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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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.