Glasses - Not Just for Myopia
It can be very difficult for a person who has never worn glasses to picture what the world looks like for someone who needs them. Similarly, across the broad spectrum of people who do wear glasses, it can be tough to understand that not everyone who they see wearing glasses wears them for the same reason. Their world may look completely different from yours when the glasses come off! The following is a quick summary of the main reasons why people wear glasses, and how their glasses work to correct their vision.
1) Myopia/Nearsightedness:
By far the most common reason you'll see someone under the age of 40 wearing glasses; nearsightedness is most simply described as the eyes being able to see well up close, but not as well in the distance. This happens because the person's eye is "more powerful" than it needs to be, and is focusing the image of what they're seeing too far in front of the retina (back of the eye) to get a clear image. Their glasses work to move that image further back in the eye.
2) Hyperopia/Farsightedness:
Less common, and much more difficult to describe, farsightedness is essentially the opposite of nearsightedness; the eyes are less powerful than they should be. Depending on the degree of farsightedness, the patient may experience no blur or strain at all if their eyes are able to overfocus and "power through" the blur. The older we get, however, the more difficult it is to compensate for this. So at some point, a farsighted person will start to notice that fine print up close is getting more difficult to read comfortably. They may start to get headaches at the end of the day. Over time, this blurriness and strain will impact distances further and further away, until eventually they will need a glasses correction to see comfortably at nearly all distances.
3) Presbyopia:
This is the technical term applied to people generally over age 40, who need glasses to see better up close. They may already wear glasses for nearsightedness or farsightedness, or perhaps they've never worn glasses until the day they turned 40. The reason for this change is that once we get into our 40's, the lens in the middle of our eyes begins to have difficulty changing shape. As children, this lens quickly and fluidly changes shape from relatively flat (to see in the distance) to more round (to see up close). Unfortunately, once presbyopia sets in, the lens has a more difficult time shifting into that round shape. The reading glasses prescription allows the eyes to focus images from close objects without requiring the lens to change shape as much. This reading prescription tends to increase at a fairly steady rate until our early 60's, when the lens is hardly able to move at all, and the reading prescription levels off.
4) Double Vision or Loss of Peripheral Vision
Occasionally, a patient may have difficulty regulating the eye muscles responsible for moving the eyeballs themselves. This can occur in one or both eyes, and will result in the eyes becoming out of sync with each other. This often leads to double vision. In many cases, this can be improved with vision therapy exercises, but in more severe cases a prism glasses prescription may be necessary. The prism moves the image that the patient is seeing, and allows both eyes to be in optical alignment, even if they still appear to be looking in different directions.
Similarly, a patient who has suffered a stroke or has had a lesion in certain areas of the brain may experience a complete loss of peripheral vision in any given direction. Prisms can help "pull" the image from the damaged part of their field into the area that's still functional.
5) Glare
There are times when a patient has minimal to no glasses prescription, but still finds themselves very sensitive to light and glare while driving at night. They may require a glasses prescription that doesn't necessarily make their vision sharper, but can cut down on the glare they experience through use of anti-reflective coatings or polarized lenses.
There are dozens more less common reasons why some people wear glasses. But it's safe to say that even if you already wear glasses, the way you see without your glasses on is more unique than you may have realized!