When you take a digital photo or stare into your webcam, light zooms into the lens. This incoming "picture" hits the image sensor, which breaks it up into individual pixels that are converted into numeric form. CCDs and CMOS chips, the two kinds of image sensor, do this job in slightly different ways. Both initially convert incoming light rays into electricity, much like photoelectric cells(used in things like "magic eye" intruder alarms or restroom washbasins that switch on automatically when you put your hands under the faucet). But a CCD is essentially an analog optical chip that converts light into varying electrical signals, which are then passed on to one or more other chips where they're digitized (turned into numbers). By contrast, a CMOS chip does everything in one place: it captures light rays and turns them into digital signals all on the one chip. So it's essentially a digital device where a CCD is an analog one. CMOS chips work faster and are cheaper to make in high volume than CCDs, so they're now used in most low-cost cellphone cameras and webcams. But CCDs are still widely used in some applications, such as low-light astronomy.

Whether images are being generated by a CMOS sensor or a CCD and other circuitry, the basic process is the same: an incoming image is converted into an outgoing pattern of digital pixels. Let's just refer to "the image sensor" from now on (and forget about whether it's a CCD and other chips or a CMOS sensor). First, the image sensor measures how much light is arriving at each pixel. This information is turned into a number that can be stored on a memory chip inside the camera. Thus, taking a digital photograph converts the picture you see into a very long string of numbers. Each number describes one pixel in the image—how bright or dark and what color it is.

Step by step

1. Light from the object (in this case, a bicycle) enters the camera lens.
2. The image sensor inside the camera splits the image up into millions of pixels (squares). An LCD display on the back of the camera shows you the image that the sensor is capturing—not an image of the object seen through a series of lenses (as with a conventional camera), but a redrawn, computerized version of the original object displayed on a screen.
3. The sensor measures the color and brightness of each pixel.
4. The color and brightness are stored as binary numbers (patterns of zeros and ones) in the camera's memory card. When you connect your camera to a computer, these numbers are transmitted instantly down the wire.