Image Storage

Film combines two discrete functions—it is both a sensing and storage medium. A digital camera separates the two functions, leaving you with the need for a storage system. Most digital cameras use flash memory to store the images they capture.

Inexpensive cameras have only built-in memory, which allows them to hold a finite number of images—typically about a dozen—before you need to download them to a computer or printer.

Better digital cameras use a memory card of some type. These cards are removable so that you can quickly exchange cards to take more pictures, much like loading a new roll of film. In fact, some vendors of memory cards call them digital film, which they are not. You can also slide out a memory card and slide it into a dedicated card reader to transfer the images into your computer. Chapter 16, "Memory," describes the various card types.

To increase the capacity of each card, most digital cameras give you the option of storing your images at two or more quality levels, which trade good looks for more compact storage. (Almost no one uses native or uncompressed mode to store digital camera images—with only a couple of shots, a memory card is full.) Although some early digital cameras distinguished their quality levels by their actual resolution, more modern cameras operate at one or two basic resolutions and trade off higher compression for greater storage capacity. When decoded, the more highly compressed images yield lower quality—they typically look fuzzier—but nevertheless display at the basic resolution level of the camera.

The compression method of choice is JPEG, which stands for the Joint Photographic Experts Group, a committee that hammered out standards for the algorithms. JPEG uses lossy compression, which means that some information gets sacrificed in the effort to shrink file size. The lost information can never be regained, so once an image is compressed, its original quality can never be restored.

At moderate levels of compression, the losses made by JPEG compression show up as a loss of image sharpness. However, if you examine a reconstructed JPEG image at high magnification, you can see patterns that arise from the compression algorithm. As you increase the JPEG compression ratio, these artifacts become obvious, the image loses definition, and it finally falls apart so that your eye can no longer make sense from it.

The storage or operating modes of most modern digital cameras correspond to the degree of JPEG compression used. The best quality mode—often called exactly that—uses the least compression, typically file size reduction by about a factor of four. The next increment down may compress by a factor of 12. Economy or low-quality mode may compress by a factor of 36 or more. Sometimes a camera's economy mode ratchets resolution down by one giant notch—for example, to VGA resolution, 640 by 480 pixels.