Simon G Levy

Consultant Eye Surgeon
Reassuring Personal Care - Advanced Technology - Specialist Cataract and Refractive surgeon

Normal sight

A miracle occurs when the eyes and brain work together to transform light into sight. There are two focusing elements in the eye, the cornea (the clear ‘front window’ of the eye) and the lens. Together these focus light sharply onto the retina at the back of the eye. The cornea does most of the focusing for objects that are more than a metre away, whilst the lens focuses for closer activities such as reading.

The retina is sensitive to light, changing it into electrical impulses sent to the brain where they are perceived as sight.

Structure of normal eye

In normal sight the image is focused accurately onto the retina

Short sight (near sight, myopia)

Vision is blurred because the eye is too long and objects are focused in front of the retina instead of on it. In short sight, distance vision is worse than near vision.

In short sight the image is focused in front of the retina

Long sight (far sight, hyperopia)

In long sight the eye is too short and so objects are focused behind the retina. Near vision is worse than distance vision but the effects vary with age. Young people may have clear vision but become dependent on glasses as they get older. Long sight is not the same as presbyopia which is discussed below.

In long sight the image is focused behind the retina

Astigmatism

Astigmatism is caused by a distortion of cornea shape. Normally the cornea is almost spherical but in astigmatism its curvature is greater in one region than another. Compare a football (spherical) to an egg or rugby ball (astigmatism). Vision is blurred at all distances. Astigmatism usually occurs with either short or long sight.

In astigmatism the image on the retina is distorted

Presbyopia

This is the dependence on reading glasses that comes to most people with age. The lens inside the eye becomes less flexible with time and so cannot focus on close objects. Presbyopia is a manifestation of getting older not a disease, but it often causes great inconvenience. It may occur on its own or with any of the other focus defects.

In presbyopia the image of close objects is focused behind the retina

Wavefront abnormalities

These are minute imperfections of focusing, as unique to every eye as a fingerprint. Wavefront abnormalities have technical names such as ‘spherical aberration’, ‘trefoil’ and ‘coma’ described collectively as ‘higher order aberrations’. Wavefront abnormalities decrease the quality of vision at all distances. Whilst they are often not corrected by glasses or contact lenses, they may be successfully treated by lens implants or laser eye surgery.

Analysis of wavefront abnormalities made by an aberrometer

Understanding your glasses prescription

  Sphere Cyl Axis Add
Example 1 -3 -1 90° -
Example 2 +2 -2 180° +2

The unit of measurement is the ’dioptre’

Sphere: The amount of short or long sight. A ‘minus’ denotes short sight and a ‘plus’ denotes long sight.

The higher the value, the more short or long sighted you are.

Cyl: The amount of the astigmatism. The higher the value, the more astigmatism you have.

Axis: Astigmatism has an orientation or ‘axis’ as well as an amount. The orientation is expressed in degrees.

Add: The strength of reading glasses required.

Example 1

Is 3 dioptres short sighted and has 1 dioptre of astigmatism oriented at 90°.

He does not need reading glasses.

Example 2

Is 2 dioptres long sighted and has 2 dioptres of astigmatism oriented at 180°.

He needs 2 dioptre reading glasses.