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Thread: How the eye/brain focusses?

  1. #1 How the eye/brain focusses? 
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    I understand the mechanics of how the eye changes its lens shape to focus, (well there are two conflicting theories anyway), but my question is: how does the brain know when the image is in focus or not?

    Autofocus cameras use phase detection, range measuring or contrast detection to focus. How does the eye/brain do it?

    OB


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    The brain automatically senses the "sharpness" or "crispness" of the boundaries of objects and lines, and it commands the eye to refocus for optimum "sharpness" or "crispness".

    This feedback system has its limitations and seems to have evolved to correct only for "depth" as a camera does. With astigmatism, some sharpness appears at one focus, and other sharpness appears at another focus. The eye continually shifts back and forth between the two, never resolving itself, which causes fatigue of the focusing muscles and the accompanying aches and pains.



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    I am not sure how we focus, but I do know that it is a learned behaviour.

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    Infant vision - Wikipedia, the free encyclopedia
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    Quote Originally Posted by jrmonroe View Post
    The brain automatically senses the "sharpness" or "crispness" of the boundaries of objects and lines, and it commands the eye to refocus for optimum "sharpness" or "crispness".
    .....But how does the brain know if something has a sharp outline but is out of focus, or that it has a feathery outline and is in focus?

    If this is the mechanism, then there must be an element of learning and memory involved with focussing.

    Binocular vision can perform a crude range finding function by sensing how far inward the eyes need to be moved to both superimpose the object - the more inward, the closer the item, but I understand that people with one eye can still focus.

    OB
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    OK.....then...question...

    How does the brain focus when the eye has been modified? i.e. surgery, implants, laser holes, Ahmed implants, cataracts, etc.?

    Need to clarify. I am not speaking Lasik when I am saying laser drill holes.
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    The brain adjusts very quickly to change. The optic nerve center and memory of a sharp image. If the eye is modified the optic center learns what works just as a new set of glasses only takes a few days to settle into.. You do not know you are doing it. As for one eye focus.. close one eye and test that...Yes.
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    Quote Originally Posted by astromark View Post
    The brain adjusts very quickly to change. The optic nerve center and memory of a sharp image. If the eye is modified the optic center learns what works just as a new set of glasses only takes a few days to settle into.. You do not know you are doing it. As for one eye focus.. close one eye and test that...Yes.
    My problem is my optic nerves. I have severe advanced and unstable glaucoma...so the 216 holes laser drilled into each of my eyes initially was to lesson the pressure on my optic nerves.... the two AMHEDS implants behind my eyes when the laser drills didn't work were for the same... were to help drain and release pressure from the optic nerve.....then they caused cataracts which we are addressing now so my vision changed.....but then again...my Ahmed's will fail within the next 18 months and I will have to attempt to do them again.....if they can...and the chemo injections that come with it....eyes that fell over your cheek......no fun....I sometimes just wanted to give up.....seriously....when you have to be led around...and you are not blind but can't distinguish things.......I really cannot explain the feeling.

    Eyes after trauma don't settle that easy....In My Humble Opinion.

    After my Ahmeds....I saw four for two days....

    Thank GOD they do one eye at a time....but saw four out of one eye....so they patched the other.....it took a few days for it to settle.....it was hell.

    I called myself The Gargoyle QUeen....because it took four months for the blood to work out of my eyes.....

    but hell....

    I was great for HALLOWEEN!
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    Quote Originally Posted by astromark View Post
    The brain adjusts very quickly to change. The optic nerve center and memory of a sharp image. If the eye is modified the optic center learns what works just as a new set of glasses only takes a few days to settle into.. You do not know you are doing it. As for one eye focus.. close one eye and test that...Yes.

    Yes but how does it do it? , that is my question. Are you saying memory?

    OB
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    Quote Originally Posted by One beer View Post
    Quote Originally Posted by astromark View Post
    The brain adjusts very quickly to change. The optic nerve center and memory of a sharp image. If the eye is modified the optic center learns what works just as a new set of glasses only takes a few days to settle into.. You do not know you are doing it. As for one eye focus.. close one eye and test that...Yes.

