You're in class. It's a huge lecture hall for like a billion people and you got a seat near the back. But wait! You're nearsighted and you forgot your glasses!
[gasp] [dramatic music]
It's okay, though, because you have a piece of paper and a pencil.
In the spirit of the Jury-Rig It contest, I'm going to make this instructable entirely in my lecture hall. I'm such a good student. For my studiousness, you should vote for me, wink wink.
Step 1: The Plan
Lenses are great and all, but they can only focus on objects at a specific distance—objects in the so-called "focal plane". But there's a range where things are still reasonably in focus. The size of the acceptably-focused region is called the depth of field. Now, I don't study optics and the factors that affect the depth of field are beyond my motivation to look up, but it's okay because we aren't going to use a lens.
There is another way to create an image, though. Instead of gathering the light with a lens, you block all the light that isn't going the right direction with a pinhole. The simplest cameras were just a light-proof box with a pinhole in it to project an image onto the film at the back of the box. They're really cool and I'd make one if I had photographic film. Alas, I don't.
Ideal pinholes have infinite depth of field. Since they don't depend on the focusing properties of a lens, all distances are in focus. They do decrease the field of view and make the image rather dim, but we care about the depth of field part.
Real pinholes have a non-zero diameter, so there are some limits to their depth of field, but in general, the smaller the aperture, the larger the depth of field.
In order to see the board from the back of our lecture hall, we're going to put a pinhole in front of both of our eyes. This will increase our depth of field enough to focus on the board (and everything else).
[Images from Wikipedia: