Memory Card Ratings
Q. I notice labels like “Class 2” and “Class 4” on Secure Digital memory cards. What do these mean?
A. The class rating specified on the Secure Digital card refers to the minimum sustained speed at which the card can continuously record video data. While this may not mean much for snapping random pictures, the sustained transfer rate is more important for camcorders and cameras that can shoot movie clips.
The number in class rating — commonly Class 2, Class 4, Class 6 or, recently, Class 10 — refers to the rate of megabytes transferred.
For example, a Class 2 card moves a minimum of two megabytes of data per second, while a Class 10 card can transfer a minimum of 10 megabytes per second. (The SD Card Association has the technical details on class ratings at www.sdcard.org/developers/tech/speed_class.)
Depending on the camera and its own specifications, using a card with a lower rating when trying to shoot high-definition video could result in error messages, dropped frames and uneven playback. Check the manual for your camera or camcorder to see what cards are recommended for your model; for high-definition video, a Class 4 card or higher may be required.
The class rating is separate from the card’s speed rating, which is also printed somewhere on the packaging or label. The speed rating looks something like “up to 15MB/s Read, 9MB/s Write” in the card’s fine print.
These numbers refer to the maximum transfer speed for data being read and written to the card. Read speed (in this case, 15 megabytes per second) is the amount of time it takes to transfer photos and videos from memory card to computer. Write speed (9 megabytes per second in this example) refers to how quickly data can be recorded onto the card from the camera.
Like class ratings, memory-card speed ratings may not matter as much for casual photography with point-and-shoot cameras.
Cards with faster speed ratings are sometimes recommended for digital S.L.R. cameras that can shoot high-resolution pictures in continuous bursts.
