Q: I like to carry a copy of my glasses power with me when travelling, but never really understood what the number mean? Can you make sense of this for me?
A: Yes! Your vision correction, or “eyeglass prescription”, had 3 or 4 parts to it. The vision Rx is like the ingredients in a recipe…..not how you make it, but simply what ingredients go in the final outcome.
- First is the “sphere power”: This will be a positive number ( + ) if you are farsighted, or a negative number ( - ) if you are nearsighted. Usually people have the same vision problem in both eyes….but not always. The bigger the number means the stronger your Rx is.
- Second is the “astigmatism power”: This will be the amount of astigmatism you have. Not everyone has astigmatism, but most people do. In Optometry this will be a negative number and again the number represents how “strong” the lens power must be.
- Third is the “cylinder axis”: Astigmatism simply means the shape of your eye is not perfectly round….like a basketball. Your eye/s are shaped more like the back of a spoon….curved more in one direction than the other. The axis tells us the orientation of this shape….like a spoon up and down, or sideways, or on an angle.
- Fourth would be the “add power”: This means you have presbyopia and need more power to help your eyes focus clearly on close up objects and print. Most people over 40 have this natural aging change, and eventually everyone does. This will always be a positive number, and again the bigger the number means the stronger the power.
I should add that uncommonly there can be more. Prismatic power is also added to lenses to treat double vision and eye alignment disorders. Some people have eye muscle issues and see double images…a very uncomfortable effect. Some people, more often children, have difficulty keeping their eyes accurately aligned when reading up close. Prismatic power can move the images through your glasses up or down, and in or out, to help your eyes work smoothly as a team.
Again, these components are the ingredients. How your optician actually makes your lenses will require measurements of your pupil centers through your new eyewear frame. And reading powers require very precise measurements where the reading power is placed in the lens. If your glasses have the right prescription, but are made incorrectly (much too common these days) your vision can feel uncomfortable or not be as clear as it should.