Spectacles with Minus (-) Number
A minus power is used to increase vision for a long distance.
A person with a minus power is called short-sighted.
Short-sightedness is technically termed ‘myopia’, and such a person is called ‘myopic’.
Short-sightedness implies a good vision for close work only. A short-sighted person may see quite well without glasses when it comes to close work like reading and writing but is blind for long distance. If his power is high, as much as -7, -10, -15, or even more in exceptional cases, he will read or write or see all near objects clearly without glasses by bringing them very close. He will not see distant objects without glasses. The wearer of minus glasses must see that he uses his glasses constantly to keep his vision normal and eyes healthy.
The minus number has a tendency to increase with age and the sight gets blurred gradually, so much so that it may go up to -6 at an alarming speed, even though the affected person is wearing glasses. The increase in number is a natural phenomenon in shortsighted eyes. It increases as a child’s height increases with the growth of bones. There is nothing one can do to stop it. In 60-70 per cent cases such a condition is hereditary. When the glasses become unsatisfactory, a check-up must be undertaken and changes made from time to time. Once the sight improves to 6/6 or 6/9 with glasses, however high the number but the eye is healthy, it should not cause any alarm.
In the case of higher numbers (-7 to -12) the person must take precautions and avoid strenuous exercises which strain the eye, and not to do intensive close work, particularly at night. This will not prevent the power from increasing but will prevent other complications. In people with numbers -12 to -20 the thickness and weight of glasses does not allow vision to reach the normal 6/6. Some inherent weakness of sight remains. These persons need utmost care and help.