Page 20 - The UK Guide to Laser Eye Surgery
P. 20

20 How the eye works
Understanding prescriptions
Optometrists measure refractive errors
- myopia, hyperopia, presbyopia and astigmatism - in units called dioptres. Dioptres are a measure of the amount of correction you need in order to see normally. One dioptre is the equivalent of a lens that can focus on an object one metre away.
The more short-sighted, long-sighted
or astigmatic you are, the higher your prescription will be in dioptres.
A typical prescription has three numbers - for example: -5.00 / -1.50 x 180.
• The first number (-5.00) identifies your degree of short-sightedness or long- sightedness. The minus sign in front of the number identifies you as short-sighted, whereas a plus sign means you are long- sighted;
• The second number (-1.50) identifies the amount of astigmatism you have. This is written with either a plus sign or a minus sign (usually minus in the UK);
• The third number (180) indicates the axis in degrees, communicating the direction of your astigmatism. An axis of 180 degrees, for example, means the astigmatism is horizontal.
So, a prescription of -5.00 / -1.50 x 180 indicates that the patient is moderately short-sighted, with a moderate degree of astigmatism in a horizontal direction.

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