In order for us to see clearly, the front part of our eye (the cornea) must bend (refract) light and focus it on the back surface of our eye (the retina) accurately. However, changes in the length of the eye, or the shape of the lens or cornea can make it difficult for the eye to focus light perfectly on the retina, making the images we see blurry.
Myopia, more commonly known as nearsightedness, occurs when the physical length of the eye is greater than the optical length. This causes light entering the eye to focus in front of the retina instead of directly on it. People who have myopia can see near objects clearly, but more distant objects are blurred.
Hyperopia, or farsightedness, occurs when light entering the eye focuses behind the retina and not directly on it. It is typically caused by an eyeball that is too short or a cornea that is too flat. People with hyperopia can see distant objects clearly while near objects are blurred.
Astigmatism is a refractive error where the cornea is abnormally curved. This keeps light from focusing directly on the retina causing blurred vision.
Presbyopia is a natural part of the aging process that will eventually affect everyone. It is a condition where, as we age, the lenses in our eyes lose the elasticity that allows them to change length or shape to focus on objects up close. This results in a gradual loss of the ability to focus on near objects.
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