The digital camera category covers a broad range of devices that use imaging chips--sensors or arrays--to capture a picture. Handheld cameras use smaller chip architecture, and use either a CCD (charge-coupled device) or a CMOS (complementary metal oxide semiconductor) sensor. Larger, more costly CCD arrays are found in studio and professional digital cameras, which may be of the single shot or scanning array type.
The image capture part of the digital camera may be integral to the camera, or in the form of a separate back that attaches to a conventional camera. The single shot usually works with sensors coated with color filters to capture an image with one exposure. A three-shot back--only used with high-end professional cameras-- works with a three-pass system--one for each color (RGB for red, green, blue). The full-color image is then processed or composited after the three passes are made.
Whatever the sensor and exposure type, digital cameras are qualified by the pixel count the sensors deliver. Those that deliver under 1 million pixels are considered best for web page (or screen only) images, and snapshot size prints. Those that yield over 1 million pixels, dubbed megapixel cameras, can be used for a wider variety of output, including moderately-sized prints, and are aimed at the avid amateur and business (real estate, insurance, etc.) user. Multi-megapixel cameras, those that are capable of delivering image file sizes over millions of pixels, are used by professional studio photographers, scientists and artists to create full-color illustrations for magazines, posters and all manner of printed images. As might be expected, multi-megapixel systems can cost many times more than the snapshot variety. Any camera offering more than 2 megapixels is good for making prints up to 8x10 inches with near-photographic quality.
Digital cameras have both mechanical and electronic shutters, though the electronic shutter is more common. Focusing systems can be manual (an option in digital SLRs and professional backs), automatic (as part of the camera's autofocusing system), or focus-free (a set focusing distance that can not be varied.).
Some digital cameras are regarded as "computer peripherals"; no image can be seen without digital image processing performed with a computer. Most handheld or point and shoot models have image processing setups built in, and can send images to the computer that are ready to print or send via the Web. In either case, however, one cannot "see" the image after processing without a viewing device. The same could be said of images produced by a conventional camera--without film processing there would be no images as well. The difference is what can be observed without the use of a machine when the processing is done.
The "film" of the digital camera is the memory card, a removable card about the size of a book of matches that holds the image information. The card can be swapped out at any time for downloading or exchange, and after use can be erased for re-use again. Digital cameras have evolved quickly to the point where even moderately priced digicams deliver power zoom lenses, on-board monitors, sophisticated metering and exposure controls, on-board menus for custom picture creation and high-density sensors that can yield photographic quality prints. There are dozens of digital cameras for general (non-professional) use on the market today. Here are some differentiating points:
Sensor or Pixel Count: Simply stated, the greater the number of pixels the sensor delivers the greater the ability to make photographic-quality prints. For those who want to just use a digital camera for Web images and an occasional small print, a VGA-type or under 1 million pixel chip in a digital camera will be good enough. If you want to make prints from your camera, and sometimes those prints will exceed 5x7 inches in size, a digital camera with over 2 million pixels is a good choice. (These cameras are often referred to as 2 megapixel cameras.)
Image Storage: There are two main divisions here--those digicams that hold images in their internal buffer and those that transfer them to a removable memory card. With a camera that does not use the card system, you have to download the images whenever the camera buffer is filled before you can make any more pictures. If you use a camera that has a removable memory card you can swap cards whenever the card in the camera is filled, and after downloading re-use the card again and again. Most people seem to consider the convenience of memory cards worth the extra price.
Monitor: If you want to preview your pictures right after they are made, a digicam with an on-board LCD monitor is the best bet. The monitor is also the place where you access the camera menu for exposure control, white balance, picture rendition options and albuming. Less expensive digicams have no monitor and do not have extensive options.
Compression Choices: One of the options available with some digicams is a range of compression choices. The more the image can be compressed the greater the number of images can be stored in the camera buffer or memory card before downloading or card swapping. Greater compression, however, means lower pixel counts on the resultant image. Some cameras allow you to make uncompressed images. While this limits the number of images you can make before downloading or card swapping, it does mean that you can get excellent quality images for large prints from a digicam. As with many options, the end use of the image is what really counts. If you're shooting just for the Web you can get away with smaller (more compressed) image files; if you're shooting for prints then less compression is desirable. If you do both, choose a camera that offers a wide range of choices. For example, you might want to get a camera that allows for various shooting modes (Fine, Good and Basic) and compression schemes that include uncompressed (TIFF), and JPEG at 1:4, 1:8 and 1:16.
Ease of Downloading: The reason you use a digicam is for instant digital images without conversion from film. To work on and send or share the image you have to get it into a computer. Some digicams allows for a direct patch to the computer via parallel, serial or USB connection, or even use a SCSI adapter. Some even offer infrared download. Perhaps the easiest method is to use a memory card and use a card reader, a drive into which you put the memory card to patch directly to the computer. This eliminates the need for direct wiring between camera and computer.
Software: Many digicams come bundled with third-party or proprietary software that helps with picture storage, editing and manipulation. While this is a bonus that should not be overlooked, it should not be the prime reason for buying a particular camera. The camera-to-computer acquisition software, however, should be considered. A clumsy interface can slow up work or make using the camera less than pleasant. Be sure to research this handshake software carefully by reading magazine or net reviews of the camera before buying.
Lens: As with point and shoot cameras, digicams come in either fixed focal length or zoom lens varieties. You may also see high zoom ratios attained through something called "digital zoom". This can come as high as 48X and higher. While this may come in handy in some situations it does not offer the quality of a true optical zoom lens. Pay more attention to the optical (prime lens) zoom ratio than the digital zoom capability.