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Thread: “Physiology of the Eye” — a return to first principles

  1. #21

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    Re: “Physiology of the Eye” — a return to first principles

    Quote Originally Posted by ic-racer View Post
    The fovea has a pretty limited angle of view, so your eye has to scan around quite a bit to make any sense of the world. The lens totally sucks when used wide open. The chromatic aberrations are pretty severe.
    Probably makes a great soft focus lens and explains why we always think our Number one squeeze actually is attractive.
    Regards
    bill

  2. #22
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    Re: “Physiology of the Eye” — a return to first principles

    Quote Originally Posted by ic-racer View Post
    The fovea has a pretty limited angle of view, so your eye has to scan around quite a bit to make any sense of the world. The lens totally sucks when used wide open. The chromatic aberrations are pretty severe.
    Yes they are (the chromatic aberrations) that is. And the refractive index of the vitreous humour varies with the wavelength of light rays passing through. This can be quite a problem when one is viewing a field with significant amounts of both red and blue which are at opposite ends of the visual spectrum.

    I've seen some professional (powerpoint) presentations done with the authors using red letters on a blue background and vice versa. Very hard to focus on their slides. Bob G.
    All natural images are analog. But the retina converts them to digital on their way to the brain.

  3. #23
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    Re: “Physiology of the Eye” — a return to first principles

    Quote Originally Posted by Leonard Evens View Post
    It is in fact impossible to see anything in the normal sense without your eye scanning the scene. An interesting experiment involves using some sort of apparatus to fix an object so that it moves with your eye. After a short while the object disappears from view.
    Yes the disappearance is quite interesting indeed. I've heard about the experiment but not participated in it.

    Nystagmus and saccadic eye movements are necessary because of the long half life of cis-retinal. The conversion of trans-retinal to the cis- isomer occurs almost instantaneously (in femtoseconds) when it is struck by photons. And this conversion is what initiates a signal in the attached nerve fiber to the brain. But the process of getting back to the trans- state takes much longer when the photons stop coming.

    So if one could keep the eye perfectly still with the same image constantly on the same part of the retina... all the receptors constantly receiving photon stimuli would become saturated (i.e., all their trans-retinal would become converted to cis-retinal) and those receptors would cease sending signals to the brain.

    Bob G.
    All natural images are analog. But the retina converts them to digital on their way to the brain.

  4. #24
    Land-Scapegrace Heroique's Avatar
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    Re: “Physiology of the Eye” — a return to first principles

    Quote Originally Posted by Leonard Evens View Post
    ...fix an object so that it moves with your eye. After a short while the object disappears from view.
    Quote Originally Posted by rguinter View Post
    Yes the disappearance is quite interesting indeed. ...The receptors...would cease sending signals to the brain.
    When this happens, what exactly fills-in the place of the “vanished” object?

    Would it look like the “blind spot” we all have on the retina?

    This is beginning to sound like magicians and rabbits.

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    Re: “Physiology of the Eye” — a return to first principles

    Quote Originally Posted by paulr View Post
    This is a way in which view cameras make photography easier. When you look at the ground glass, you perceive it as a picture that's already been flattened to a 2-dimensional projection.

    For some reason when you look through a viewfinder (even though the viewfinder may actually be based on a ground glass, like in an slr) the sense of looking through a window at a 3-dimensional world persists.
    I'm starting to understand that. I think the important difference is looking at the flattened image with 2 eyes is more informative than looking at the flattened image through one eye.

    ...Mike

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    Re: “Physiology of the Eye” — a return to first principles

    Quote Originally Posted by Heroique View Post
    When this happens, what exactly fills-in the place of the “vanished” object?

    Would it look like the “blind spot” we all have on the retina?

    This is beginning to sound like magicians and rabbits.
    My guess is yes it probably would.

    The blind spot is a true "blind" spot for vision. It is the off-center position on the back of the retina where the optic nerve is attached.

    There are plenty of text books with diagrams that can be used for demonstrating the blind spot. A google search will pick some up. Me I used a picture of an orange and an apple when I ran seminars in human-factors. While focusing on the orange with one eye open and moving the picture around a bit one sees the apple disappear.

    While doing this one can actually become aware of what the brain does constantly... it overlays adjacent visual information to fill in the spot. Much the way a "clone stamp" tool works in photoshop.

    With this process we are never aware of the blind spot except for those few moments when doing the demonstration I described above.

    Cheers. Bob G.
    All natural images are analog. But the retina converts them to digital on their way to the brain.

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