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Once you’ve viewed your image through the viewfinder, composed and focused it with your lens, and captured the photons with the sensor, the final step is to store the digital image semipermanently so it can be transferred to your computer for viewing, editing, or printing. While the kind of storage you use in your camera won’t directly affect the quality of your image, it can impact the convenience and versatility of your dSLR, so storage is worth a short discussion.

Once converted to digital form, your images first make their way into a special kind of memory called a buffer, which accepts the signals from the sensor (freeing it to take another picture) and then passes the information along to your removable memory card. The buffer is important because it affects how quickly you can take the next picture. If your camera has a lot of this very fast memory, you’ll be able to take several shots in quick succession, and use a burst mode capable of several pictures per second for five or six or ten consecutive exposures.

Many digital SLRs provide a viewfinder readout showing either how many pictures can be stored in the remaining buffer or, perhaps, a flashing bar that “fills” as the buffer fills and gets smaller as more room becomes available for pictures. When your buffer is completely full, your camera stops taking pictures completely until it is able to offload some of the shots to your memory card.

The buffer is such a limitation on sequence photography that Nikon has introduced a dSLR that crops the center out of an image (creating an 6.8 megapixel picture out of a 12.4 megapixel photo) simply because the smaller images can be moved through the buffer more quickly. Nikon touts this feature as part of its faster burst mode.

The memory card itself has its own writing speed, which signifies how quickly it can accept images from the buffer. There’s no standard way of expressing this speed. Some card vendors use megabytes per second. Others label their cards as 40X, 80X, and so forth. Some prefer to use word descriptions, such as Standard, Ultra, Ultra II, or Extreme. I’m not going to tell you which cards are fastest here, because memory card technology and pricing is changing with blinding speed.

Google the Web for sites that have comparisons of speeds for various current memory cards before you buy. In recent months, the trend has been toward faster and faster memory cards at lower prices. That’s the main reason for the dropping price tags on those 1GB cards I bought. Both were older “standard” cards that were considered outmoded in a time when the leading vendors were pushing 2GB and larger “ultra” cards.

For standard shooting, I’ve never found the speed of my digital film to be much of a constraint, but if you shoot many action photos, sequences, or high-resolution (TIFF or RAW) pictures, you might want to compare write speeds carefully before you buy. A card that’s been tested to write more quickly can come in handy when you don’t have time to wait for your photos to be written from your camera’s buffer to the memory card. What I always recommend is to buy the fastest memory card you can afford in a size that will hold a decent number of pictures.

Then, purchase additional cards in larger sizes at bargain prices as your backups. For example, if you’ve got a lot of money to spend, you might want to buy a 2GB “ultra” card as your main memory card for everyday shooting, and stock up on slower, but dirt-cheap 1GB cards to use when your main card fills up. Or, if your budget is limited and you don’t need a high-speed card very often, spend your money on a 1GB or larger standard card, and treat yourself to high-speed media in a more affordable size, such as 512MB. That way, if you do need the extra-fast writing speed of an ultra card, you’ll have it without spending a bundle on a high-speed/high-capacity memory card. And you’ll have plenty of capacity in your standard digital film at an economical price.

You certainly won’t be choosing your digital camera based on the kind of storage it uses. Digital SLRs generally rely on CompactFlash for the most part, because CompactFlash media is small enough to be carried around easily and always seems to be on the leading edge of capacities.

Smaller form factors, such as the postage stamp-sized Secure Digital card, are not found in as many advanced cameras, and are annoyingly easy to misplace. However, SD cards are rapidly catching up to CompactFlash in capacities, and I’d expect many digital SLRs to have slots for both in the future, similar to the Canon EOS-1Ds Mark II. Among non-SLR, point-and-shoot digital cameras, the various film card formats are proliferating at an alarming rate. Only the first three formats listed below are likely to see widespread use in digital SLRs, however.

■ CompactFlash. Among all digital cameras, CompactFlash is the second-most-favored format in the United States and first among digital SLRs. Although larger in size than Secure Digital (SD) cards, CompactFlash cards are still very small and convenient to carry and use. As larger capacities are introduced, they usually appear in CF format first. As a bonus, the CompactFlash slot can also be used for mini hard drives, such as those from IBM, with capacities of a gigabyte or more.

■ Secure Digital. The SD format overtook CompactFlash as the most popular memory card format in digital cameras with one of the last bastions, Nikon, finally adding an SD card camera to its compact point-and-shoot line. Most other vendors had long since converted for their compact digitals, although Canon continues to offer beginner cameras that use CompactFlash. The postage-stamp-sized SD cards allow designing smaller cameras, are available in roughly the same capacities as CompactFlash, and cost about the same. The chief drawback (to date) is that there are no mini hard drives in the SD format. If you want to use a mini hard disk, you’ll need a camera with a CompactFlash slot. Some digital cameras can also use the similar, but slower Multimedia Memory card (MMC). Figure 2.18 shows both CompactFlash and SD memory cards.

■ Mini hard drives. For a long time, mini hard drives were your only option when you needed more than a gigabyte of storage. If you’re using a 6 megapixel or better camera and like to save your images as TIFF files or in another lossless format, you need more than a gigabyte of storage. However, with CompactFlash cards now available in 4GB to 8GB sizes, the mini hard drive is losing its capacity edge, and they have always cost more than the equivalent silicon memory card. Although not excessively prone to failure, mini hard drives do have moving parts and must be handled with more care than memory cards.