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Thread: focal length

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    Photographer; for hire!! shiato storm's Avatar
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    focal length

    here's one i've been pondering over the past few weeks and still can't work it out. one for the photographic buffs, or biologically intrigued...

    at what distance does the human eye assume a focal length of 'infinity'?

    assume, if you will, the eye as similar to a camera lens in that it can focus in on something not more than 3 inches from it and also expand its focus when looking out over a view, for example, when pretty much the whole scene - beyond a certain distance - all becomes 'in focus'. on some lenses this is above about 20feet or so. so, where does the human eye go from focusing with a relatively narrow depth of field (at close to the face) to an 'infinity' setting, where all beyond said distance is 'in focus'...?...and how might one find out (without the need of token pretty assistant and large field)
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    Resident abit mourner BUFF's Avatar
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    I don't know the answer to your question but what you want is not the focal length it is the the lens-to-subject distance. Perhaps an optician would know?
    Apparently focussed at infinity focal length of the human eye is approx 25mm.

    Interesting article http://www.minoxlab.com/PZ051897/peterd.htm

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    Photographer; for hire!! shiato storm's Avatar
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    interesting article...cheers

    sort of helps me understand, i mean, i get the fact that the iris changing size etc and reading the rest of the article kind of helps in terms of depth of field the human eye has - roughly speaking of course...wonder if any where out there has info on the infinity thing ...
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    Now with added sobriety Rave's Avatar
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    Ahah! An interesting question, and one that I have pondered myself a little bit. In fact the answer is: it depends! The depth of field of a camera lens varies with the apeture setting; the smaller the hole, the greater the depth of field. Now the eye has an iris a lot like a camera lens, so your effective depth of field varies according to how bright or dark it is, and consequently how large your pupil is. You can sort of see the effect if you close one eye and focus on your finger, held about a foot away. The background will be more blurred at night than it would be during the day.

    The thing is though, even if you did the measurements at a constant brightness level, the depth of field would still vary from person to person, because some people's retinas are better than others. If your eyes focus properly (i.e. you're not long or short sighted), you still might not have as good a retina as someone else. Your effective depth of field would be higherm because although at the edges of the focus range the image would be getting a bit blurred, your retina wouldn;t be good enough to tell the difference, so in effect it wouldn't matter. I don't need glasses (yet), whereas my wife is very short sighted, and I mean very, her glasses give me a headache within seconds. With her contacts on though, she can read a bus number further away than I can!

    Rich :¬)

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    Photographer; for hire!! shiato storm's Avatar
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    mmm...
    interesting point. yeah, being interested in photography - hence the reason of my initial questions - i figured that the sharpness is def. effected by the light availability etc. and if you quint really hard you can sort of see how your eye focuses on pretty much everything regardless of distance due to limited light...as in a camera.
    poses the interesting question that, assuming the analogy of the eye being like a camera, what shutter speed to we opperate at? I know above abour 50 frames/sec things can pass us by but within the visual cortex and regions of the brain that control eye relfexes there must be a 'set speed' sensor that controls the rapid responsiveness of the iris, through it is assumed the activity of optical neurons (from the retina). i.e. too much light gives a huge response being sent down the optic nerve, this is sensed and the iris rapidly closes to bring the level of light hitting retina down to a 'sustainable' (or undamaging) level...

    but i'm still fuddled by the point at what distance, for the human eye, becomes irrelevant and whatever is beyond that point - say 20 meters - is all in focus, as if I were to set the lens on my camera to 'infinity'...thus imagine the picture once developed where things from distant mountains right up to the object 20 meters away you wanted are in focus...
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    Now with added sobriety Rave's Avatar
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    Quote Originally Posted by shiato storm
    but i'm still fuddled by the point at what distance, for the human eye, becomes irrelevant and whatever is beyond that point - say 20 meters - is all in focus, as if I were to set the lens on my camera to 'infinity'...thus imagine the picture once developed where things from distant mountains right up to the object 20 meters away you wanted are in focus...
    Like I said, the 'hyperfocal distance' of your eye will vary depending on how bright it is. You could probably do your own experiments; on a starry night, go out and focus on the roof of your house, then see if the stars are in focus. If they're not, walk away until they are, then check the distance. You could do the same during the day using an aeroplane. I would guess though the hyperfocal distance is rarely more than about 10 metres.

    Rich :¬)

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    Photographer; for hire!! shiato storm's Avatar
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    fair enough, I shall go and experiment and come back with the answer ...
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