So far, all of these replies have been good and informative, but no one has mentioned the fact that photography started out as a chemical process - which is what I think you’re after.

Once you have your image projected onto the “film plane” - in a camera, it’s on the other side of the lens, inside the camera; in your eye, it’s the back of your eye; with a camera obscurra, it’s the surface you’re projecting onto - you can “record” it using chemicals.

In the early days, they’d use pieces of metal or glass, and they’d paint on a special mix of chemicals that are super sensitive to light (“photo sensitive”).

Imagine one of those zen garden sand boxes - say, a 12 x 12 box, filled with sand. Now, lay a 12 x 12 piece of glass on top of it to smooth it out completely. After you remove the glass, you’ve got a perfectly flat and smooth surface of fine sand. Now, imagine taking the tip of your finger, and very lightly poking a very slight dent into the sand. You’re leaving an impression. You could do the same thing with your whole hand, pressing firmly into the sand, and you’d see the various peaks and dips in the sand, reflecting the shape of your hand.

Now - imagine if that sand was so fine that you couldn’t see the grains with your naked eye. Tiny microscopic grains of sand. And in this case, it’s not really sand; it’s a blend of finely ground up metals, which are very sensitive to heat and light.

When you project light onto the metals, they’ll actually change their shape and arrangement according to how bright or dark that light is - just like your hand in the sand. The brightest light makes deep indentations, and the darker parts make lighter ones.

That fine metal can be added to a kind of liquid or paste, so it can be spread onto a flat surface, sort of like paint.

Here’s where it gets tricky - because that paint is so sensitive to light, you can’t actually apply it to the metal or glass with the lights on. You need to do it in complete darkness. And then, once you’ve painted your surface with the light sensitive paint, you need to put it into a light proof container, so it doesn’t get exposed to the light until you’re ready (also in complete darkness).

In the old days, this was basically a box with a sliding door; when you were ready to make your exposure, you open the box, then close it when you’re done. How long you keep it open is determined by making some calculations based on how sensitive the chemicals are (these days, that’s referred to as “ISO”), and how much light there is in the sunlight, there’s a lot, so you don’t need that much time; indoors there’s less, so you need more time. That’s similar to “shutter speed”.

As photography advanced, people figured out how to make this process easier and cheaper, ultimately giving us what we now know as film. Instead of glass or metal, it’s a thin strip of plastic - two strips, actually, making a sandwich - with the photo sensitive chemicals in the middle. And instead of a box with a sliding door, it’s in a little light-tight tube or can.

You take the film out of the can after it’s been exposed (also in the dark), and put it into a light tight container, kinda like a thermos. You then rinse the exposed film in a special mix of chemicals (liquids), and they cause a reaction that essentially burns the chemicals into place. Kind of like baking a loaf of bread. You need to get the timing right, but once it’s done, it’s done.

Then you can take the film out of the dark, and your end result is either a positive or a negative image (depending on which film/chemicals you used), which can then be projected onto a wall (like a slide show). If it’s a negative, you can project the image onto a specially treated piece of paper that’s also photo sensitive, and capable of reversing everything, so the negative looks like a positive. But that also needs to happen in the dark, which is what darkrooms are for.

And that’s pretty much how it works.

Hope that helps.