First, let’s talk about the minus sign and what that means. The minus sign on the prescription before the power tells us that your eyes are myopic, or nearsighted. This means that you see better up close than you do far away.
This is typically someone who starts wearing glasses in elementary school and needs their glasses for seeing in the distance. A person becomes nearsighted when the eyeball is longer than average, which causes light to come into focus before it reaches the back of their eye.
This is processed as blurred vision. The glasses and/or contact lenses then cause the light to shift to the back of the eye for clear vision.
A positive prescription is just the opposite. This is someone who is hyperopic, or farsighted. This person tends to see better far away than they do at near. Depending on how high the power is, people who are farsighted might not need glasses while they are young and may just need reading glasses later in life.
A hyperopic eye is typically shorter than average, causing light to fall behind the eye. When light falls behind the eye, we are able to bring it into focus on the back of the eye resulting in clear vision.
Now when we reach a certain power of farsightedness, we no longer have the ability to bring the light into focus on the back of our eye. If we cannot bring the light into focus on our own, then glasses are required.
There is not a certain power that is the cutoff for needing glasses or not, it is all dependent on the person.
There are other types of prescriptions including those for astigmatism and those for presbyopia (people who need reading glasses or bifocals), but we’ve covered the basics of glasses and contact lens prescriptions!