The amount of detail that a camera can capture is called the resolution, and it is measured in pixels. The more pixels a camera has, the more detail it can capture and the larger pictures can be without becoming blurry or "grainy." High-end consumer cameras can capture over 12 million pixels. Some professional cameras support over 16 million pixels (megapixels), or 20 million pixels for large-format cameras. For comparison, it has been estimated that the quality of 35mm film is about 20 million pixels.



Exposure and Focus

Just as with film, a digital camera has to control the amount of light that reaches the sensor. The two components it uses to do this, the aperture and shutter speed, are also present on conventional cameras.

Aperture: The size of the opening in the camera. The aperture is automatic in most digital cameras, but some allow manual adjustment to give professionals and hobbyists more control over the final image.

Shutter speed: The amount of time that light can pass through the aperture. Unlike film, the light sensor in a digital camera can be reset electronically, so digital cameras have a digital shutter rather than a mechanical shutter.

These two aspects work together to capture the amount of light needed to make a good image. In photographic terms, they set the exposure of the sensor.

In addition to controlling the amount of light, the camera has to adjust the lenses to control how the light is focused on the sensor. In general, the lenses on digital cameras are very similar to conventional camera lenses -- some digital cameras can even use conventional lenses. Most use automatic focusing techniques.

The focal length, however, is one important difference between the lens of a digital camera and the lens of a 35mm camera. The focal length is the distance between the lens and the surface of the sensor. Sensors from different manufacturers vary widely in size, but in general they're smaller than a piece of 35mm film. In order to project the image onto a smaller sensor, the focal length is shortened by the same proportion.

Focal length also determines the magnification, or zoom, when you look through the camera. In 35mm cameras, a 50mm lens gives a natural view of the subject. Increasing the focal length increases the magnification, and objects appear to get closer. The reverse happens when decreasing the focal length. A zoom lens is any lens that has an adjustable focal length, and digital cameras can have optical or digital zoom -- some have both. Some cameras also have macro focusing capability, meaning that the camera can take pictures from very close to the subject.

Digital cameras have one of four types of lenses:

1) Fixed-focus, fixed-zoom lenses - These are the kinds of lenses on disposable and inexpensive film cameras -- inexpensive and great for snapshots, but fairly limited.

2) Optical-zoom lenses with automatic focus - Similar to the lens on a video camcorder, these have "wide" and "telephoto" options and automatic focus. The camera may or may not support manual focus. These actually change the focal length of the lens rather than just magnifying the information that hits the sensor.

3) Digital-zoom lenses - With digital zoom, the camera takes pixels from the center of the image sensor and interpolates (alters) them to make a full-sized image. Depending on the resolution of the image and the sensor, this approach may create a grainy or fuzzy image. You can manually do the same thing with image processing software -- simply snap a picture, cut out the center and magnify it.

4) Replaceable lens systems - These are similar to the replaceable lenses on a 35mm camera. Some digital cameras can use 35mm camera lenses.

Storage of Images

Most digital cameras have an LCD screen so you can view your picture right away. This is one of the great advantages of a digital camera -- you get immediate feedback on what you capture. Of course, viewing the image on your camera would lose its charm if that's all you could do. You want to be able to load the picture into your computer or send it directly to a printer. There are several ways to do this.

Although most of today's cameras are capable of connecting through serial, parallel, SCSI, USB, or FireWire connections, they usually also use some sort of removable storage device. Digital cameras use a number of storage systems. These are like reusable, digital film, and they use a caddy or card reader to transfer the data to a computer. Many involve fixed or removable flash memory. Digital camera manufacturers often develop their own proprietary flash memory devices, including SmartMedia cards, CompactFlash cards and Memory Sticks. Other removable storage device include floppy disks, hard disks (external, or microdrives), and writeable CD's and DVD's.

Regardless of what type of storage they use, all digital cameras need lots of room for pictures. They usually store images in one of two formats -- TIFF, which is uncompressed, and JPEG, which is compressed. Most cameras use the JPEG file format for storing pictures, and they sometimes offer quality settings (such as medium or high).

To make the most of their storage space, almost all digital cameras use some sort of additional data compression to make the files smaller. One compression routine takes advantage of patterns that repeat. The image can be reconstructed exactly as it was recorded, reducing the file size no more than 50%, often much less. Another compression routine called irrelevancy eliminates some of the more meaningless data, taking advantage of the fact that digital cameras record more information than the human eye can easily detect.