How can defective vision be corrected

Without correction, myopic people see objects that are far away unfocused. Close-up, they not only see well, they even see better than people without defective vision under the same conditions. With short-sightedness, the eyeball is often too long; that is why the rays of light already meet in front of the retina. When images seen from a distance reach the retina, they appear unfocused. Short-sightedness can have a variety of causes. The most frequent is axial myopia which is caused by the eyeball being too long.

Axial myopia is normally inherited. Refractive myopia is less common and is caused by excessive refraction of the cornea or the lens. This is the case when the cornea or the lens has increased curvature or if the core of the lens is opaque a cataract, for instance, can be responsible for the latter.

Without correction, hyperopic people see objects far away focused. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Astigmatism: This defect is when the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea.

A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred.

Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis. Eye related problems:. Cataracts: A cataract is a clouding of the lens, which prevents a clear, sharp image being produced. A cataract forms because the lens is sealed in a capsule and as old cells die they get trapped in the capsule, with time this causes a clouding over of the lens.

This clouding results in blurred images. The top layer of the cornea is surgically peeled back and the underlying tissue ablated by multiple bursts of finely controlled ultraviolet radiation produced by an excimer laser. Lasers are used because they not only produce well-focused intense light, but they also emit very pure wavelength electromagnetic radiation that can be controlled more accurately than mixed wavelength light.

The nm wavelength UV commonly used is extremely and strongly absorbed by corneal tissue, allowing precise evaporation of very thin layers.

A computer controlled program applies more bursts, usually at a rate of 10 per second, to the areas that require deeper removal. Typically a spot less than 1 mm in diameter and about 0. The corneal flap is replaced; healing takes place rapidly and is nearly painless. Skip to main content. Vision and Optical Instruments. Search for:. Vision Correction Learning Objectives By the end of this section, you will be able to: Identify and discuss common vision defects.

Explain nearsightedness and farsightedness corrections. Explain laser vision correction. Example 1. Correcting Nearsightedness What power of spectacle lens is needed to correct the vision of a nearsighted person whose far point is Strategy You want this nearsighted person to be able to see very distant objects clearly. Discussion The negative power indicates a diverging or concave lens, as expected.

Example 2. Correcting Farsightedness What power of spectacle lens is needed to allow a farsighted person, whose near point is 1. Strategy When an object is held Conceptual Questions It has become common to replace the cataract-clouded lens of the eye with an internal lens. This intraocular lens can be chosen so that the person has perfect distant vision.

Will the person be able to read without glasses? If the person was nearsighted, is the power of the intraocular lens greater or less than the removed lens? If the cornea is to be reshaped this can be done surgically or with contact lenses to correct myopia, should its curvature be made greater or smaller? Also explain how hyperopia can be corrected. If there is a fixed percent uncertainty in LASIK reshaping of the cornea, why would you expect those people with the greatest correction to have a poorer chance of normal distant vision after the procedure?

A person with presbyopia has lost some or all of the ability to accommodate the power of the eye. What is the near point of a person whose eyes have an accommodated power of What is the range of diopters for spectacle lenses that this person might need after LASIK procedure?

How do you know? What was the previous far point of a patient who had laser vision correction that reduced the power of her eye by 7. A severely myopic patient has a far point of 5. By how many diopters should the power of his eye be reduced in laser vision correction to obtain normal distant vision for him?

How far is the board from his eyes? How far from her eyes is the feature being examined? A young woman with normal distant vision has a What is the closest object she can see clearly? The far point of a myopic administrator is A very myopic man has a far point of What power contact lens when on the eye will correct his distant vision? Repeat the previous problem for eyeglasses held 1.

What is her far point? Repeat the previous problem for glasses that are 1. The contact lens prescription for a mildly farsighted person is 0.