Seeing Clearly – Visual Acuity

Near Acuity - the eye’s ability to distinguish an object’s shape and details clearly at near distances – usually 16 inches (40 cm).

Distance Acuity - the eye’s ability to distinguish an object’s shape and details clearly at a far distances – usually 20 feet (6 meters).

Snellen Chart

“20/20” – is the measurement of the sharpness of sight. The first number is the distance from the eye chart, which is usually 20 feet. The bottom number is the size of the letter (a bigger measurement means poorer vision, because you can’t see small detail clearly). You can also think of the bottom number as the distance at which a person with normal eyesight could read the line. For example, if you see 20/40, that means that the line you correctly read at 20 feet could be read by a person with normal vision at 40 feet – twice as far away!

The Snellen chart, which consists of letters, numbers, or symbols, is used to test visual acuity (sharpness of eyesight) in older children and adults. Teller Acuity testing, which has stripes of different sizes, can be used to check the visual clarity of infants.

We are not born with 20/20 vision. We have to experience detail to develop sharp eyesight.

Normal Visual Acuity Development is expected to be as follows:

• 4 – 6 months: 20/300 or better
• 6 – 12 months: 20/150 or better
• 12 – 24 months: 20/100 or better
• 30 – 35 months: 20/63 or better
• 36 – 47 months: 20/50 or better
• 48 – 59 months: 20/40 or better
• 60 – 72 months: 20/32 or better
• 6 years & up: 20/20

What if you can’t see 20/20 clarity? A refraction test is used to determine the amount of prescription to put in glasses or contact lenses to correct for refractive errors such as astigmatism, myopia (nearsightedness), or hyperopia (farsightedness).

Astigmatism

astigmatism

Sometimes people mistakenly call this “stigmatism”. Astigmatism is caused by an abnormally shaped cornea (front surface of the eye) or lens, and can cause blurred or distorted vision far away and up close. You can think of the surface of the eye being the shape of a football instead of a basketball. To see clearly, you need a stronger power of lens in one direction and a weaker power of lens in another direction. This condition is corrected by a cylindrical (“cyl”) eyeglass or (“toric”) contact lens. Astigmatism often causes squinting, eyestrain, headaches, and fatigue and affects 28% of school children.

For more information: http://www.aoa.org/Astigmatism.xml

Myopia

myopia (nearsightedness)

This is the medical term for “nearsighted or short sighted”. Nearsighted eyes are too long, so the eye is unable to focus clearly far away. It is easier for someone who has myopia to see up close (therefore the eye is “nearsighted”). As children grow, their eyes grow longer, and the amount of myopia increases. This is called “myopia progression” and can make it difficult to see the board at school. Nearsightedness runs in families and is common in premature babies. Myopia is corrected with a minus lens and affects about one-third of Americans.

For more information: http://www.aoa.org/myopia.xml

Hyperopia

hyperopia (farsightedness)

This is the medical term for “farsighted or long sighted.” Small amounts of farsightedness are normal in children. As children grow, their too-short eyes grow longer, and the amount of farsightedness can decrease. However, large amounts of farsightedness (+3.00 or more) are NOT outgrown and need to be corrected with a plus lens. This condition is rarely screened for at the pediatrician’s office or at school, even though it can easily be detected during an eye examination. A hyperopic eye has to focus more than normal to see clearly, especially at near distances, so it is easier to focus at far distances (therefore the eye is “farsighted”). If the eyes have a high amount of farsightedness, or different amounts of farsightedness, it is very common for vision to be very poor (amblyopia). Farsightedness can cause headaches, eyestrain, and trouble reading and occurs in about 25% of the US Population.

For more information: http://www.aoa.org/hyperopia.xml