    Yes but how does it do it? , that is my question. Are you saying memory?

    OB
    ~ If I knew how the human brain actually worked.. I might be someone special.. I am not that. ~
    People with damaged nerve pathways and or eyes that just do not see as everyone else does. Have a massive disadvantage as they attempt to negotiate the world we see..
    ~ As a astronomer of both optical telescopes and radio spectrum signals I need to be able to see.. That my sight is adjusted by the simple wearing of prescription lenses.. I fortunately see very well. We are not all so fortunate, and as I read of 'Babe's' issues I feel for the fragile optic issues she has.. When I said memory I did so because a sighted person has built a image of what the world looks like. We train our minds to interpret what we see. Look at a toddler learning to navigate without falling..
    A child trains it's own mind to what it see's.. It's how we learn and remembering to recognize objects familiar is learning.
    Focus is a issue of depth perception and each eye can work interdependently.. Our focus is adjustable so as we can shift our best vision to a closer abject or a distant object.. With the two eyes working together a depth of field is established as a true 3d image is built.. The brain learns to read distance.. your hand eye coordination adjusts in a instant most of the time, and you do not knock the wine glass over..
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    Astromark, I am grateful for the scientific research that is ongoing that has kept my vision where it is today.

    30 years ago? I would not have been as fortunate!
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    Quote Originally Posted by One beer View Post
    Yes but how does it do it?
    I don't have specific knowledge, but the most likely mechanism is feedback.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    This is interesting.

    Normally a question on this forum is answered by a direction to Wiki or other resource, but that has not happened here yet.

    We all just know when something we are looking at is in focus or not.

    Yes, feedback will be involved, and auto focus cameras use phase detection, contrast detection or range measurement to determine focus. The brain probably uses one or a combination of these methods, and I would imagine that memory is involved too. There must have been research into this?

    I can't find anything in Wiki, but It might be there somewhere.

    OB
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    The brain doesnt 'know' if something is in focus. Focus entirely depends on the length of the eye which is a developmental process; the interaction between the retina and the sclera (emmetropisation) during a childs development ensures that the eye is the correct length in order for the light to be focussed correctly on the retina. If the eye is too long it causes myopia because the light falls short of the retina and if the eye is too short it causes hyperopia with the light focus falling behind the retina. All babies are hyperopic and need to be exposed to seeing long distances throughout development (ie into teen years) in order for the eye to develop correctly. Thats why too much close work causes myopia so children that only ever look at telly/computers etc develop myopia and are therefore at risk of glaucoma and retinitis pigmentosa.

    The signals from the photoreceptors and retinal pigment epithelials transduces the focussed light into signals the brain can use but if the focus is wrong the signal the brain receives is wrong and it does not attempt to correct it in any way. The brain has nothing to do with focus - that all occurs in the eye.
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    Quote Originally Posted by One beer View Post
    This is interesting.

    Normally a question on this forum is answered by a direction to Wiki or other resource, but that has not happened here yet.

    We all just know when something we are looking at is in focus or not.
    No - we dont.

    [QUOTE=One beer;571307]
    Yes, feedback will be involved, and auto focus cameras use phase detection, contrast detection or range measurement to determine focus. The brain probably uses one or a combination of these methods, and I would imagine that memory is involved too. There must have been research into this? [QUOTE]

    Focus occurs entirely in the eye.
    Last edited by LuciDreaming; May 29th, 2014 at 04:54 PM. Reason: quotes in wrong place
    "And we should consider every day lost on which we have not danced at least once. And we should call every truth false which was not accompanied by at least one laugh" Nietzsche.
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    Quote Originally Posted by LuciDreaming View Post
    The brain has nothing to do with focus - that all occurs in the eye.
    Are you forgetting about the crystalline lens?
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    Quote Originally Posted by LuciDreaming View Post

    We all just know when something we are looking at is in focus or not.

    No - we dont.

    Focus occurs entirely in the eye.
    I am not making myself clear.

    How does the brain/eye 'know' when it is focussing correctly on the subject of interest? What is the mechanism by which the eye/brain knows when it has focussed correctly?

    I know how the lens and eye ball physically adjust to focus, but that is not my question.

