Science Questions

Why does a hole in a piece of paper correct my poor vision?

Mon, 28th Sep 2015

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Question

Ben Hermann asked:

 

Hi there! 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

 

 

Answer

We put Ben's question to Dr Chris Smith

Chris - Yeah, you can do this yourself. If you have any kind of eye problem and you look through a very tiny hole into the distance, you will notice that you can see extremely well through that tiny hole. What you have effectively done is Holeto create a pinhole camera and this is exactly how pinhole cameras work. The idea of a pinhole camera and if you haven't made one, itís very easy to do. If you get a cardboard box, make a tiny hole in one side and put a sheet of grease proof paper or white paper on the opposite inside surface of the box, you can then turn the box to look at something and you'll see an image of the thing you're looking at. It will be upside down or back to front but it will be on that white piece of paper. The downside is, it will be very, very dim. So, you donít get very much light coming in through your pinhole, but thatís actually how it works. So, what's happening is, when you look at a distant object, individual spots of light will be coming to the pinhole. Theyíll go through the pinhole and then theyíll be arriving at the screen as an individual spot of light. If you do that enough times, with lots of little spots of light, you'll build up a nice perfect picture of the thing you're looking at regardless of how far away it is. The reason itís dim is because most of the light coming from the object will hit the box. It won't go through the hole. Now the reason you see blurs when you wear glasses is because your eye, the pupil which is your own pinhole in your eye actually is collecting a lot of light in order to make it a good balance between how bright the object is and so on. And so, when you gather lots of light, what the eye then does is to focus lots of light from a target into one place on your retina. So, you see a clear picture of lots of spots of light brought together. So, you see a nice bright but in focus object. The downside of the pinhole is that because you're throwing away lots of the light, although itís extremely exquisitely well-focused, itís not very bright. So those early cameras, you had to do very long exposures to get a good picture, but it would be nice and crisp and sharp without needing a lens.

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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.

Pecos_Bill, Thu, 6th Aug 2015

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. alancalverd, Thu, 6th Aug 2015

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