This writeup will be about an interesting use of the camera obscura, also called a pinhole camera, but I've found that the currently existing writeups don't adequately explain what is meant by that and how it works, thus I'll give a short explanation of the principles. If you already know the principles, you can skip to beneath the horizontal ruler.
One form of a camera obscura is a box in which the front and back ends have been replaced by white paper. A small hole (think needle point or smaller) is made in the front side paper, and a candle is lit in the room. The rest of the room is darkened as much as possible. If you now place the box with the front towards the candle, you can observe an image of the flame on the back paper. The image will be upside down, and not as bright as the candle flame itself.
What's causing this image? Well, imagine the light beams from one point of the candle. Normally, they would travel everywhere reaching the wall at pretty much every possible point, along with light from other parts of the flame. This doesn't create an image, but rather the glow of light you expect from a candle. When a few beams of light travel towards the box, however, a very limited amount is let through (it's easier to think of it as if just one beam was let through). That light beam creates a tiny dot on the back paper. The light beams from other parts of the candle make different dots in different locations on the back paper since they come in on different angles. Like this an image is created.
Why is the image upside down? Imagine a light beam from the top of the candle. It will be traveling downwards when it goes through the hole, ending up very low on the back paper. A light beam from the bottom of the flame will be traveling up, ending up at the top of the paper. This effectively turns the image upside down.
Why is the image so dark? Normally when you make an image, you use a lens which focuses an area of light into a point. Now you just take one light beam. This means a great reduction in light intensity.
That should cover the basics, but what application could this possibly have, in an age where cameras are common good? Well, that's easy. What does a camera obscura do? It creates a sharp image, so that's the application. But it's not just any camera obscura I'm talking about.
Have you ever seen people who normally wear glasses squint their eyes trying to read a billboard when they've forgotten their glasses? Squinting your eyes has a similar effect as the camera obscura, it reduces the amount of light beams that enter your eyes, hopefully making your image a little sharper. As someone who needs glasses, I can say that it's not really effective. It also gives you a headache.
Recently, when I was on the bus and couldn't read the monitor displaying the next stop, I was squinting and straining one of my eyes with my hand to read it, when suddenly I noticed I could read the display clearly. I had, without realizing it, created a camera obscura with my thumb and 2 fingers. The letters were clearly readable and not at all dim (a monitor emits its own light, that helps).
This is how you do it:
Place your right thumb with its base against your right eye socket, with the point of your thumb beneath your eye pointing in the direction you're looking at. Now place your index and middle finger next to each other with the tips on top of your thumb, but don't press down too hard. This should create a small triangle between your three fingers. Look through this triangle, and make it as small as possible while you're still able to look straight through it from behind. It might take some practice at first.
If you're nearsighted (Myopia), you should now see a significant increase in sharpness. Try opening both eyes for comparison. If you aren't nearsighted, you probably won't see much difference. The reason you don't see everything upside down is because your eyes get the same light beams as they normally do, except now just one beam per point.
What's intriguing me about this method, is that it's lens-independent. If you can consciously control it, try to shift your focus to near-vision instead of infinity. You'll see that the image will stay sharp, it will only get a little bit smaller. Another thing to note is that this works for nearsightedness, but it also works for farsightedness (Hyperopia), if you can't view anything sharp that's under 30 cm away from you, you can with this method. For me, this doesn't really work. Because I'm nearsighted I can view things sharp up to 5 cm away, meaning I won't see a difference in sharpness when using the method until about 3 cm from the object, leaving me with little room to maneuver.
Things to note:
- Because your eyes can adjust to the amount of light coming in, you'll usually get a pretty good picture of your surroundings. Still, it works better with lots of light.
- You could technically make glasses from paper with a pinpoint opening in them. This looks really silly though, and you'll probably not have them with you all the time. I never forget to take my fingers with me though. At the moment my glasses are broken and I don't want to wear my contact lenses too much, so I use it now and then.
- You can use the light reduction to your advantage. If you make the hole small enough, you can look right into a light bulb without hurting your eyes (Don't try the sun though. That's probably still no good. Check the camera obscura node for an experiment with a camera obscura and the sun).
- montecarlo mentioned that this method also works when you use the thumb and index fingers of your both hands, creating a small diamond between your fingertips. Like that it's actually easier to make a small hole! It's also easier to explain to people.
That's about all I can tell about it. It takes practice to get it right immediately, but it's a nifty thing to be able to do (Like raising one eyebrow, but more useful). Have fun with it!