As an individual with perfectly adequate vision, I’ve been fascinated with the idea of how people with astigmatism, long- and short-sightedness, and color-blindness view the world. This fascination is by no means unique, in fact it’s almost cliché. Many of the people in my class ‘Structure and Function of the Nervous System’ were very excited about our lab on vision this week because it included sections on how those with regular vision can try to perceive the world with astigmatism or color-blindness.
The most exciting part of the lab was the second or so when I saw the world as a red-green colorblind individual does. I "bleached" my vision by holding a piece of green transparency film over a light and staring into that light for 20 seconds then immediately looking at a image that test for colorblindness, such as this image:
The next stop in lab was a table with glasses correcting astigmatism. The simplest way to test for astigmatism is to look at the following image:
An individual with astigmatism cannot sharply focus all the lines in the above fan simultaneously; while all the lines are of equal thickness an individual with astigmatism will perceive the lines as having varying thickness. At first I didn’t believe that, but then I put on glasses that correct for astigmatism and suddenly "I’m a believer!" In your average person the cornea is radially symmetric, which allows the light to be focused at one point. But when the cornea’s radial symmetry is distorted to a shape more closely resembling a football (that’s an American football, of course) the light in the two different planes are curved into the eye to different degrees, so they don’t meet tightly at one point which leads to that lack of sharp focus.
The most shocking section of lab was a special meter stick for measuring the distance from the eye that one can focus an image. We had to hold the meter stick to our cheek bone, hold a piece of paper above the stick and move it to the closest and furthest point on the stick that we could resolve the writing. I can read images as close as 10 cm from my eye and furthest is much farther than a meter. I learned that this is normal from my age as along one edge of the meter stick were centimeter marks while the other edge listed ages. There is, on average, a correlation between age and the distance from the eye one can resolve an image. For your average 20 year old the closest image you can resolve is about 10 cm from the eye. For younger individuals that distance is less, and for older individuals the distance gets larger and larger with age! It looks like I’ll need reading glasses at 40 or so…