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Digital Camera: A Revolution in Photography

April 2001 IBEW Journal

The celebrated photographer Ansel Adams (1902-1984) once said, "We must remember that a photograph can hold just as much as we put into it, and no one has ever approached the full possibilities of the medium." If he were alive today, Adams would surely marvel at the emergence of digital imagery and the technology behind the current generation of cameras used by professional and amateur photographers alike.

It is nearly impossible today to open a newspaper or magazine without seeing a digital photograph. The digital camera is rapidly gaining popularity across North America and the rest of the world. The unit sales of digital cameras are expected to exceed those of film cameras by 2002. The convenience of digital cameras makes it easy to understand why they are becoming so commonplace.

Digital vs. Traditional Cameras

Digital cameras offer an array of features not found in traditional film-based cameras. For one thing, digital cameras offer immediacy. Users can immediately view the photos they have taken, delete those they dont want, and reshoot the pictures that didnt turn out the first time.

Digital photos can be modified with computer software that allows users to edit the pictures in powerful ways. Users can crop out unwanted scenery, adjust the brightness and color balance, eliminate flash-induced red-eye, and touch up details before printing the image, posting it to the web, or attaching it to an e-mail message.

Because digital photographs can be manipulated on a computer, they are in some ways more versatile than film-based photos that require negatives. However, negatives will probably be printable far into the future. In contrast, a digital image file may not be compatible with the software and hardware in use 20 years from now, unless the file is regularly updated.

One of the biggest selling points for these cameras is the elimination of the time and cost of film developing. Digital cameras store snapshots on disks or memory cards rather than on film. These storage devices can be used again, once the photos are downloaded to a computer. The storage devices will hold several images, depending on their memory capacity and the size of the images themselves.

Digital Technology

Like conventional cameras, digital cameras have a lens, aperture, and shutter (see sidebar), but they dont use light-sensitive film. Digital cameras use a solid-state device called an image sensor to capture and store images. Image sensors are silicon chips containing millions of light-sensitive diodes known as photosites. When the shutter of a digital camera opens, each photosite on the image sensor records the brightness of the light that strikes it and accumulates a charge. The more light that strikes the sensor, the greater the charge. The brightness levels recorded on each photosite are converted to digital numbers that can be manipulated by the computer to which the digital image is transferred. The computer translates the recorded brightness into the picture elements or pixels displayed on the computer screen.

The image sensor that converts patterns of light into electrical impulses and useful digital information is either a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS). CCDs were developed by Bell Labs in the late 1960s and have been used in applications such as the first solid-state video camera, space-based telescopes, scanners, and video camcorders. Although CMOS image sensors are less expensive to fabricate and have lower power requirements, they do not perform as well as CCDs in low-light conditions.

Digital photographs consist of hundreds of thousandsor even millionsof tiny pixels. Each photosite on the image sensor captures one pixel when the photo is taken, so that a digital photo contains a mosaic or map of pixels. The number of pixels in a digital image affects the quality or optical resolution of the printed or displayed image. The maximum number of pixels that a digital camera can capture depends, in part, on the number of photosites on the image sensor. Some software packages can enhance the resolution of a digital image by adding pixels based on a mathematical calculation, creating interpolated resolution. To a professional eye, the image quality produced by a digital camera is still not as fine as that obtained by film-based cameras. Film can still capture far more detail in the highlights and shadows.

Digital cameras at the lower end of the market have resolutions around 640 x 480 pixels. This resolution creates images that are 640 pixels wide by 480 pixels tall, containing about 307,200 pixels. Megapixel cameras produce images with one million or more pixels, and multi-megapixel cameras produce images containing more than two million pixels. Camera costs, photo resolution, and file sizes increase as the number of pixels increases. Some digital cameras allow the user to select the resolution of each individual photo. Many of the lower-cost digital cameras in use now produce images suitable for publishing to the web, but not of a resolution high enough for printed material.

From Gray Scale to Color

Photographic images are usually based on the three primary colors of light: red, green and blue (RGB). This is an additive color system, because combining the three colors of light in equal quantities creates white light. Each pixel in a digital image carries a single color, formed from a combination of red, green and blue. But the image sensor that records the image in the digital camera can record only the gray scale, a series of steps from pure white (maximum brightness, or maximum light) to pure black (absence of brightness, or absence of light). There are several ways to capture the colors. These techniques use filters:

1. A camera might take three separate exposures, using a different filter for each exposure (i.e., red, green and blue). The three files are then merged to form the full-color image.
2. A camera might use the three separate image sensors, each with its own filter. Each sensor would capture the image through a red, green or blue filter.
3. Individual photosites on the image sensor can be covered with their own filters. This technique allows one-third of the image to be captured through each color filter.

With the second and third technique, interpolation is used to calculate the two colors that an individual photosite did not record. The color of neighboring pixels affects the calculation, and there must be enough information in those pixels to contribute to the final result.

Area-array sensors are found in most digital cameras. These image sensors arrange their photosites in a grid pattern so that they can capture the entire image. They are used in a variety of ways:

1. One-chip, one-shot cameras use different color filters over each photosite. Red, green and blue are captured with a single exposure. This form of image sensor is common in digital cameras used by the average consumer.
2. One-chip, three-shot cameras actually take three separate exposures of a shot: one each for red, green, and blue. The image sensor uses a different colored filter for each color. These cameras are more commonly used for studio photography, where the subject is not moving.
3. Two-chip cameras, use one image sensor to capture red and blue light and a second to capture green light. Less interpolation is required to generate the true colors of the image being photographed because the two sensors capture more information in each shot.
4. Three-chip cameras, as the name implies, utilize three image sensors. Each sensor utilizes a filter to make it sensitive to red, green or blue light. Inside these cameras, a beam splitter divides incoming images into three, full-frame images, with one image directed to each sensor. This type of digital camera is larger and more expensive but yields images with excellent color and high resolution.

NOTE:  This article offers a general introduction to digital camera technology. It is not intended to be an exhaustive study of optics or digital imaging technology. Digital imaging can encompass not only photography but also areas such as videography, medical imaging, computer graphics, movie animation, and scientific instrumentation, to name a few.

Glossary of Terms

Aperture: the adjustable opening that limits the amount of light passing through a lens.

Lens: a glass or plastic optical instrument that can refract light.

Shutter: the mechanism that opens and closes the lens aperture of a camera.