Digital cameras

Summary: Rapid advances in digital camera technology have led to their widespread use.

From the invention of modern photography in the 1800s to the rise of digital photography in the twenty-first century, the function of the camera has been the same: to record patterns of light. The word “photography,” coined by Sir John Herschel in 1839, is from the Greek phos (light) and gráphein (to write). Simple pinhole cameras were described as early as the fourth and fifth centuries b.c.e. by Chinese philosopher Mo Ti and Greek mathematicians Aristotle and Euclid of Alexandria. Mathematician and physicist James Maxwell created the first color photograph in 1861. Not long after, American inventor and Kodak founder George Eastman developed inexpensive equipment and film that made photography practical for common use. Until recently, cameras recorded images on media coated in photosensitive compounds. Incoming light was registered as a chemical change that could be seen upon development in specialized photochemistry. Digital cameras use an electronic chip that is sensitive to light. The chip, either a charge coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS), converts the light into an electrical signal, and a small computer in the camera then transforms that signal into the “ons” and “offs” (or “1”s and “0”s) of binary code for storage on a digital storage device. The digital information that represents an image can easily be copied onto a computer, manipulated, published electronically, and printed. Researchers also investigate mathematical questions like how many images one should shoot in order to be reasonably confident that no person in the photograph blinks. For groups under 20 people, the number of images is approximately equal to the number of people divided by one-half.

The Lens and the Shutter

Like most cameras, a digital camera begins the process of taking a photograph by letting light in through a lens (a curved piece of glass or plastic) that bends light through the principle of refraction and focuses the image. The light then passes through an opening called an “aperture” whose size can be adjusted to let more or less light pass. Apertures are described by an f-stop number, which is proportional to the focal length of the lens over the diameter of the entrance pupil. Since it is a ratio, larger f-stop numbers refer to smaller apertures. Each doubling or halving of the f-stop number translates to a change in amount of light let in by a factor of four. Thus, an f-stop of 11 lets in four times more light than an f-stop of 22. Finally, the light comes to a shutter that opens for a period of time when the shutter release is triggered, allowing light into the camera body. Usually, a shutter speed is a fraction of a second, though long shutter speeds can be used in low light, or for a variety of effects. With especially long shutter speeds, heat can build up in the CCD or CMOS, causing electrical interference that interferes with accurate, binary recording of the image, resulting in error or “noise,” though camera manufacturers are developing a number of processes that have made this less of a problem over time.

The CCD or CMOS

In order to capture an image, the light that comes into the camera falls on the CCD or CMOS chip, which changes the image into electric current. A CCD is made up of tiny regions called “picture elements” or “pixels” that will correspond to the points in the photograph. A CMOS works similarly, though the specific underlying technology is a bit different. Some cameras have one CCD for all three primary colors of visible light, red, green, and blue; each pixel records only color of light from the scene. More advanced cameras use three different CCDs, one for each primary color, resulting in a more accurate image. Ultimately, the electric current from the CCD is encoded by a small computer in the camera into a stream of binary information in the form of “ons” and “offs” that ultimately will be stored on a flash memory card.

Sensitivity and ISO

In film cameras, different formulations of film are used for different light conditions, with more sensitive films employed in low light. In a digital camera, the signal from the CCD can be boosted to handle low light levels; however, doing so introduces noise in the signal. The setting for camera sensitivity is described as its “ISO number,” an international standard for measuring the speed of color film. It uses both an arithmetic and logarithmic scale to combine two previous film standards. In the arithmetic scale, which is commonly the only one given, each doubling of the ISO representing a doubling of the sensitivity. Thus, a camera set to ISO 100 will be half as sensitive to light—and will require twice as long an exposure for a given scene to achieve the same result, given the same f-stop setting—as one with an ISO of 200.

Pixel Dimensions

One of the factors that determines the picture quality of an image produced by a digital camera is the number of pixels it records. This is especially relevant when images are blown up to large dimensions, as the individual pixels begin to become visible. Pixels are the individual binary units into which the image is “broken up” and stored during the electronic conversion process by the camera’s chip. For example, the Droid Incredible phone, released in 2010, contains an 8 megapixel camera, which means its photographs are composed of about 8 million individual pixels, with each picture having a possible resolution of roughly 3264 pixels wide by 2468 pixels high. However, in practice, there are many factors that affect picture quality. The size of the electronic chip plays a large role. When the photosensitive regions of a camera’s chip are packed too tightly together, they create electronic interference in their neighbors, potentially affecting the binary storage, and ultimately affecting the accuracy and quality of the stored image.

Bibliography

Stone, J., and B. Stone. A Short Course in Digital Photography. Upper Saddle River, NJ: Prentice Hall, 2009.

Svenson, Nic. “Velocity Science in Motion: Blink-free Photos, Guaranteed.” http://velocity.ansto.gov.au/velocity/ans0011/article‗06.asp.