Refractive Errors

What is refraction?

Light consists of photons that travel through space like a stream of tiny particles. In general, anything in that space (including air) may affect the direction of the light by either bending it, filtering it, or blocking it altogether. Refraction refers to the change in direction (bending) of light caused by whatever it travels through (the transmission medium).

Regarding vision, refraction refers to the bending of light as it travels through the eye to reach the retina, where the light is converted into electrical impulses that are transmitted through the optic nerve to the brain. The brain interprets these impulses into the images we experience as sight.

What are refractive errors?

An eye that has no refractive error when viewing distant objects is said to have emmetropia (or be emmetropic), meaning the eye is in a state in which it can focus parallel rays of light from distant objects on the retina.

For eye exams, a distant object is defined as one located 20 feet from the eye, since the light from these objects arrives as essentially parallel rays, considering the limitations of human perception.

Visual acuity is usually measured with a Snellen chart. Snellen charts display letters of progressively smaller size. “Normal” vision is 20/20. This means that the patient sees the same line of letters at 20 feet that a person with normal vision sees at 20 feet.

20/40 vision means that a patient sees at 20 feet what a person with normal vision sees at 40 feet. Another way of saying this is that a person with 20/40 vision has vision that is only half as good as typical; objects must be at half the normal distance for them to be visible.

What causes refractive errors?

How you use your eyes does not affect your refractive error. Refractive errors occur when the shape of your eye does not bend light correctly. This could be due to the length of the eyeball (longer or shorter), changes in the shape of the cornea, or aging of the lens.

What are the types of refractive errors?

The main types of refractive errors are myopia (nearsightedness), hyperopia (farsightedness), presbyopia (loss of near vision with age), and astigmatism.

Click on each section below to view its contents:

Myopia (nearsightedness)

Myopia (close objects are in focus, and distant objects are blurry)

Myopia (nearsightedness) is caused by a cornea or lens with excessive curvature or an eyeball that is too long. The light focuses in front of the retina, rather than directly on it. Myopia can be corrected with a concave lens.

Myopia is usually inherited and often discovered in childhood. Myopia frequently progresses during the teenage years, when the body is growing rapidly.

People with high degrees of myopia have a higher risk of retinal detachment, which may require surgical repair.

Hyperopia (farsightedness)

Hyperopia (close objects are more blurry than distant objects)

Hyperopia (farsightedness) is due to a cornea or crystalline lens with not enough curvature or an eyeball that is too short. In mild hyperopia, distance vision is clear while near vision is blurry. For people with significant hyperopia, vision can be blurry for objects at all distances. Hyperopia can be corrected with convex lenses, which cause light rays to converge before they reach the cornea.

Hyperopia can also be inherited. Some people may not notice any vision problems, especially when they are young.

Presbyopia

Presbyopia (close objects gradually become blurry with age)

Presbyopia is an age-related condition in which the lens inside the eye becomes more rigid and can no longer change shape sufficiently. Presbyopia affects most adults over the age of 35, when it gradually becomes more challenging to read at close range. Presbyopia can be corrected by reading glasses, bifocal, or progressive lenses.

This is a normal aging process of the lens that can also be combined with myopia, hyperopia, or astigmatism.

Astigmatism

Astigmatism is a condition in which the front surface of the eye, the cornea, has an asymmetric curvature.

Typically, the cornea is smooth and equally curved in all directions (like a round ball), and light entering the cornea is focused equally on all planes, or in all directions. In astigmatism, the front surface of the cornea is curved more in one direction than in another. This abnormality may result in vision that resembles looking into a distorted, wavy mirror. Usually, astigmatism causes blurred vision at all distances.

A person with astigmatic refractive error sees lines of a particular orientation less clearly than lines at right angles to them. This defect can be corrected by refracting light more in one meridian than the other. Cylindrical lenses serve this purpose.

How does ‘cylinder’ relate to astigmatism?

Cylindrical errors occur when the shape of the eye across the meridian (up and down, like longitude) causes the cornea to have a more cylindrical or football shape instead of a round one. The angle between that meridian and the horizontal is known as the axis of the cylinder.

How are refractive errors diagnosed?

An eye doctor can diagnose a refractive error during a comprehensive eye examination, and it is usually measured by your ability to identify letters or numbers on a standardized eye chart from a specific viewing distance.

How are refractive errors treated?

Refractive disorders are commonly treated using corrective lenses, such as eyeglasses or contact lenses. LASIK refractive surgery can also be used to permanently change the shape of the cornea and correct some refractive disorders. Presbyopia, in the absence of any other refractive error, can often be treated with over-the-counter reading glasses. There is no way to slow down or reverse presbyopia.

Eyeglasses are the easiest and safest method of correction. Your eye doctor can prescribe appropriate lenses to give you optimal vision.
Contact lenses cause a more precise refraction or focus by becoming the first refractive surface for light rays entering the eye. In many cases, contact lenses provide clearer vision, a wider field of vision, and freedom from glasses. When fitted and used correctly, they are a safe and effective option.
LASIK refractive surgery permanently alters the shape of the cornea. This change in eye shape restores the focusing power of the eye by allowing light rays to focus precisely on the retina, resulting in improved vision.

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