To test the clarity of one’s vision, experts usually use something known as a Snellen chart and you’ve probably seen it at your local eye doctor’s office. This chart is the most commonly used method of measuring how well you can see at what distance. Snellen charts are named after the Dutch ophthalmologist Hermann Snellen who developed the chart in 1862. Scientists now use a variation of this chart, designed by Ian Bailey and Jan Lovie.
The traditional Snellen chart is printed with eleven lines of block letters. The first line has one very large letter, which may be one of several letters, for example E, H, or N. All the rows that follow have increasing numbers of letters that decrease in size. A person taking the test covers one eye (or the doctor covers one eye), and then reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the visual acuity in that eye.
The symbols on the chart are formally known as “optotypes” (standard symbols for testing vision). In the case of the traditional Snellen chart, the optotypes appear as block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer’s font. They have a particular, simple geometry whereby the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter “C.” In addition, the height and width of the optotype is five times the thickness of the line. Only the letters C, D, E, F, L, N, O, P, T, and Z are used in the traditional Snellen chart.
The smaller letters on the chart are designed to give an accurate measure of how good your vision is. If you notice the big letter this is an excellent sign, because it implies that your overall eye vision is nice and healthy. But the smaller letters are what are used to give an even more accurate picture of how well your eyes are. If you wish to read well in addition to see signs from a far distance then you will need to be in a position to read small letters.
The largest letter on an eye chart often represents an acuity of 20/200 (6/60), the value that is considered “legally blind.” Some individuals with moderate myopia may not be able to read the large E without glasses, but have no problem reading the 20/20 line (line which is supposed to represent a person’s perfect visual ability without eyeglasses at a distance of 20 feet) with glasses. In contrast, people who are legally blind have a visual acuity of 20/200 (6/60) or less when using the best prescription eyeglasses.
When you are exploring the Snellen chart you should not strain your eyes to determine the letters. If you need to strain your eyes to see the letters then chances are how well you see may not be just like you may like it to be.
If you can see all of the letters on the Snellen chart clearly then you can leave your doctor’s office without any problems BUT if you have to struggle to see any of the letters, then you’ll probably end up needing prescription glasses which are made very specifically according to your latest eyeglasses prescription. With these prescription lenses fitted into fashionable eyeglasses frames, you will not only be able to see clearly, but will also be adding a new fashion accessory to your already trendy style.