Let's talk about how you might make an image. So, here's an idea, let's just take a piece of film, or a sensor, and, let's just expose it to the light. Do we get a reasonable picture, well, and the fact that the title says image formation, bad method should give it away.

Well, this point on top of this cartoon pine tree, well it projects, a ray of light to some point on the film. But of course [COUGH] it also projects a ray of light all over the film. Likewise, another point projects all over the film. So, one of the problems is that every location on the film is seeing light from all sorts of different places, there's no single, value of light that would, that comes from a single place in the world. So how might we fix that? Well, here's an easy way.

Let's put in a barrier, a barrier with a very small hole in it. So here we have this little, a hole, and it's called an aperture. Okay. And we're going to use the word aperture to mean a couple of different things, but generally aperture is a small hole, that let's light in it, you might change the size. But here we have an aperture, and what that aperture makes is that each area on the film plane or on your sensor plane is seeing light from only one, point on the object, so it is showing you the light that just came from one place in the world, which is what we're after. By the way, something to note here, and we're going to see this a lot more, is that the image over here on the film is inverted from what's on the object. So the pine tree would be upside down, if you will.