The easiest way to think about 20/20 is that it means that the eye can see at 20 feet what the average, healthy eye can see at 20 feet. It would follow that 20/200 (poor) vision means that the eye can see at 20 feet what an average, healthy eye can see at 200 feet, or that an eye that sees 20/10 (exceptionally well) sees at 20 feet what an average, healthy eye can see at 10 feet.
Having 20/20 vision does not necessarily mean that your eyes are healthy. The presence of glaucoma or many other eye conditions may not affect central visual acuity until late in the disease. Healthy eyes, though, usually do see 20/20 or better.
If an eye does not see 20/20, it is up to the ophthalmologist to determine the cause. It is also important to test each eye individually.
Did you also know? The standard eye chart used by most doctors today is known as the Snellen Eye Chart, developed by Dutch ophthalmologist Herman Snellen in 1862.
image: wikimedia commons
Did you know? Patients frequently ask why it's necessary to dilate the pupils during a complete eye examination. It's simple, really. Imagine the difference between looking into a room through a keyhole, and looking in through a large window. If you really want to get a good view inside the room, the large window gives a much more complete view.
Similarly, dilating the pupil enables the ophthalmologist to view the entire back of the eye. What are we looking for? There are hundreds of things that can go wrong inside the eye, (many of which have no symptoms). We're checking to see if everything is normal. We look at the optic nerve, the blood vessels, and the retina, including the central retina, called the macula. If you have a specific complaint, like floaters, or a family history of glaucoma, or blurred central vision, obviously we will pay particular attention to the part of the eye which is most likely involved.
Here is a diagram which shows the advantage of a dilated pupil. The diagram is incorrect, though. With a dilated pupil, and the proper equipment and technique, the entire back of the eye can be examined (including the periphery), not just the part shown in the diagram.
(courtesy of the National Eye Institute)
Did you also know? In 1823, a Dr. Purkinje was the first to ever look inside a living human eye, when he devised the optics to do so. It was not until 1850 that another ophthalmologist, Hermann von Helmholtz, invented the ophthalmoscope independently and publicized his device that other doctors finally were able to look inside the eye and more accurately diagnose its conditions. Read more about the history of the ophthalmoscope here.
Did you know? Until 1905 there was no objective way to measure pressure within the eye. In that year, Norwegian ophthalmologist Hjalmar Schiotz invented the first tonometer, which now bears his name, the Schiotz tonometer.
Until this time, doctors estimated pressure within the eye by pushing on the eye through the eyelid, and guessing, much like you might push on a bicycle tire with your thumb and guess the pressure. The Schiotz tonometer actually works in much the same way, it measures the weight necessary to indent the cornea a standard amount.
Knowing the pressure more accurately allows us to better diagnose and treat glaucoma, a disease usually caused by elevated eye pressure damaging the optic nerve.
The Schiotz tonometer is still used in many parts of the world, and when I trained, it was what most doctors used as a portable tonometer.
Did you also know? Dr. Schiotz was also responsible for introducing the concept of sterilizing surgical instruments (by boiling) to Norway.
Did you know? There is a story about Abraham Lincoln seeing his ghost while looking into a mirror from his bed, shortly after being elected in 1860. By studying numerous photographs and portraits of Lincoln, ophthalmologists have determined that Lincoln had a weakness of a muscle in his left eye (the trochlea), which made the eye unable to look downward in certain positions, including when trying to look into a mirror from a supine position. This would explain his double vision during the incident. Others have ascribed this story to a mirror having two reflecting surfaces, but I find that less convincing.
Did you also know? Physicians say there’s a 50/50 chance that Lincoln had Marfan’s Syndrome, which causes elongation of the hands and jaw during adulthood, due to an excess of human growth hormone. (Marfan's Syndrome can also can dislocation of the lens within the eye).
photo courtesy wikimedia commons
Did you know? Benjamin Franklin (1706-1790) really did invent bifocals! Franklin was a prolific inventor. He grew tired of switching from distance glasses to reading glasses, and suggested to his optician that each lens be cut in half and the distance lens be placed atop the near lens. Thus, bifocals were born! A detailed summary of Franklin's invention exists here.
image: wikipedia public domain photos
Did you also know? My favorite Franklin invention is the glass harmonica. It consisted of crystal glasses arranged in a manner that they can be rotated and "played" by a wet finger. Mozart even wrote a concerto for glass harmonica! Here's a picture of Franklin playing the glass harmonica, wearing his bifocals.
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