COMMON MYTHS

MYTH :: The 20/20 letter chart is the best way to test my everyday vision.

FALSE. Many people believe the familiar 20/20 letter chart is the best way to test everyday vision. Most letter charts, however, only gauge a person's ability to detect varying sizes of crisp black letters on a white background -- a high contrast situation. What about low contrast situations? People who have low contrast sensitivity may be able to read the small print on an eye chart but may still experience trouble seeing objects at night or in a dimly lit room.

Contrast sensitivity testing provides a more comprehensive assessment of everyday vision. By using sine-wave gratings with varying degrees of contrast and spatial frequencies, contrast sensitivity charts test the ability of the visual system to distinguish between the light and dark parts of test targets (see Sine-wave Gratings for more details). This testing technique better predicts everyday visual performance as everyday life is full of high contrast situations, low contrast situations and everything in between.

MYTH :: My vision is 20/20. I have perfect vision.

FALSE. 20/20 vision is not equal to perfect vision. When your eye doctor tests your eyes using a standard 20/20 letter chart, your vision is rated based on the distance at which you can correctly identify small black letters on a white background.

How the 20/20 scoring system works:
This method of testing and scoring depicts how well a person can identify black letters on a white background at a certain distance away from the test chart. 20/20 vision, which is considered normal vision, means that a person is able to identify a line of small sized letters at a distance of 20 feet. A score of 20/10 means that a person can identify at 20 feet what a person with "normal vision" must be at 10 feet to identify.

The 20/20 letter chart reveals problems with near and far-sightedness, which is a valuable measure of your vision. It does not, however, paint a complete picture -- our functional vision is much more complex. In seeing everyday objects, our eyes collect several bits of information using various visual channels (see The Visual System and The Channel Model). These visual channels collect information about size, shape, contrast, color and motion. Each channel collects and feeds this information individually to our eye/brain system. The 20/20 letter chart only tests one of these visual channels, therefore, 20/20 vision does not equate with perfect vision. Contrast sensitivity testing that examines the full range of visual channels is needed to complete the picture.

MYTH :: The standard 20/20 letter chart is effective in the early detection of diseases such as cataracts and glaucoma.

POSSIBLY. The standard 20/20 letter chart used by most vision clinics is not effective in the early detection of diseases such as cataracts and glaucoma. However, other commonly performed tests (such as the slit lamp, intraocular pressure testing and visual fields) can detect certain vision abnormalities caused by such diseases. This is good, but now we can do better. Years of research and clinical trials has provided a more comprehensive, noninvasive technique for disease screening -- contrast sensitivity testing.

Contrast sensitivity testing can help in early detection of a wide range of visual problems including amblyobia, cataracts, glaucoma, Alzheimer's, AIDS, macular degeneration, diabetes, and exposure to toxic agents. Incorporating contrast sensitivity testing during routine optometric eye exams would arm clinicians with valuable information about their patient's visual condition. This information can further aid in proper patient care, referrals and treatment regimens.

MYTH :: My 70-year-old grandmother just had her eyes tested for her driver's license. With her glasses, she has 20/40 vision, so her license was renewed. She complains that she has trouble seeing at night, but I guess it is OK for her to continue driving at night because she passed the eye test.

FALSE. Drivers over 65 who pass the 20/20 letter chart test often have age related contrast sensitivity impairments which makes it more difficult to drive at night. The standard 20/20 letter chart tests the eye's ability to depict black letters on a white background at a distance. This test helps in testing of the optics of the eye-- for example, determining a spectacle correction. This test doesn't, however, account for how well one can see the varying contrasts and sizes of objects encountered in everyday life -- in this case, how well grandma can see road signs at dusk or objects in foggy weather.

Our eyes use many different "visual channels" (see The Channel Model) to see varying sizes, colors, contrast, motion and shapes. The effects of aging often impact our visual system, which in turn impacts how well our eyes see and process information about the objects we see everyday. Older people often experience the inability to see in low contrast situations such as driving at night. The standard 20/20 letter chart test is unable to detect this problem because it only tests one visual channel.

 

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