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Understanding the Eye Chart


We all know why we need to wear sunglasses and sunscreen in the summer. Winter, however, can be deceiving. It's an illusion to assume that we are safe from sunburns during the colder season.

Snow acts as a powerful mirror for sunlight and magnifies the effects of UV rays which would otherwise be absorbed by the ground. As a result, the eyes are exposed to both the UV radiation bouncing back from the snowy carpet and the rays shining down directly from the sun.

If your family is skiing or snowboarding up in the mountains, you need to be even more careful! UV rays are more powerful at higher altitudes. Another important factor to remember is that ultraviolet radiation penetrates through clouds, so even if the sun is hidden behind them, it can still damage your eyes.

Prevent overexposure to sunlight by wearing sunglasses that absorb at least 95% of ultraviolet radiation when you go outside, no matter what time of year it is. Even though you want to look great in your shades, the most important part of choosing sunglasses is making sure they provide adequate protection against UV. Make sure the lenses are 100% UV blocking by looking for an indication that they block all light up to 400 nanometers - UV400. The good news is you don't necessarily have to pay more for full coverage for your eyes. Dozens of reasonably priced options exist that still provide total ultraviolet protection.

Another important factor in selecting sun wear is the size of the lenses. You want to make sure your glasses cover as much of the area around your eyes as possible. The more coverage you have, the less harmful radiation will be able to penetrate. Lenses that wrap around the temples will also prevent UV waves from entering from the sides.

If you like to ski or frolic in the snowy hills, you should be aware that the sun's rays are stronger at higher elevations, so you need to be especially careful to keep your eyes shaded on the slopes. In addition to sunglasses, it's a good idea to put on a wide brimmed hat that covers your eyes.

Make a point to be knowledgeable about proper eye protection throughout the year. Don't forget to wear your sunglasses.

Eye charts of different variations have become a standard in vision screenings and eye exams. One of the most familiar charts associated with vision is the Snellen eye chart, designed by Dutch ophthalmologist Hermann Snellen in 1862 to measure visual acuity- how well you can see at various distances.

Although there are variations of the Snellen chart used today, a traditional Snellen chart has eleven lines of block letters. The first line has one very large letter, which is one of several letters, for example E, H, or N. The following rows have increasing numbers of letters that become smaller in size as you read from the top to the bottom of the chart. The letters used on the chart are C, D, E, F, L, N, O, P, T, and Z.

When taking a vision exam, one eye is covered and you are asked to read the letters of each row aloud beginning at the top of the chart. The smallest row that you can read correctly indicates the visual acuity in the eye being tested.

The chart is positioned at a distance of 20 feet in the United States or 6 meters in the rest of the world. The term 20/20 vision is used to indicate the clarity and sharpness of your vision measured at a distance of 20 feet. If you have 20/20 vision, you can see clearly at 20 feet objects that can normally be seen at that distance. If you have 20/40 vision, it means that you need to be as close as 20 feet to see what a person with normal vision can see at 40 feet. The largest letter on an eye chart often represents an acuity of 20/200 which is associated with the term "legally blind."

You will be asked to read the letters one eye at a time. Some people can see well at a distance, but are unable to bring nearer objects into focus, while others can see items that are close, but cannot see them far away. By having you read the chart, your eye doctor is able to ascertain whether you have difficulty with distance vision and can determine which corrective lenses can be used to improve it. Near vision problems or other vision and eye health issues may not be detected with the Snellen eye chart alone, so a comprehensive eye exam is always recommended.

The next time you hop into the chair at your optometrists' office, you'll be able to understand why you have to read the letters on the chart in front of you and what the results mean for your vision.