This image is part of the standard test for color blindness. Individuals with normal color vision will see the number 57, while those with red-green deficiencies will see the number 35. Color blindness, an inability to distinguish between red and green and sometimes between blue and yellow, is caused by a defect in one of the three color-sensitive cells in the retina. Color blindness affects approximately one person in thirty.

 Eye examinations are performed by an ophthalmologist, a medical physician trained to diagnose and treat eye disorders, or an optometrist, an eye-care specialist trained to examine the eye and prescribe eyeglasses or contact lenses.

 These vision tests measure visual acuity using the Snellen chart. Refraction tests determine if a patient has distorted vision that can be corrected with eyeglasses or contact lenses, such as nearsightedness, farsightedness, and astigmatism. In visual field tests, which indicate problems with peripheral vision, one eye is covered while the other eye is directed to a point straight ahead. Lights are projected onto a screen at various locations in the periphery of vision and the person indicates when a light is seen.

 Children often do not know that they have faulty vision. Parents may notice that their child does not seem to see distant objects clearly or that an eye appears crossed. An eye examination can uncover the cause of the problem. The child may only need corrective eyeglasses to see things far away, such as the blackboard in school. Crossed eyes, known as strabismus, may be corrected with eyeglasses. Other children with this condition may be given eye exercises to strengthen weak eye muscles. They may wear a patch over the eye with better vision to force the weaker eye to see well. If exercises and eyeglasses do not correct the problem, a surgeon may operate to strengthen the weak muscles and straighten the crossed eye.

 Another common cause of damaged vision in young people is injury from sharp objects, such as scissors, or head injuries from automobile or bicycle accidents. Eyes can be protected from injuries by the use of safety equipment—such as wearing a helmet for cycling or wearing shatterproof goggles during fast-moving ball games.

 Some vision problems are the result of illnesses, such as high blood pressure or diabetes mellitus. Glaucoma is an eye disease caused by faulty drainage of normal eye fluid from inside the eye. The pressure in the eye slowly rises and over many years may cause damage to the optic nerve, eventually resulting in blindness. Macular degeneration is a serious eye condition that is usually associated with aging. The macula is vital for clear, sharp sight. In people with macular degeneration, deteriorating cells or abnormal blood vessel growth in the macula cause blurred vision in the central area of focus. Vision loss associated with macular degeneration cannot be corrected with standard eyeglasses or contact lenses.