Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: thedoc on 06/08/2015 17:50:03
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Ben Hermann asked the Naked Scientists:
I discovered that, when I look through a tiny hole e.g. in paper, it serves as an optical lens correcting my eyesight enough to read. The same works in uneven holes formed by a curled finger. How is that possible as no optically effective material through which I would be looking is involved? Thanks for looking into this! ;-)
Cheers ,Ben
What do you think?
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Let us consider what happened when Pecos-Bill visited the world famous Huntingdon Gardens near Pasadena to photograph the famous "boojum" plant in the desert section. I believe, you have something of the sort at some place called "Qew"
To really capture the boojumness of the boojum on a 2 dimensional plane, you must - wait for it - adjust the camera aperture down to around, say, f22 so that you have boo-coo depth of focus.
That's what a pinhole does to your eye. It augments your poor old presbyopic peeper to let you read the damned menu, etc.
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A pinhole camera doesn't have a lens at all. In principle it can form a perfect image on a film if you wait long enough.
The "hole in paper" phenomenon relies on the same process. Primary visual defects are caused by the lens of your eye being less than perfectly shaped, so light passing through the edges of the lens is not focussed to the same point as light passing through the centre, and the image on your retina is blurred. The pinhole cuts out the offcentre light, and the image is formed only by rays that pass through the centre of the lens, so the apparent focus improves.
We use the same phenomenon to adjust "depth of focus" in a camera. With the lens wide open, only rays from a certain fixed distance will be focussed on the film and everything else will be blurred by the spherical aberrations of the lens, and as you reduce the aperture, so the blurring of other objects becomes less significant.
Lens apertures are calculated as "f numbers", the ratio of the focal length of the lens to the diameter of the aperture, so an "f/1.4" camera lens will have a very shallow depth of focus but good light-gathering power, and "stopping down" the same lens to "f/64" will give reasonable sharpness to everything from a few feet to infinity, but will require a much longer exposure.