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Refraction

Refraction is the part of a comprehensive ocular examination that evaluates the optical properties of the eye. The primary purpose of a refraction is to determine whether glasses are needed to improve vision. However, a refraction also is useful to evaluate the status of other diseases that may be affecting vision such as cataract, macular degeneration or glaucoma. A decrease in vision may be due to the need for a change in glasses but it also may indicate a worsening of a disease process within the eye.

Optically the eye can be compared to a camera in which the iris (and pupil) resemble the shutter of the camera, the lens is the focusing device in both and the retina in the back of the eye is the recording film. The cornea, the clear front portion of the eye, actually is the chief refracting surface of the eye.

Parallel light rays entering the eye will be brought to a point focus on the retina by the refracting power of the cornea and the lens. Variations in the curvature of these structures, along with the length of the eyeball, determine the eyes ability to focus light rays on the retina. Heredity is one factor that influences the optical properties of the eye, hence the refraction.

Some of the common refractive categories are:

Myopia (Nearsightedness)

In this refractive condition, parallel light rays entering the eye are brought to a point focus in front of the retina. This is the result of a greater than normal converging power of the refractive surfaces (cornea and lens) and/or a longer than normal eyeball. These individuals have decreased distance vision but can read without glasses, often into their advanced years. Myopia tends to progress in the teen years.

Vision in a nearsighted person is improved by a concave shaped lens in the form of glasses or contact lenses. However, the progression of the myopic will not be altered significantly by wearing (or not wearing) the glasses.

Hyperopia (farsightedness)

Parallel light rays in the hyperopic eye theoretically are brought to an imaginary point focus behind the retina. The eyeball is shorter than normal or the refracting surfaces lack sufficient converging power to bring these rays to focus on the retina. However, a third factor called accommodation influences vision in the farsighted eye. Accommodation is the ability of the lens within the eye to change shape, become more convex, increasing the refractive power. Just like a camera, the ability of the lens to change shape permits the eye to see clear in the distance, and then change focus (become more convex) in order to read. A farsighted eye must accommodate all the time in order to see clearly, both at distance and at near. This is not a problem early in life as the eye has remarkable accommodate power. In the third and fourth decades this ability to change focus decreases. Initially, this is manifested by difficulty with reading at an earlier (than normal) age but subsequent distance vision also is affected. In this situation a bifocal correction becomes “the lesser of the evils”.

Astigmatism

This term is derived from the Greek language and means “without a point”. The refracting element of the eye (usually the cornea) is not the same in all meridians resulting in a configuration that is not spherical. In other words, the cornea is egg shaped as opposed to a perfectly round sphere like a golf ball. Hence, parallel light rays entering the eye will have two focal points instead of one, resulting in a blurred image. Astigmatism is corrected by a cylinder lens, which optically collapses the two points into one.

Presbyopia

This condition develops when the accommodative ability of the lens to change shape (become more convex) is no longer adequate for near visual demands. In the average eye, this will occur between 42-45 years. It will occur earlier in the farsighted eye (because more accommodation is necessary) and later in the under corrected nearsighted eye. If vision is good at distance, then the convex (converging) power can be placed in a full reading lens. However, this will blur distance vision. The “half glass” is another option in this situation. However if correction is needed for distance vision, then the bifocal is generally preferred as opposed to two pair of glasses.