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In the back of your camera, directly behind the lens, is a small computer chip that has an area of light- sensitive material on it. This is the camera's image sensor, and it's what actually captures light and makes an image. It's the digital equivalent of a piece of film, or the back of your eyeball. The surface of the image sensor is divided into a grid, with one cell for each pixel that the sensor can capture. So if you have a 10-megapixel camera, the sensor is divided into a grid of 10 million cells.
Each one of these cells contains a type of metal in it that emits electrons when struck by light. The more light that strikes an area, the more electrons that get emitted. By reading the voltage at each one of these cells, and these cells are called photosites, by reading the voltage, we can find out exactly how much light has struck each pixel. In other words, we can capture an image. Now here is the kicker. There is no color in this image. All that the sensor detects is how much light has struck each pixel.
We know nothing about what color the light was. In other words your camera is inherently a black-and-white device. Now, color is interpolated using something about a hack. Each photosite on the sensor is covered with a different colored filter. Some filters might be red, some might be green, some might be blue; different companies use different colored filters. In this case, there are twice as many green filters as red or blue, because the eye is more sensitive to green than to any other color.
So after the image is exposed, we know how much red filtered light has struck some of the cells, how much blue filtered light has struck others, and so on. We still don't know the exact color of any particular pixel, but we can interpolate the color from these filtered pixels. Here is a very over-simplified version of what I'm talking about. Let's say we're trying to figure out the color of that middle pixel there, which has a green filter on it. We know that it has received a full amount of light, as have all the pixels around it.
And those have various combinations of red and blue filters over them. Because we know that equal amounts of full-strength red, green, and blue make white, the odds are that the center pixel is white. "But," you might be thinking, "what if that's just a single white pixel in the middle of a field of other colors?" Well, the pixels on your image sensor are so tiny that the odds are that there would ever be a single-pixel row or column of colors is just incredibly small. Just to be safe though, there is actually a filter in front of the image sensor which blurs the image a little bit to smear that colors together to ensure that there are no single stray pixels of one odd color.
Now, obviously this whole color-interpolation process is much more complicated than the very simple version that we've seen here, but this should give you an idea of how it works. Now curiously enough, we think of the digital camera as a somewhat recent invention, but all this image sensor technology was invented back in the '60s. And we can thank Einstein for our understanding of metals that emit electrons when they are struck by light. He won the Nobel Prize for his study of the photoelectric effect, which is what that process is called. Your camera might have a black-and- white mode on it, and you might think, "Oh, that probably just reads that luminance information and doesn't bother with the color conversion stuff," but that's actually not how it works.
When you shoot in black-and-white mode, your camera goes ahead and generates a color image, and then it converts that back to black and white. To get the best results from your camera and its sensor, you need to consider some particular camera settings, and that's what we are going to look at in the next chapter.
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