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1. There Are Two Different Speed Ratings That Matter To Still Photos And Video:

When you’re shooting stills, pay attention to the maximum transfer speed, expressed in megabytes per second. But for video, what counts is the Speed Class rating, listed as Class 2, 4, or 6. This designates the minimum sustained write speed for video applications, whether you use a camcorder with flash-memory or the video function of a digital compact camera or DSLR. For standard definition, Class 2 is all you need, but if you’re shooting high-def, spring for Class 4 or 6 (which cost $10-$15 more).

2. If You Have The Right Camera, A Faster Card Can Boost The Burst:

Some DSLRs, such as the Nikon D90, can take more successive images when loaded with a fast card, particularly the 30MB/sec SanDisk Extreme III SDHC card. You could also potentially get a faster burst rate with the right card/camera combo. For example, with any brand of UDMA CompactFlash card, the Sony Alpha 700 captures images at 5 frames per second, instead of 4.8 fps with a non-UDMA card.

3. You Can’t Take Full Advantage Of The Fast Speed Of A Memory Card Unless You Have A Fast Enough Card Reader, Too:

Sure, a top-rated card will allow your DSLR to recover more quickly so you can go back to shooting after a big burst of images. But the other main benefit of these cards is their faster transfer speed when uploading images to a computer. For instance, the Lexar Multi-card USB 2.0 Card Reader ($24, street) transfers images at up to 480Mb/sec, sufficient for many types of consumer CF, memory stick, and SD cards, while the Lexar Professional UDMA FireWire 800 Reader ($75, street) transfers data at up to 800Mb/sec, better for a Lexar Professional UDMA 300x card or other Professional CF cards.

4. Always Format Your Memory Card For The Camera You’re Using:

If you swap the card between cameras, you could lose what’s on the card, or risk corrupting the new pictures you’re taking. And, of course, before you format, make sure you’ve offloaded the photos you just took.

5. Then Again, Your Pictures Might Still Be There, Even If It Looks Like They’re Gone:

While reformatting your card wipes all the data-deleting doesn’t. So if you think you’ve lost some images but you haven’t formatted your card since the mess-up, rescue software might save them. If your card comes with it, use that. With a SanDisk Extreme Card, you can download free RescuePRO data recovery software from Lexar Image Rescue 3 ($29, direct; free with some pro-level cards; works with any brand of memory card.