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

29 September 2015

Question

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 to 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, it's 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 don't get very much light coming in through your pinhole, but that's 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. They'll go through the pinhole and then they'll 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 it's 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 it's extremely exquisitely well-focused, it's 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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