally applies to both eyes, because the underlying change is a property of ageing tissue rather than of an individual eye's shape.

Prism is a separate correction

A prism value shifts the image sideways or vertically and is prescribed where the eyes do not align properly with each other.

It has a direction as well as a magnitude, described by the base direction, and it addresses double vision or eye strain rather than blur.

Prism appears in a minority of prescriptions and generally follows a specific assessment of how the two eyes work together.

What the prescription does not include

Lens material, coatings and frame choice are not clinical values and are where most of the price variation in glasses sits.

Pupillary distance, the separation between the centres of the pupils, is needed to make the lenses but is sometimes recorded separately from the prescription itself.

A prescription is also a snapshot with an expiry, and a sudden change in vision is a reason to be examined rather than to order new lenses.