    OB
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    ~ Having found my way to the ships nav., and coms., room ( and a WiFi friendly lap top.. mine ) and with BA else to do I wish to help 'One Beer' get a answer.. a answer might be the answer I hope he wants.. Yes by the process of memory of image recognition and clarity..
    Just as when we Open the Telescope we need to adjust focus.. The mechanics are the same ( not the question ) with the scope we use a rack that moves the focal distance point forward or back.. adjusting clarity.. While the human ( and others ) eye adjusts the fluid pressure to change the shape of the eyes lens.. we have a memory of optic clarity we recognize when it's getting better or worse in a instant ( you never notice a wrong focus..you just fix it) .. This process for the human eye takes about a fifth of a second.. Your life depends on it.. when you are driving.. At the optical telescope we tell people that to make the image sharp and clear. Smaller is correct. Turn that focus knob the wrong way will make the image bigger and just plain awful.. hope to have helped.. Calm waters and clear skies.. Mark.
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    I would think it involves feedback and memory of past vision experience. My eyes have gotten worse with age, because of myopia and also dry eyes that sometimes causes slight double vision or ghosting. I know should be able to read a certain size of letters clearly and can't. But there are also times when I'm actually not sure whether an image is blurry or it's just me, or when I thought I was looking at, for example, two stars and there was actually only one.

    I've also heard of little kids who didn't realize they had any vision problem until someone plopped a pair of glasses on their head. So the brain doesn't necessarily know what it is supposed to see.

    I wonder if we were ever able to experience vision like an eagle, whether we'd all be terribly dissatisfied with our own.
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    Quote Originally Posted by One beer View Post
    Quote Originally Posted by LuciDreaming View Post

    We all just know when something we are looking at is in focus or not.

    No - we dont.

    Focus occurs entirely in the eye.
    I am not making myself clear.

    How does the brain/eye 'know' when it is focussing correctly on the subject of interest? What is the mechanism by which the eye/brain knows when it has focussed correctly?

    I know how the lens and eye ball physically adjust to focus, but that is not my question.

    OB
    The brain doesnt 'know' when it has focussed on something correctly or not - the brain doesnt 'know' about anything going on outside your body (or inside for that matter). It receives sensory information and acts upon it - there is no 'knowing'.
    "And we should consider every day lost on which we have not danced at least once. And we should call every truth false which was not accompanied by at least one laugh" Nietzsche.
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by LuciDreaming View Post
    The brain has nothing to do with focus - that all occurs in the eye.
    Are you forgetting about the crystalline lens?
    Not sure what you mean - the crystalline lens is also in the eye.
    "And we should consider every day lost on which we have not danced at least once. And we should call every truth false which was not accompanied by at least one laugh" Nietzsche.
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    Quote Originally Posted by LuciDreaming View Post

    The brain doesnt 'know' when it has focussed on something correctly or not - the brain doesnt 'know' about anything going on outside your body (or inside for that matter). It receives sensory information and acts upon it - there is no 'knowing'.

    OK, fair enough, wrong word, (which is why I put it in inverted commas), but semantics really.

    The brain has many mechanisms, for example; adjusting heart rate, breathing rate, etc by making various measurements and sending out signals accordingly.

    The brain, or eye, or both has a mechanism for adjusting visual focus until the image is sharp - but what is interesting is that there seems to be no knowledge of how the eye/brain perceives WHEN the image is sharp. Most of us can see instantly if an image is sharp or not, but what measurements are our eye/brains making to determine this?

    It would seem logical that it might be partly learned, probably around the same time that the brain is 'realising' (sorry Luci, I don't know how else to put it), that it needs to flip the upside-down image on it's retinas.

    Or perhaps there is no measurement process and maybe the eyes are constantly "bracketing" focus?

    But there still needs to be a way that the brain or eye's visual circuitry feeds back a signal to either adjust focus or not.

    So by what physical property or measurement is the correct focus arrived at? What is the sensory information being used?

