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While the evolution of photography has gone on for a very long time — with the first film cameras appearing in the 1800s — the fundamental principles behind it have actually been known for much, much longer. We’re talking thousands of years. All the way back to at least the 5th Century BC, where the Chinese philosopher Mozi made the first historically documented mention of the concept.

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Photo film wasn’t a thing over two thousand years ago, of course, but some people understood the importance of light and reflections well enough to use something called a “camera obscura” to view and sometimes record images. But since film didn’t exist yet, it was used more like a rudimentary projector to position an image of a scene or subject, often onto a sheet of paper, then someone would trace over it to recreate the shot.

Modern cameras don’t have a tiny person inside making incredibly fast and detailed drawings of what’s in front of the lens, but a lot of these core ideas haven’t changed in all this time.

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The Latin definition of camera obscura just means “dark chamber,” so any dark room or container could technically be considered a kind of camera obscura, but there’s realistically a bit more to it than that. A proper, functional camera obscura is, yes, a dark box, container, room, or what have you, but also with a single small opening on one side that allows light in.

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It’s that tiny bit of light that makes the real difference, as it causes the scene in the lit area to be projected onto the interior camera obscura wall — inverted (i.e. flipped horizontally and vertically), but an image does appear all the same. It works particularly well when the wall receiving the projection is white, so the image shows up more clearly, but it doesn’t have to be a light color.

Really, it doesn’t have to be a flat surface, or even an intentional camera obscura. Thanks to the fascinating weirdness of science, just about any situation where bright light passes through a narrow opening into a larger space can cause this kind of visual trick. There might even be instances where you’ve noticed a somewhat blurry but still identifiable upside down and backwards image of the outside projected onto a wall in your home thanks to a partially opened blind. That’s basically a camera obscura, too!

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What’s really interesting about all of this is that we kind of always have these images projected and reflected off of every surface that isn’t in complete darkness — it’s just that those reflections are exceedingly out of focus, blown out, and all layer on top of each other. By metaphorically squeezing the light that causes those reflections through a small opening, it brings these images into a much clearer focus.

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The more complicated explanation is that almost everything reflects light, and those reflections bounce outwards off of objects in multiple directions. By positioning a surface (like a wall or the inside of a box) across from a lit object, then narrowing the opening that’s allowing light to reach this surface (basically a kind of photography aperture) to block out most of the reflections other than the ones that are more straight-on, those reflections create an image of the object. The narrower the opening, the more visual “noise” gets removed, and the clearer the resulting reflection.

And the image appears upside-down because light travels in a straight line (outside of extremes, like black holes). The reflected light can’t simply narrow down, pass through the opening, then expand again. Instead, light bouncing off of the top of the object passes through at a downward angle and ends up on the bottom of the receiving surface. It’s why all of the illustrations explaining these principles always use what look like mirrored cones to represent light.

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