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Tuesday, June 19, 2012

DPP Solutions: Decoding High-Speed Memory Cards

You need the right card to perform up to your camera’s full capabilities

Left to Right: PNY 64 GB SDXC card; Lexar 32 GB CF card; Hoodman 16 GB CF card

High-megapixel continuous still shooting, especially RAW, and full HD video require fast memory cards. If your card is too slow, continuous still burst rates will bog down and video recording may just stop. We'll go through the specs to help you find the right card for your needs.

X Ratings
There's a common way to quantify memory card speeds (although not all card manufacturers use it): the X rating. One "X" is the old standard CD transfer rate of 150 KB/sec., or 0.15 MB/sec. Thus, 100X is 15 MB/sec., 200X is 30 MB/sec., 400X is 60 MB/sec., 600X is 90 MB/sec., and 1000X is 150 MB/sec. Note that these are maximum read rates; often, the maximum write rate is slower—and write rate is what you're concerned with in-camera.

Bear in mind that system speed depends on both the camera and the card. Putting a slow card in a fast camera will slow things down, but putting a fast card in a slow camera won't speed things up. For best burst and video performance, you want to make sure you use the fastest cards your camera can handle, but there's no point in using a card that's faster than your camera's writing rate. However, you can always use a fast card in your next, faster camera.

CompactFlash cards were introduced in the mid-1990s, and they're the largest of the media used in current DSLRs at 43x36mm. There are two types: I (3.3mm thick) and II (5mm thick). All cameras that accept CompactFlash can use Type I cards, but not all can use the thicker Type II. Newer CF cards are all Type II, including the fastest ones. Currently, CF cards are available in capacities of 2 GB to 256 GB, with speeds of up to 1000X (150 MB/sec. read, 80 MB/sec. write).

The original SD cards came out in 1999 in 2 GB and 4 GB capacities. In 2006, the SDHC (high-capacity) cards were introduced, with the same small size, but capacities of 4 GB to 32 GB. In 2009, SDXC (extended capacity) cards were introduced, with capacities from 32 GB to a potential 2 TB (128 GB is the highest capacity currently available).

SDHC and SDXC cards have two different speed rating systems, one based on High Speed Bus (Class 2, 4, 6 and 10) and one based on Ultra High Speed Bus (UHS-I, UHS-II). Cards rated Class 6 or 10, and UHS-I or UHS-II, are ideal for full HD video and high-speed still shooting. Theoretical maximum read and write rates for SDHC and SDXC are 104 MB/sec. (UHS-I) and 312 MB/sec. (UHS-II), but the top speed to appear in a card so far is 633X (95 MB/sec. read, 90 MB/sec. write). UHS-I and Class 10 card speeds range from 30 MB/sec. to 95 MB/sec., so check the particular card you're buying for specifics. Class 6 is 20 MB/sec.

Sony Memory Stick
Sony introduced its own Memory Stick media in 1998. Current Sony DSLRs and mirrorless cameras can use Memory Stick PRO Duo and PRO-HG Duo media, both measuring 31x20x1.6mm, with capacities of up to 32 GB and speeds of up to 50 MB/sec. These cameras also will accept SD, SDHC and SDXC memory cards via the same card slot.

The newest memory card type is XQD, introduced in 2011 and currently used only by the Nikon D4 DSLR (which also has a slot for CompactFlash cards). XQD cards are fast—up to 125 MB/sec. read and write speeds—and are available in 16 GB and 32 GB capacities (only from Sony so far). Dimensions are between CF and SD: 38.5x29.8x3.8mm.


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