Nearsightedness, also called Myopia, is when a person experiences blurred vision for far away distances and clear vision while reading up close.
Farsightedness, also called Hyperopia, is when a person experiences clear vision for far away distances and blurred vision while reading up close. (some exceptions apply to hyperopia though).
Nearsightedness (myopia): image is over focused to a point in front of the retina
Dr. Beach says, "I like to think about the eye's visual system like a projector system at the movie theater. In order for your vision to be clear, it needs to be focused directly on the retina which would be equivalent to the screen at the movie theater. If the projector system over-focused, the image would be blurry. If the projector system under-focused it would also be blurry. The focus has to be just right. " People who don’t need glasses or contacts have visual systems which accurately focus images on the retina.
People with myopia, or nearsightedness, essentially have a vision system that is too strong or is over-focusing. The image focuses before the retina. Why does this happen? One possible reason is the the cornea on the front surface of the eye is too steep in curvature. This is the equivalent of turning up the focusing knob on a movie projector. The movie on the screen is now blurry.
Another reason for myopia is the eye may be too long. Sounds strange, but the visual system of the eye is designed to focus at a certain distance between the cornea and the retina. Imagine that same movie theater in which the engineers have designed a projector system focused on a screen 75 feet away. But, what if someone knocked out the back wall of the movie theater and moved the screen to 100 feet? The images would be blurry because the projector is over-focused to 75 feet, but the screen is another 25 feet further.
Farsightedness (hyperopia): image is under focused to a point behind of the retina
Hyperopia, or farsightedness, is the exact opposite. The visual system is essentially too weak and focuses images somewhere behind the retina instead of directly on the retina. This may happen if the curvature of the cornea is too flat (weak) or if the eye is too short.
With either myopia or hyperopia, glasses and contact lenses can be used essentially like the fine focusing knob on the movie projector. The prescription can be adjusted so that the image of vision system focuses perfectly on the retina, changing the eye's blurry vision to clear images.
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