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  1. May 28, 2007 #1
    I was thinking about the eye one time, and the question arose to me of why it is that the eye needs to refocus its lens to see things at different distances. This would seem to imply that the light coming from an object 3 feet away is somehow different from the light coming from an object 30 feet away. Why would this be? Sorry if this is a noob question, but i haven't seen an explanation anywhere.
  2. jcsd
  3. May 28, 2007 #2


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    Staff: Mentor

    Lenses (any lenses) need to make the light rays come to a point to focus. When the distance from the object changes, the angle between the rays of light changes, so the focus must change to bring the rays back to a point.

    http://en.wikipedia.org/wiki/Lens_(optics [Broken])
    Last edited by a moderator: May 2, 2017
  4. May 28, 2007 #3


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    The light sensors in your eyes are the retinas, and to see clearly, your lenses need to be able to focus the light from objects near and far on those sensors. The light from 3' and 30' is not different in any way - the need to refocus is a matter of optics and engineering. Think of a camera lens as an analogue for the human eye. If you have plenty of light, you can use a small aperture, and objects over a great range of distances can be in focus. If you have to work with little light, the aperture of the lens has to be quite large to allow enough light to the film/sensor to record an image, but the resultant depth-of-field will be very shallow. Only objects in a narrow range of distances will be in focus. To prove this to yourself, on a sunny day, tightly curl your index finder and thumb and peek through the little gap in the center. Although diffraction will blur the image, you will find that things very close and very far are all in reasonably sharp focus.
  5. May 28, 2007 #4


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    The light rays from an object go through multiple paths in a lens. The paths in the middle involve very little bending, while the paths on the outer edges of the lense involve significant bending. All of these paths need to rejoin at the back of the retina for proper "focus". When they don't converge to a point, the result is that the rays are scattered on the retina, causing the image to be blurry. As mentioned, under bright conditions, where only the center of the lens is used, the smaller amount of bending of the rays of light allows a larger range of reasonably focused objects.
  6. May 28, 2007 #5
    Thanks for your comments, I think I finally get it now!
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