A Snellen eye chart is used to determine how "normal" your vision is. It sets a standard for what most people should be able to see when they stand 20 feet away from the chart. 20/20 vision just means that when you stand 20 feet away from a Snellen eye chart, you see what a normal human being can see. If you see 20/40, that means that when you stand 20 feet away from the chart, you see what a person with "normal" vision sees when standing 40 feet away from it. The higher the second number, the worse your vision is. 20/200 (you see at 20 feet what a person with "normal" vision sees at 200) is the number for legal blindness in the United States.
20/20 vision isn't perfect, it's just "normal." You can have better vision than 20/20. If you have 20/10 you see at 20 feet what most people see at 10. Some animals, like hawks, might have 20/2 vision!
You can use our Snellen eye chart* to compare vision within your family or with your friends. (This will only give you an approximate idea of your vision. Your optometrist has much more precise tools to find out exactly how well you can see.) Each line of the chart is labeled on the left side. The second to last line is 20/20.
Tape the eye chart to a wall, making sure it is in plenty of light. Stand twenty feet away from the chart and begin reading each line. Have a family member or friend watch to see that you are reading each letter correctly. The last line that you are able to read will give you an approximate idea of your vision. If you can read the very bottom line, your vision is 20/10! Now try covering one eye and just testing the other one. Is one eye better than the other?
Have all of your family members try reading the chart. Do some of you have better vision than others? If you wear glasses, what is your vision with them on and what is it without them?
*Instructions for downloading: The Snellen Eye Chart PDF is 11" x 17", so to print correctly you will need to set your print options to "tile." Printer choices will vary, but you should do something similar to this. Open the PDF and choose Print. Under the page scaling options, select "tile all pages." This should print the chart on four sheets of paper. You will need to trim the edges so the pieces match up, and then tape or glue them together. (You can also order an already-printed 11" x 17" copy of our Snellen Eye Chart.)