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Medical Why do we have to focus?

  1. Sep 23, 2010 #1
    Visual information arrives via photons of visible light. These, we are assured, all travel at the same speed and therefore have the same impact. Why then, do those that travel a little further require a change in the shape of the receiving lens? In particular, nearby objects at distances of, say, 3 feet and 6 feet, require a considerable change of focus. This cannot be due to binocular vision as monocular vision also has to be focused.
  2. jcsd
  3. Sep 23, 2010 #2


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    Staff: Mentor

    It's just basic optics. See the Lens Equation part-way down this page:


    1/S1 + 1/S2 = 1/f
  4. Sep 23, 2010 #3

    Take a optical lens with refractive index similar to the Eye lens, place an object (on one side of the lens ) at two different distances from the lens . For the image of the object to fall on a fixed point or same point both times on other side of lens, the lens has to moved or curvature of the lens has to be changed.

    But in a eye, the lens and the retina ( part that absorbs light converts it to signals that is transmitted to the brain ) is fixed, it cannot move. so for objects at different distances for the image to fall on the retina (fixed point ) only the curvature of the lens can be changed (since the lens cannot move).
  5. Sep 26, 2010 #4
    No, I think you're missing the point entirely, berkeman. It has to do with information.

    The more information one has at one's disposal, and the more information one has at one's disposal that one is able to process accurately, the greater the liklihood that the organism will be able to survive.

    All things else being equal, of course.

    The maximum visual acuity of a human is thought to be around 20/10. Hawks, however, have a visual acuity of 20/2, about five times better than we humans.

    However, we see better at night than they do, so there's the compromise.

    There's always a compromise... Yes, we've simply reached that evolutionary state whereby advancements in one area require compromise in another! And I'm damned glad for it, as it levels the playing field when it comes to arguments concerning the future of mankind, or, for that matter, all living species.

    So, "why do we focus?" Because visual acuity long ago became a prime factor in our abilities to both procure our game, cultivate our food, and defend ourselves from our enemies.

    Those who'se focal acuity couldn't hack it, died!

    Thankfully, given our brains, we figured out many different sociological and physiological ways to help one another overcome poor eyesight, including glasses as well as the differentian of duties not specific to one's acuity. As a result, they continued to contribute to society, so both they and they're progeny survived.

    Our society is very unique among those throughout most of the animalia kingdom, in that our blind not only survive, but thrive.

    There are nevertheless a few totally blind species of animals which have managed to eak out an existence in the total void of light.

    Here's the difference: Our blind do so at our best wishes and support, not our competition! Our human nature has afforded our blind amble opportunity to make their own way.

    As a personal note, I've worked with the blind for 24 years, off and on. My uncle hiked the Appalachian Trail with the first blind person to do so, and I rejoiced in their achievement!

    So before you throw me off the platform and onto the tracks... Best know where I'm coming from. For goodness sakes, I'm barely, and yet not quote over a moderate corneal abrasion, not to mention the fact I've worked with the blind on multiple occasion, so yes, I understand fulll well what's at stake!

    I am simply not the technical expert, here. I have an idea as to who might be, and will get in touch with him as soon as I can. No promises. Just directions of desire to make things right.
  6. Sep 28, 2010 #5
    Thank you for your replies. Whilst I do realise that we have to focus, my question is why. After all, the light photons have travelled some 90 million miles with no change, and yet two identical objects, at slightly differing distances, viewed viewed outside in the same light, requires the eye to alter focus. What is it that is different about the light photons from the separate objects; ie, why is one image sharp and the other blurred? The eye cannot know that the objects differ in distance, therefore the information received from the objects must differ. How?
  7. Sep 28, 2010 #6


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    If the light from a given object were received through an ideally 0 diameter aperature, then there would be no need to focus.

    This is important, so i'll repeat it. As the aperature of (any) imaging device approaches zero, focusing becomes less and less necessary. (Pinhole cameras need no focus.)

    But a small aperature cuts out a vast portion of the light available from an object. (pinhole cameras need a bright subject).

    Our eyes have an iris that lets in light through a large rarea, thus allowing better resolution etc.

    But the downside to all this extra light is that it does not all arrive at a zero angle from the object. If the lens did not change shape, most of the rays would fall in the wrong place on the retina.

    So, focusing is a compromise between very sharp and very dim imaging.

    An example:

    If I ran around all day with my camera set on F22 (tiny aperature), I would virtually never need to focus at all. This works.

    But, come evening, I'm out of luck. That tiny aperature lets in far too little light to image anything but broad daylight scenes. Now what do I do?

    I widen the aperature. Lots more light come in, but I need a way to correct for the fact that the light rays are coming from a range of angles.
    Last edited: Sep 28, 2010
  8. Sep 29, 2010 #7
    It has to do with ability of the lens to be able to focus light from the object on the retina.
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