    OB
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    OK - the brain is not involved in focus at all. Focus is an entirely mechanical process which involves only the eye - if the brain were involved there wouldnt be a need for glasses because any mechanical defect would be adjusted by the nervous system. Whether the eye can focus correctly or not depends on how the eye develops as I stated before. Your brain doesnt need to know when an object is in focus - it either is or it isnt.

    There is no feedback loop because the eye focuses light and not objects - adjusting the lens to focus light is controlled by the muscles in the eye and the muscles are calibrated by experience during development - if you really want to you can consciously adjust the lens in your eyes to make things in focus go out of focus.

    The brain doesnt reverse the image made on the retina - I might be wrong but you seem to be under the impression that there is some sort of output from the brain via the eye and that isnt the case. Sensory information comes in via the eye and the brain reacts accordingly and thats it - there is no back out again.
    "And we should consider every day lost on which we have not danced at least once. And we should call every truth false which was not accompanied by at least one laugh" Nietzsche.
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    Quote Originally Posted by LuciDreaming View Post
    OK - the brain is not involved in focus at all. Focus is an entirely mechanical process which involves only the eye - if the brain were involved there wouldnt be a need for glasses because any mechanical defect would be adjusted by the nervous system. Whether the eye can focus correctly or not depends on how the eye develops as I stated before. Your brain doesnt need to know when an object is in focus - it either is or it isnt.

    There is no feedback loop because the eye focuses light and not objects - adjusting the lens to focus light is controlled by the muscles in the eye and the muscles are calibrated by experience during development - if you really want to you can consciously adjust the lens in your eyes to make things in focus go out of focus.

    The brain doesnt reverse the image made on the retina - I might be wrong but you seem to be under the impression that there is some sort of output from the brain via the eye and that isnt the case. Sensory information comes in via the eye and the brain reacts accordingly and thats it - there is no back out again.
    I understand that the physical act of adjusting the eye's optics to focus at different distances is changed by muscles in the eye, but that is NOT my question. That is the 'hardware' if you like, I am asking about the 'software'. What is the 'software' process that controls the eye to focus correctly?

    How does the human focussing "software" determine whether the light on the retina is from a long way away - in which case focus to infinity, or if it is coming from 20cm away, in which case focus close?

    Perhaps I can put it this way: If you had a camera with an auto focussing lens but no software, how would you write the software to tell it how to recognise when it was in focus? You are saying the eye's focus is calibrated from experience during development; can you elaborate?

    You say the brain does not reverse the image on the retina, are you sure? I have seen documentaries on this. Owing to the lens of the eye, the image on our retinas is upside down. The brain "realises" this after a few days of life and flips the image internally before "displaying" it internally to our conscious self. I am not talking about visual feedback to the eye, I was talking about focus feedback to the eye muscles (if it is the brain which contains the focus algorithm) - you imply that this focus algorithm is in the eye itself; can you elaborate?

    If the eye produced an out of focus image, no amount of post processing could make this truly sharp, because it could not reproduce the fine detail that was lost due to the out of focusness of the eye. This is why glasses, or corneal modification is needed.

    As I've said, autofocus cameras use either phase detection, contrast detection or range finding to determine correct focus. Does anyone know what method the eye uses?

    OB
    Last edited by One beer; May 30th, 2014 at 12:42 PM.
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    I found this on physlink.com:



    Question


    How do we see things upright if the image formed on the retina in our eye is an inverted one?


    Asked by: Shweta Kala


    Answer


    It is true that the images formed on your retina are upside-down. It is also true that most people have two eyes, and therefore two retinas. Why, then, don't you see two distinct images? For the same reason that you don't see everything upside-down. One of our most remarkable tools - the brain - is hard at work for us at this task.


    Processing visual information is a complex task - it takes up a relatively large portion of the brain compared to other senses. This is because your brain performs several tasks to make images 'easier' to see. One, of course, is combining the two images, which is helped by the corpus callosum, the tiny part of your brain which joins the two big hemispheres. The other part is handled in the optic part of your brain itself, and part of its job is to make images right-side-up. It does this because your brain is so USED to seeing things upside-down that it eventually adjusts to it. After all, it's a lot easier to flip the image over than it is to try and coordinate your hands and legs with an upside-down world! As a result, though, it is believed that for the first few days, babies see everything upside-down. This is because they have not become used to vision.


