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What is Visual Acuity? What do all those numbers mean?

With every visit to the eye doctor, most of us want to know how well we see. People with 'normal' vision grow accustomed to being told they see 20/20, and this stays the same from one exam to the next, unless some eye disease occurs. A lot of confusion exists, however, as to what these numbers mean.

A standardized eye chart is constructed so that the 20/20 line subtends 5 seconds of a degree angle, when measured from the center of the eye. If the person moves closer to the chart, that angle increases and the letter appears larger, changing the reading. If the person moves farther away, that angle decreases and the letter appears smaller, also changing the reading, this is why using a set distance is important. That set distance is 20 feet (or for those in the UK, 6 meters). The 20 foot letters are about the average that someone with perfect vision would see. The notation 20/20, therefore, means that at a distance of 20 feet, the person reads the 20-foot letter.

Logically, a 40-foot letter is twice the size of a 20-foot letter (subtending 10 seconds of a degree angle). So if a person can read this, they see 20/40, and so on with all the other letter sizes.

Understanding where the notation comes from is nice, but exactly what do all those numbers mean for those with low vision, and why do the numbers often vary from one exam to the next? In the field of low vision, the numbers mean very little. This is because subtle changes in light, the patient's mood that day, blur interpretation, and other difficult to control variables impact the readings. They do have certain uses, however.

They give a ballpark idea for the doctor to monitor for change. A small change, perhaps one line or two, may not be significant, as that change could very well be different at the next visit, but a more substantial change definitely warrants investigation.

They give those who don't have vision impairment a gauge to approximate what their loved one sees.

They are used to determine eligibility for state (or provincial) and federal aid.

With regard to low vision, the numbers may be very misleading, as they do not relate to functional vision. Those with vision impairment often learn adept blur interpretation, that is they may not be able to visually resolve the letters on a sign, but by knowing the context of the situation as well as the general shape of the word, they know what it reads. This skill tends to be more advanced in those with congenital or longstanding vision loss than those who have just recently lost vision (and those with 'normal' vision who take their glasses off). In this respect, the functional vision of someone with low vision may be 20/200, but in real life situations they may actually perceive much more than that number insinuates. On the flip side of this coin, some with low vision function as nearly completely blind with this identical visual acuity. The visual skills and blur interpretation of these individuals is significantly lower.

J. Hensil