Using a memory card with a “write” speed rating faster than what your camera supports doesn’t increase how quickly your images are processed and stored. Not only does the card have a maximum speed rating, but so does your camera. When combined, it’s the slower of the two write speeds that determines your maximum throughput.
So it might seem like using a card that’s faster than your camera is overkill. Well, not if you consider the process of downloading images from the card to your computer. Here, the “read” speed comes into play, and the faster the card, the better (provided you use a card reader and not the camera to download your images). This is where the read speed matters.
Memory-card speeds are generally rated in terms of “x”—40x, 133x, 266x. Each x equals a minimum sustained transfer rate of 150 KBps, and all other speeds are expressed as multiples of that speed. For example, 40x is 6 MBps (150 KB x 40 = 6,000 KB or 6 MB), 133x equals 20 MBps, and 266x is 40 MBps. But pay attention to how that x rating is defined. Some memory-card manufacturers describe it as read or write speed, while others mean both and call it a “data transfer” rate. Read-speed times are often higher than write-speed times.
Say, for example, your camera shoots 3.6 MB images in highest-quality JPEG mode. A 40x card would transfer 1.67 images per second and a 133x card at 5.5 images per second, if your camera can handle those rates. Most D-SLRs today can, but if your camera is older, you should check the manual or go to the manufacturer’s website to determine the maximum capacity. This isn’t as easy as it sounds. Most camera makers tell you what kind of card format to use, but they don’t tell you what speed.
Conventional wisdom says that if you’re using a digital camera with up to around a 5-megapixel sensor, a standard-speed card rated at less than 45x will work best. Newer and higher-end cameras with image sensors over 5 megapixels benefit from cards rated over 45x. Besides fast write times, these cards perform better when shooting in continuous burst mode or recording high-res videos.