    Your brain CAN be retrained though. In one psychological study, participants were asked to wear inverting lenses - lenses that invert the image BEFORE they get to your eye, so that when your eye inverts it, it's right-side-up. At first, everything appeared upside-down to the participants. But, after a few days, people began to report that everything appeared right-side-up! As a second part of the study, the people were asked to take the glasses off. Because they were now used to the lenses, their NORMAL vision appeared upside-down!! Within a day, though, their vision returned to normal. The reason you don't see everything upside-down, then, is simply because it's easier to think about right-side-up!


    Answered by: Michael Brady, Computer Engineering Undergrad., NCSU, Raleigh
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    Quote Originally Posted by LuciDreaming View Post
    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by LuciDreaming View Post
    The brain has nothing to do with focus - that all occurs in the eye.
    Are you forgetting about the crystalline lens?
    Not sure what you mean - the crystalline lens is also in the eye.
    What do you suppose controls the muscles that change the shape of the lens, if not the brain?
    Ciliary body - Wikipedia, the free encyclopedia
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    Of course it does but not because it knows it needs to focus on something.
    "And we should consider every day lost on which we have not danced at least once. And we should call every truth false which was not accompanied by at least one laugh" Nietzsche.
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    Well, it "knows" it consciously or subconsciously, or it would be able to find the correct focus. Here's some more information about it.
    Accommodation reflex - Wikipedia, the free encyclopedia
    Information from the light on each retina is taken to the occipital lobe via the optic nerve and optic radiation, where it is interpreted as vision. The peristriate area 19 interprets accommodation, and sends signals via theEdinger-Westphal nucleus and the 3rd cranial nerve to the ciliary muscle, the medial rectus muscle and (via parasympathetic fibres) the sphincter pupillae muscle.[2][3
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    Quote Originally Posted by Harold14370 View Post
    Well, it "knows" it consciously or subconsciously, or it would be able to find the correct focus. Here's some more information about it.
    Accommodation reflex - Wikipedia, the free encyclopedia
    That is a start.
    Almost 1/3 of the brain is involved with vision in one way or another so there is a lot of complex processing going on.
    I tried looking through some material on Neuro-Ophthalmology hoping to see a clear explanation.
    However their vision seems to be restricted to describing what goes wrong instead of how the brain actually does it.
    I stopped trying see such a passage for I could observe no light at the end of that tunnel.
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    Quote Originally Posted by One beer View Post
    part of its job is to make images right-side-up. It does this because your brain is so USED to seeing things upside-down that it eventually adjusts to it. After all, it's a lot easier to flip the image over than it is to try and coordinate your hands and legs with an upside-down world! As a result, though, it is believed that for the first few days, babies see everything upside-down. This is because they have not become used to vision.
    This can't possibly be correct. It's like saying that if I turn a desktop computer on its side, the image being displayed on the monitor will turn on its side.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by LuciDreaming View Post
    the brain doesnt 'know' about anything going on outside your body (or inside for that matter). It receives sensory information and acts upon it - there is no 'knowing'.
    How is "receives sensory information" not the same as "knowing"?
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    You people are entertaining.. " You have a lifetime of sight to argue with " .. No. Your image IS inverted both upside down and left is right..
    Ask any astronomer about the telescopes we all use. Up is down. Left is right. That we get so accustomed to this we do not even notice..
    A child at the eye piece asks why is the Moon the wrong way around, and we quickly remember inverted..
    The eye is a adjustable single lens.. the Image projected to the back of your eye is inverted.. Your brain soon learned this as you discovered your eye sight.. It's a fact. Just as the focus is automated so is inversion done without thought..

    NB; Only binoculars have the double prisms to invert the image to be as your eye would expect..
    and before you launch into a argument.. do go look at the 'Wikipedia' about imaging.. lenses..
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    Harold, thank you for the link to the accommodation reflex.

    It does not exactly answer my question, but it gives me a lot of information about focussing that I didn't have.

    Thanks for researching too Dan.

