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Sometimes, for different reasons, the eye doesn't focus quite right:


The surfaces of the lens or cornea may not be smooth, causing an aberration that results in a streak of distortion called astigmatism.


The lens may not be able to change its curve to properly match the image (called accommodation).


The cornea may not be shaped properly, resulting in blurred vision.


Most vision problems occur when the eye cannot focus the image onto the retina. Here are a few of the most common problems:


● Myopia (nearsightedness) occurs when a distant object looks blurred because the image comes into focus before it reaches the retina. Myopia can be corrected with a minus lens, which moves the focus farther back.


● Hyperopia (farsightedness) occurs when a close object looks blurred because the image doesn't come into focus before it gets to the retina. Hyperopia, which can also occur as we age, can be corrected with a plus lens. Bifocal lenses, which have a small plus segment, can help a farsighted person read or do close work, such as sewing.


● Astigmatism is caused by a distortion that results in a second focal point. It can be corrected with a cylinder curve.


In addition, lenses can be made to correct for double vision when the eyes do not work together ("crossed eyes"). The lenses do this by moving the image to match the wayward eye.


Corrective lenses, then, are prescribed to correct for aberrations, to adjust the focal point onto the retina or to compensate for other abnormalities. You can read more about vision problems in How Refractive Vision Problems Work.


How a Lens Works


The best way to understand the behavior of light through a curved lens is to relate it to a prism. A prism is thicker at one end, and light passing through it is bent (refracted) toward the thickest portion.



A lens can be thought of as two rounded prisms joined together. Light passing through the lens is always bent toward the thickest part of the prisms. To make a minus lens (above on the left), the thickest part, the base, of the prisms is on the outer edges and the thinnest part, the apex, is in the middle. This spreads the light away from the center of the lens and moves the focal point forward. The stronger the lens, the farther the focal point is from the lens.




Refraction: the bending of light

Diopter (D): the refractive power of a lens; the higher the number, the stronger the lens

Compound lens: a lens having both a spherical and a cylindrical component

Cylindrical curve: a curve that radiates along a straight line, like a pipe cut lengthwise

Spherical curve: a curve that is the same in all directions, like a basketball cut in half


Placing the correct type and power of lens in front of the eye will adjust the focal point to compensate for the eye's inability to focus the image on the retina.


Determining Lens Strength


The strength of a lens is determined by the lens material and the angle of the curve that is ground into the lens. Lens strength is expressed as diopters (D), which indicates how much the light is bent. The higher the diopter, the stronger the lens. Also, a plus (+) or minus (-) sign before the diopter strength indicates the type of lens.