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The following terminology will help you understand how your eye works. Refractive Error If the eye does not accurately focus rays of light onto the retina at the back of the eye then there is a refractive error. Accommodation Accommodation is where the lens within the eye changes its shape to focus distance and near images. In the young it allows a wide range of vision from very close to far distance. From approximately 45 years of age the lens becomes less elastic and the range of focus decreases. Presbyopia Presbyopia is the normal ageing change where the eye reduces its capacity to change focus. This means that for the eye that is normal sighted uncorrected reading vision becomes difficult. Reading or bifocal glasses are usually needed at this stage. Dioptre This is a measurement used to record the amount of refractive error present in the eye. It is used to record the degree to which light converges or diverges. Assessing Your Refractive State Spectacles or contact lenses to correct myopia are characterised by the lenses being thicker at the edge of the lens than the centre. Lenses for hyperopia are thicker in the centre than the edge and magnify close objects. Low Myopia less than -4.0 D Moderate Myopia -4.0 D to -7.75 D High Myopia -8.0 D to -12.0 D Extreme Myopia over -12.0 D
Astigmatism
Astigmatism is where the eye does not focus light evenly, usually due to the cornea of the eye being more curved in one direction than another
Emmetropia (normal sight) 
Emmetropia is the refractive condition of the eye in which no refractive error is present
Hyperopia (hypermetropia, long or far-sight) 
Hyperopia is where the length of the eye is too short causing the light to focus at a point beyond the retina. The lens within the eye will compensate to a certain extent, but reading glasses are normally needed at a relatively early age. Later distance glasses are also needed
Myopia (short-sight or near-sight) 
Myopia is where the length of the eye is too long, causing light to be focused in front of the retina. Distant objects are blurred but near objects are seen clearly. Glasses or contact lenses are needed to clearly focus on images in the distance.
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Page last updated on Saturday, 25 June 2005 12:36:27
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Highgate Ophthalmic Practice
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