    OB

    PS Luci, you might want to have a look at the accommodation link too.
    Last edited by One beer; May 31st, 2014 at 06:32 AM.
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by One beer View Post
    part of its job is to make images right-side-up. It does this because your brain is so USED to seeing things upside-down that it eventually adjusts to it. After all, it's a lot easier to flip the image over than it is to try and coordinate your hands and legs with an upside-down world! As a result, though, it is believed that for the first few days, babies see everything upside-down. This is because they have not become used to vision.
    This can't possibly be correct. It's like saying that if I turn a desktop computer on its side, the image being displayed on the monitor will turn on its side.
    You're joking, right?

    Any basic physics textbook about optics will show you that the image projected by a single lens is inverted and swapped left to right.

    The human - and presumably any - brain connected to eyes with lenses, flips the image in the 'software' to present it the right way round to the internal imaging system.

    (if you turned your computer monitor on it's side, of course the image would be sideways too, unless it was a tablet with no rotation lock)

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  35. #34  
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    Quote Originally Posted by One beer View Post
    Quote Originally Posted by KJW View Post
    Quote Originally Posted by One beer View Post
    part of its job is to make images right-side-up. It does this because your brain is so USED to seeing things upside-down that it eventually adjusts to it. After all, it's a lot easier to flip the image over than it is to try and coordinate your hands and legs with an upside-down world! As a result, though, it is believed that for the first few days, babies see everything upside-down. This is because they have not become used to vision.
    This can't possibly be correct. It's like saying that if I turn a desktop computer on its side, the image being displayed on the monitor will turn on its side.
    You're joking, right?

    Any basic physics textbook about optics will show you that the image projected by a single lens is inverted and swapped left to right.

    The human - and presumably any - brain connected to eyes with lenses, flips the image in the 'software' to present it the right way round to the internal imaging system.

    (if you turned your computer monitor on it's side, of course the image would be sideways too, unless it was a tablet with no rotation lock)

    OB
    No, I'm not joking. The orientation of the image on the retina has no bearing on our perception of orientation, any more than the orientation of the bitmap image in the video card has any bearing on the orientation of the image in the monitor.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    In the 1890s George Stratton conducted an experiment where he wore glasses that inverted the image on his retina. After 7 days, he was able to adjust and things felt normal. After taking the glasses off, it took him a while to adjust back to normal vision.
    George M. Stratton - Wikipedia, the free encyclopedia
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    One beer and LuciDreaming, the act of focusing does occur in the eye, but it requires feedback, which comes from the brain. The circuitry within the eye is not sophisticated enough to determine focus.

    What we perceive as "vision" is extremely complex. There are different parts of the brain that recognize the different components of vision — colors, movement, boundaries, shapes, depth, etc, and there are parts of the brain that combine all these components to give us what we perceive as "vision".

    With focus, we're talking boundaries and we're talking overall crispiness of the boundaries in an image. If you think of focus as a single variable along the x axis of a chart, the function would look something like a bell curve. If the curve is very narrow and peaky in the middle, then optimal focus is easy to discern by the brain and easy to produce by the eye. If the curve is rather broad and flat in the middle, optimal focus is not so certain. The focusing system hunts around continuously for optimal focus. If the curve is narrow and peaky, the system doesn't hunt far in either direction. If the curve is broad and flat, the system hunts farther afield. Too much hunting tires the focusing muscles, giving us eye strain, headaches, etc.
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    I think I've found the answer to my own question. This is from the oculist.net website: (I assume it is OK to quote?)


    STIMULUS FOR ACCOMMODATION

    The nature of the optical stimulus for reflex changes in accommodation has been debated for more than 50 years. The standard view is that accommodation is a closed-loop negative feedback system that alters focus to maximize or optimize the luminance contrast of the retinal image. In this view, contrast is reduced both for underaccommodation and overaccommodation, and feedback from changes in defocus blur is an essential part of the accommodative process. However, recent experiments confirm that accommodation responds in the absence of blur feedback and that the stimulus on the retina has directional quality that distinguishes myopic from hyperopic focus. Along these lines, Fincham suggested that accommodation responds directly to the vergence of light at the retina, using the effects of chromatic aberration and the Stiles-Crawford effect. At spatial frequencies above approximately 1 cpd, chromatic aberration ensures that the contrasts of long-, middle- and short-wavelength components of the retinal image are different. For example, the relative contrasts red>green>blue specify focus in front of the retina. In one model of the process, the refractive state of the eye is determined by comparing relative cone contrasts, measured separately by L-, M- and S-cone classes. In addition to the effects of chromatic aberration, the waveguide nature of directionally sensitive foveal cones could play a role in the accommodative process, but the notion remains largely unexplored.



