The Human Eye

[two-columns][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.

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With normal vision (emmetropia), images are brought to a clear focus 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).

Images are focused on the retina by the cornea (the clear part of the front of the eye) and the natural lens (the crystalline lens suspended within the eye just behind the pupil). About half the UK population requires spectacles or contact lenses to see clearly.

Short sight, long sight and astigmatism

Short sight

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short_sight

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Long sight

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long_sight

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][Short sight occurs when the eyeball is too long and/or the cornea is too steep, and the image is focused in front of the retina, causing blurring of vision. With long sight, the eyeball is too short and/or the cornea too flat, and images come to a focal point behind the retina. Spectacles and contact lenses compensate for these defects in the eye’s natural focusing power, which are called refractive errors.

Refractive errors are measured in units of lens power, known as dioptres (D) and are represented in your spectacle or contact lens prescription by a number prefixed by a sign (e.g. +1.00D or –2.50D). 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 focusing power (astigmatism).

Imagine that the cornea is shaped more like a rugby ball than a football. The extent to which the difference in focusing power leads to a distortion of the image is the extent of astigmatism.

This is shown in your spectacle prescription by a second number and an angle (e.g. –4.00D at 80°) indicating the focusing power and the angle of correction required to correct the astigmatism.][Refractive errors are measured in units of lens power, known as dioptres (D) and are represented in your spectacle or contact lens prescription by a number prefixed by a sign (e.g. +1.00D or –2.50D). The sign indicates whether the spectacle correction required is for long (+) or short (-) sight.

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astigmatism

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Younger people are able to increase focusing power, or accommodate, to see near objects clearly. This is due to the flexibility of the natural lens. As we get older, the lens becomes less flexible and the ability to accommodate diminishes, a condition known as presbyopia. This is why normally sighted people need reading glasses from their mid-40s on. The final part of your spectacle prescription describes the difference between your prescriptions for distance and reading glasses (known as reading addition). Typically this varies from +1.00D in your mid-40s to a maximum level of +3.00D by your late 50s.

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