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You create a real image with a converging lens, and you record the light pattern of this real image on a layer of light-sensitive material. Conceptually, this is all that's involved in taking a picture. But to capture a clear image, you have to carefully control how everything comes together.
Obviously, if you were to lay a piece of film on the ground and focus a real image onto it with a converging lens, you wouldn't get any kind of usable picture. Out in the open, every grain in the film would be completely exposed to light. And without any contrasting unexposed areas, there's no picture.
To capture an image, you have to keep the film in complete darkness until it's time to take the picture. Then, when you want to record an image, you let some light in. At its most basic level, this is all the body of a camera is -- a sealed box with a shutter that opens and closes between the lens and film. In fact, the term camera is shortened from Camera Obscura, literally "dark room" in Latin.
For the picture to come out right, you have to precisely control how much light hits the film. If you let too much light in, too many grains will react, and the picture will appear washed out. If you don't let enough light hit the film, too few grains will react, and the picture will be too dark. In the next section, we'll look at the different camera mechanisms that let you adjust the exposure.
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