Now you have a prescription… what’s it mean?
You just finished your eye exam and the doctor said you need glasses, so you get this piece of paper covered in numbers, what do they mean?
Those numbers tell the optician if you are myopic, near sighted meaning you can see up close but not far away. Hyperopic, far sighted meaning you need help up close, presbyopic, also known as old age/short arm disease, or have astigmatism, which messes your vision across all distances. Or if your unlucky possibly some combination of all the above.
The standard way a prescription is written, the right eye is on top and the left eye is on the bottom. They can be denoted by either an “R” or “OD” for the right eye or “L” or “OS” for the left eye.
The first column will either be labeled “Sphere” or “Sph”. If the number has a “+” sign in front of it or nothing that means you have hyperopia, and your lens will magnify things. A “-“ sign means you have myopia and your lens will shrink things.
The second column will be “Cylinder” or “Cyl” and that is for your astigmatism correction. Very simply put, astigmatism means your cornea is shaped more like a football than a baseball. The effect that cylinder correction has is very similar to that of a fun house mirror. If it lines up one way everything looks squashed horizontally or stretched vertically or if at an angle a square door looks skewed. Those sensations normally only last a short while till your eyes and brain gets accustomed to the correction. After all they have been working to give you clear vision and with the glasses now doing the hard part they have to learn to relax and just fine tune your focus.
The third column will be labeled “Axis”. You will only have a number in this column if you have astigmatism correction. It tells the optician how to lay out the cylinder curve for the proper correction.
There may also be a column labeled “Add”, this is if you need reading correction above and beyond what the distance portion of your glasses give you. If you are buying reading glasses the “Sphere” power and “add” power are added together. For example if you have a +1.00 sphere correction and a +2.50 add power your readers would be +3.50. On the other hand if your “Sphere” power is -2.00 and your “add” is +2.50 then your reading power would be +0.50.
Hopefully this takes some of the mystery out of what all those numbers mean and how they make your glasses work.