When your own lens is removed during cataract surgery or clear lens extraction, we have the opportunity to change your glasses prescription to whatever you desire. If you have ever been to optician, you may have noticed that your prescription has 2 elements if you are less than 45 (pre-presbyopic) and 3 elements if you are older (presbyopic).
The first element is the spherical correction required to bring light into focus. For patients who are short sighted, the number will be negative. A low number like -1 or -2.5 means you have mild short sight or myopia, whereas a higher negative number like – 6 means you have high myopia. Plano (often represented by an infinity sign) means you are emmetropic and can see in the distance clearly without glasses. Finally, positive numbers mean you are “long sighted.”
The second element specifies how much astigmatism you have, which broadly equates to whether the front of the cornea (and/or less commonly the front of the lens) is shaped like the surface of a football or rugby ball. 15% of patients have more than 1.5 dioptres of astigmatism.
In presbyopes, the prescription will have an “add” factor, which specifies how much extra help is required to focus images for near.