With traditional cameras, the film both records and stores the image. With digital cameras, separate devices perform these two functions. The image is captured by the image sensor, then transferred to a storage device of some kind. These devices are only designed for temporary storage. At some point you transfer the images from them to a computer, erase the device, and reuse it.

Older and less expensive cameras have built-in fixed storage that can't be removed or increased. This greatly reduces the number of photos you can take before having to erase them to make room for new ones.

Almost all newer digital cameras use some form of removable storage media, usually flash memory cards, but occasionally small hard disks. Whatever its form, removable media let's you remove one storage device when it's full and insert another. The number of images that you can store in a camera depends on a variety of factors including:

• The number of storage devices you have and the capacity of each (expressed in Megabytes or Gigabytes).
• The resolution or image format used to capture images.
• The amount of compression used.

The number you can store is important because once you reach the limit you have no choice but to quit taking pictures or erase some existing ones to make room for new ones. How much storage capacity you need depends partly on what you use the camera for. If you're used to shooting 5 or 6 rolls of standard film on vacation, your camera should be able to store the same number of images or you'll be out of luck.

There is an old set up line for a joke that begins "I have good news and bad news." The good news is that we have these memory cards at all. The bad news is that they come in a variety of formats that are not interchangeable. Once you have a sizable investment in memory cards, you are locked into using only those cameras that support your format or buy a new set of cards.

Over the past few years a variety of memory cards have come and gone. At the moment there are two types in widespread use— Compact Flash and Secure Digital (SD). These flash memory cards store your image files on memory chips that are similar to the RAM chips used inside your computer but there is one important difference. They require no batteries and don't loose images when the power is turned off. Your photographs are retained indefinitely without any power to the memory card. These chips are packaged inside a case equipped with electrical connectors and the sealed unit is called a card. Flash memory cards consume little power, take up little space, and are very rugged. They are also very convenient; you can carry a number of them and change them as needed.

• CompactFlash (CF) was developed by SanDisk Corp and these cards are about the size of a matchbook.
• Secure Digital (SD) cards are smaller than CompactFlash cards and are used in over half of current camera models. However there are compatibility problems since more recent 2GB and larger cards look the same as older cards, but use an SDHC (Secure Digital High Capacity) format that isn't supported by older devices.
• MultiMedia (MMC) cards are also small cards used in a few pocket cameras.
• Memory Stick™, a proprietary format from Sony Corporation, is shaped something like a stick of gum. These cards are used only in Sony products.
• Hard drives like Hitachi's Microdrive and Sony's Compactvault are high speed, high capacity hard disk drives. These drives are so small they can be plugged into a Type II CompactFlash slot on a digital camera or flash card reader. (Type I CompactFlash slots are thinner).
• One-time use flash cards were introduced with the idea that flash memory was so inexpensive and users so clueless that they'd rather leave their photos on a card then copy them to a computer. If you are as careless as I am, in a few years you'll have a very large investment in a pile of unlabeled cards and images you can't find and can't share.

One thing to consider is the "speed" of a card. Many companies sell regular and high-speed versions. The high-speed versions, with their high profit margins, usually benefit the manufacturer more than the user. Unless you are a Sports Illustrated photographer shooting large images in continuous mode at the Super Bowl you may be better off investing elsewhere in your system.

When you first buy a memory card or use it in a different camera you should format it. Every camera that accepts these cards has a Format command listed somewhere in it's menus. Formatting prepares the card for use in a camera and reformatting it when you change cameras just ensures the card will be accurately written to and read in that specific camera. You may also find that formatting a card that has developed problems will also fix it. Just be aware that the Format command will erase all of the images from the card. Should you ever do this by mistake, there is digital image recovery software available.

Some cameras come with software that lets you connect the camera to the computer (called tethering it) and operate it from there. Captured images can be stored on the computer's hard drive instead of on the camera's memory card. Although this approach is frequently used in the studio, it's also occasionally used by landscape photographers when they want to immediately evaluate images on the computer's much larger screen.