Of course to look good, we have to be able to see! How else can you apply makeup correctly or do your hair? Not to mention the fashion statement that glasses make. However, how many of you understand what is happening in your eye that causes you to be nearsighted or farsighted? Do you know how your glasses or contacts work? Do you know what the numbers on your prescription mean?
In a healthy eye, the light from an image comes through the eye and goes to the lens. The lens works just like a lens for glasses or contact lenses and refracts (bends) the image so that it can be focused on the retina and your optic nerve can read the data and your brain can recognize the image.
I have nearsightedness, also known as myopia (which, frankly, is easier to say. lets all say myopia instead of nearsightedness). So what is happening in my eye is that my lens is focusing the image in front of my retina.
If you have myopia, your diopter (the number in your prescription) is negative. The higher the number, the stronger your prescription. The diopter is a characteristic of the lens itself, related to the focal length of the lens. Lenses for myopia are concave. Concave lenses cause the light to diverge, therefore extending the focal length so that the image is focused on the retina.
Hyperopia, or farsightedness, is the opposite condition. The lens in your eye focuses the image behind your retina. Lenses for hyperopia are convex, which shortens the focal length of your eye. Diopter values are positive.
If you have astigmatism (like I do), your eye is not shaped like the eyes above. Your cornea has a slight "football" shape to it, which causes the images to refract differently than a normal eye and also causes standard contacts to not fit properly. Your eye literally focuses at 2 points instead of 1. This is corrected with lenses that have special "cylindrical" values in the prescription (If you have cylindrical and spherical values in your prescription, you have astigmatism. If you only have spherical values, you do not have astigmatism.).
The PD length that is written in your prescription is the pupilary distance - or the distance between both of your pupils. This is necessary for glasses so that they are made correctly. For example, I had an old pair of glasses that gave me terrible headaches every time I wore them. My contacts were fine, but my glasses were torture. By the time I finally took them back to the doctor, it was too late for them to fix them (they were out of warranty), but the PD length was wrong. Basically my glasses were trying to focus my pupils outward instead of straight ahead, and it was giving me headaches.
It is VERY important that you take care of your eyes (after all, you only get 2) and see a qualified eye care professional. You should NOT get headaches from your glasses or contacts. If you do, see your doctor because it can be one of many problems - even something as simple as an incorrect prescription or incorrect PD length.