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Forum index » Medicine forums » vision
How big is our normal blind spot?
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CGeorge
medicine forum beginner


Joined: 12 Jul 2006
Posts: 3

PostPosted: Wed Jul 12, 2006 7:09 pm    Post subject: How big is our normal blind spot? Reply with quote

How big is it if you look at things from, let's say, a 5 yard distance?
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Mike Tyner
medicine forum Guru


Joined: 24 Mar 2005
Posts: 1299

PostPosted: Thu Jul 13, 2006 12:46 am    Post subject: Re: How big is our normal blind spot? Reply with quote

"CGeorge" <cgeorge@hotmail.com> wrote in message
news:PNbtg.39$yn3.13@fe08.lga...
Quote:
How big is it if you look at things from, let's say, a 5 yard distance?

About as big as your hand.

-MT
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serebel
medicine forum addict


Joined: 12 Aug 2005
Posts: 81

PostPosted: Thu Jul 13, 2006 1:18 am    Post subject: Re: How big is our normal blind spot? Reply with quote

Close your eyes and it's huge. Smile
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Mike Tyner
medicine forum Guru


Joined: 24 Mar 2005
Posts: 1299

PostPosted: Thu Jul 13, 2006 3:13 am    Post subject: Re: How big is our normal blind spot? Reply with quote

"Mike Tyner" <mtyner@mindspring.com> wrote

Quote:
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
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Don W
medicine forum addict


Joined: 26 May 2005
Posts: 90

PostPosted: Thu Jul 13, 2006 11:02 pm    Post subject: Re: How big is our normal blind spot? Reply with quote

"CGeorge" <cgeorge@hotmail.com> wrote in message
news:PNbtg.39$yn3.13@fe08.lga...
Quote:
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.
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Don W
medicine forum addict


Joined: 26 May 2005
Posts: 90

PostPosted: Thu Jul 13, 2006 11:04 pm    Post subject: Re: How big is our normal blind spot? Reply with quote

"Mike Tyner" <mtyner@mindspring.com> wrote in message
news:kJGdnRjDGbziJyjZnZ2dnUVZ_oqdnZ2d@giganews.com...
Quote:

"Mike Tyner" <mtyner@mindspring.com> wrote

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

Exactly how did you do that?

Don W.
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Mike Tyner
medicine forum Guru


Joined: 24 Mar 2005
Posts: 1299

PostPosted: Thu Jul 13, 2006 11:39 pm    Post subject: Re: How big is our normal blind spot? Reply with quote

"Don W" <dwilgus@prodigy.net> wrote
Quote:
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
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Mike Tyner
medicine forum Guru


Joined: 24 Mar 2005
Posts: 1299

PostPosted: Fri Jul 14, 2006 1:17 am    Post subject: Re: How big is our normal blind spot? Reply with quote

"Don W" <dwilgus@prodigy.net> wrote

Quote:
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
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Don W
medicine forum addict


Joined: 26 May 2005
Posts: 90

PostPosted: Fri Jul 14, 2006 2:37 pm    Post subject: Re: How big is our normal blind spot? Reply with quote

"Mike Tyner" <mtyner@mindspring.com> wrote in message
news:As2dnYl4H9MzbSvZnZ2dnUVZ_vqdnZ2d@giganews.com...
Quote:

"Don W" <dwilgus@prodigy.net> wrote

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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Google

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