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Medical Forum / General / Vision / July 2006How big is our normal blind spot?
How big is it if you look at things from, let's say, a 5 yard distance? > How big is it if you look at things from, let's say, a 5 yard distance? About as big as your hand. -MT Close your eyes and it's huge. :) > About as big as your hand. Sorry. I just checked and mine is about as wide as my hand, at five _feet_. Three hands at 5 yards. -MT >> About as big as your hand. > [quoted text clipped - 4 lines] > > -MT Exactly how did you do that? Don W. > Exactly how did you do that? I picked a bright-printed title on a book in the bookcase about five feet away. I counted books to the right of the bright spot until the spot disappeared, then I kept counting until the bright spot reappeared. The subtended set of books was a little wider than my hand. -MT > How big is it if you look at things from, let's say, a 5 yard distance? You really should be talking about subtended angles. The blind spot is 5 degrees wide and 7 degrees high according to Bennett and Rabbetts. An old astronomer's trick for measuring angles is the width of a closed fist at arm's length is 10 degrees. So my question is, how are you measuring the blind spot? Don W. > astronomer's trick for measuring angles is the width of a closed fist at > arm's length is 10 degrees. In my same setting, a closed fist covers about two hands' worth of books. -MT >> astronomer's trick for measuring angles is the width of a closed fist at >> arm's length is 10 degrees. > > In my same setting, a closed fist covers about two hands' worth of books. > > -MT If I take the conventional 3" x 5" card and put a "dot" and a "+" on it (couple of inches apart) to check the blind spot, I get the "+" to disappear at about 8 inches looking at the "dot" . Now if I just run a vertical line thru the "+" to cover the entire card, I can get the "+" to disappear, but the line stays intact continuous, but no horizontal part of the "+"). Interesting computational fill in. So if you are looking at a 10" book at 5 feet, that covers 9 degrees, which exceeds the blind spot dimension. So I would say it was lucky your book disappeared. Don W. |
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