    So it is thought that luminance contrast is what is used to determine accommodation, (which I've called focus). In addition, the difference in contrast of red, green and blue light caused by the chromatic aberration of the cornea and lens is used to determine which way to adjust the focus.


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    I am not convinced that helps you at all.. Cones and rods.. are terms for the light receptors of the eye. In dime light conditions humanity needed to detect movement and does so by the rods.. Which do not detect colour tones.. but are very good for detecting movement. That explanation start to get into intensities of light and wave lengths of light as in the colour and tone.. of light.
    It needs to be understood that you have learned to interpret what your eye see's. Your brain has learned to interpret receptors signals.. Some other animals see very differently than do we.. Many birds have a focus cone that is simply incredible.. To see a field mouse from over a thousand feet above it.. Eagle eyed. Is the term we know.. It's true. The domestic Cat sees in the very poor light conditions of a Moonless night.. We have poor eyesight when compared, but we have other advantages from our larger brain.. We recognize a image quickly. Is it prey or are we..?
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    Okay, the "luminance contrast" in your quote refers to the difference in brightness between the sides of boundaries. The maximum contrast occurs with luminance perceived (ie, received by the retina) as a step function (ie, the sharpest) at a boundary, and a lesser contrast occurs with luminance perceived as a ramp-like function (ie, some amount of blurriness).
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    Frankly....I only care about continuing to being able to see......they explain stuff to me all the time.

    I know that isn't very scientific.....but it's just plain dang honest.

    I just want my eyes to work for a long long long long time.

    Losing my sight scares the hell out of me.
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    Quote Originally Posted by astromark View Post
    I am not convinced that helps you at all.........

    On the contrary, it is EXACTLY the answer to my original question. The visual system uses contrast to determine when the eye is in focus. That's all I was asking.

    I know how the eye is constructed and what form all the components take and what their function is* but my actual question was not about all that.

    OB


    *In biology at school, then more study of the human eye as part of my television broadcasting training.
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    Quote Originally Posted by One beer View Post
    Quote Originally Posted by astromark View Post
    I am not convinced that helps you at all.........

    On the contrary, it is EXACTLY the answer to my original question. The visual system uses contrast to determine when the eye is in focus. That's all I was asking.

    I know how the eye is constructed and what form all the components take and what their function is* but my actual question was not about all that.

    OB


    *In biology at school, then more study of the human eye as part of my television broadcasting training.
    You are correct, which is why glare can distort vision. Contrast!
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    Quote Originally Posted by KJW View Post

    ....... The orientation of the image on the retina has no bearing on our perception of orientation, any more than the orientation of the bitmap image in the video card has any bearing on the orientation of the image in the monitor.
    The brain must "realise" that the image impinging on its retina is upside down, because when the human moved its head up and down, the image on its retinas would move in the opposite direction. So the visual software would quickly determine that the image was arriving at the visual cortex inverted.

    This does beg the question though, how do newly born baby horses, (that presumably have a similar optical system to our own) manage to run from predators within hours of birth if their vision has not yet flipped? Perhaps the flipping only takes hours not days?

    OB
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    The answer is; Yes, the young fowl has a quicker brain eye connection than a human baby..
    or the nonsense answer; The fowl has a set of gyros in it's head.. It knows whats up and down.. cos it just does..

    ~ Oh dear me.. Thanks One Beer .. yes fowl is foal...
    Last edited by astromark; June 1st, 2014 at 03:29 PM.
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    Quote Originally Posted by astromark View Post
    The answer is; Yes, the young fowl has a quicker brain eye connection than a human baby..
    or the nonsense answer; The fowl has a set of gyros in it's head.. It knows whats up and down.. cos it just does..

