There are generally two parts to your eyeglasses prescription, distance and close up. Your distance prescription is largely determined by the curvature of your cornea. If the curvature is perfect in relation to the length of your eyeball then you do not have a distance prescription. Your close up prescription is a result of aging of the lens as described in the article about cataracts.
If you do require distance spectacles then this is due to either the cornea being too curved or not curved enough. If it is too curved, it is too powerful and you are short sighted. If it is not curved enough, it is too weak and you are long sighted. The cornea can be curved differently in two directions; if this is the case then you have astigmatism.
Your prescription will have three boxes for each eye. One will be the sphere (Sph), one will be the cylinder (cyl) and the other will be the axis. The lens in your eyeglasses needs to be curved a precise amount in order to bend light so that it focuses perfectly on your retina. The sphere part of your prescription is the main curvature required and the cylinder is the second one. You can only have two curves and they are always at 90 degrees to one another. The axis tells the specs maker how to orientate the lens in the glasses frame.
If you have a plus sign in the sphere part of your prescription you are long sighted, If you have a minus sign you are short sighted (a large value in the cyl box can change this). The cyl box can be either a plus or a minus. The axis is a number between 0 and 180. Lenses for long sight are thick in the middle and thin at the edges. Lenses for short sight are thin in the middle and thick at the edges. This has implications for your choice of frame. Look through your current eyeglasses and you will notice that what you are looking at through them has changed in size. If it is smaller you have minus lenses. If it is larger you have plus lenses.
For the sphere part, between zero and about 2.00 would be considered a low prescription. Between 2.00 and 5.00 a medium prescription, and above 5.00, a large prescription. Above 10.00 is a very large prescription. For the cyl part, below 1.00 is low. 1.00 to 2.50 is medium and above that it is high. With high prescriptions, choosing the right designer frame is all important.
Craig Leaver is a profesisonal optician and owner of https://www.eyewearcanada.com/ an online shop for designer glasses and Eyeglasses.
Craigs dedication to caring for his patients visual health and their pocked drives him on his daily quest for caring quality.