    I know the moderators don't like us correcting other people's typos, but a fowl is a bird - I know you meant foal.

    Yes, of course, the brain uses the motion sensors in the ear to determine which direction the head is moving, and if that does not correlate with the vision from the eye, it can flip the vision until it does.

    I've just realised the subtlety of what KJW was saying; the photons striking the retina carry no up/down information themselves, that is true. However, the brain correlates head or eye movement with the image movement to adjust and align the overall vision system.

    I would suggest that this process has more priority in newly born herd animals' brains, where the ability to run accurately and safely away from predators is cruicial to survival.

    OB
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    Quote Originally Posted by One beer View Post
    I've just realised the subtlety of what KJW was saying; the photons striking the retina carry no up/down information themselves, that is true.
    One way to understand what I'm saying is to consider rotating the eyeballs by 180 within their sockets. It is expected that this would rotate the perceived image by 180. But the image on the retina would still be inverted, thus indicating that the change in perception is not due to any change in the orientation of the image on the retina (which does not change). What changes is the relationship between the image and the individual receptors of the retina, and in turn the neurons of the visual cortex. Thus, while it is expected that the inverted orientation of the image would affect the connection between the retina and the visual cortex via the optic nerve, it wouldn't affect the perceived orientation of the world. Note that my main criticism was about the statement that a newborn baby would see the world upside down because the image is inverted on the retina.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    I don't know how the brain interprets image inversion at birth. As far as I know, there's a 50/50 chance that the brain interprets it upside up (correctly) instead of upside down.

    I don't know why we should assume the worst case scenario.

    The lens likewise flips the sides, right for left and left for right, and no one here is concerned about that (yet).
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    Quote Originally Posted by jrmonroe View Post
    I don't know how the brain interprets image inversion at birth. As far as I know, there's a 50/50 chance that the brain interprets it upside up (correctly) instead of upside down.

    I don't know why we should assume the worst case scenario.

    The lens likewise flips the sides, right for left and left for right, and no one here is concerned about that (yet).
    What would you mean by upside down? For when I think of something as being upside down everything else is the right way around.
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    Quote Originally Posted by astromark View Post
    You people are entertaining.. " You have a lifetime of sight to argue with " .. No. Your image IS inverted both upside down and left is right..
    Ask any astronomer about the telescopes we all use. Up is down. Left is right. That we get so accustomed to this we do not even notice..
    A child at the eye piece asks why is the Moon the wrong way around, and we quickly remember inverted..
    The eye is a adjustable single lens.. the Image projected to the back of your eye is inverted.. Your brain soon learned this as you discovered your eye sight.. It's a fact. Just as the focus is automated so is inversion done without thought..

    NB; Only binoculars have the double prisms to invert the image to be as your eye would expect..
    and before you launch into a argument.. do go look at the 'Wikipedia' about imaging.. lenses..
    To test this, Hold a magnifying glass at arms length and note the image is inverted.. That your eye does this and your brain sorts it out..
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    Quote Originally Posted by astromark View Post
    Quote Originally Posted by astromark View Post
    You people are entertaining.. " You have a lifetime of sight to argue with " .. No. Your image IS inverted both upside down and left is right..
    Ask any astronomer about the telescopes we all use. Up is down. Left is right. That we get so accustomed to this we do not even notice..
    A child at the eye piece asks why is the Moon the wrong way around, and we quickly remember inverted..
    The eye is a adjustable single lens.. the Image projected to the back of your eye is inverted.. Your brain soon learned this as you discovered your eye sight.. It's a fact. Just as the focus is automated so is inversion done without thought..

    NB; Only binoculars have the double prisms to invert the image to be as your eye would expect..
    and before you launch into a argument.. do go look at the 'Wikipedia' about imaging.. lenses..
    To test this, Hold a magnifying glass at arms length and note the image is inverted.. That your eye does this and your brain sorts it out..
    In this case it is only the image produced by the lens that is seen upside down. Your brain keeps that image upside down and doesn't correct that bit..
    Last edited by Robittybob1; June 4th, 2014 at 04:58 PM.
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