SD/SDHC card speed class ratings – what they mean
While SDHC cards show they capacity very clearly, their speed can be a little harder to understand.
SDHC cards are sold by their capacity and their write speed. The write speed is measured in megabytes per second (MB/sec). And the “C” symbol on the card opposite tells you its speed class rating – in this card, this card is a “Class-2″ type.
The class rating number indicates its minimum rated write speed in MB/second. So a Class-2 card has a minimum sustained write speed of 2MB/sec. It may well be faster than that but it will be at least 2MB/second.
At present, there are three major class ratings – Class-2 (2MB/sec), Class-6 (6MB/sec) and Class-10 (10MB/sec).
There’s also a Class-16 (16GB/sec) option coming shortly.
If you’re buying for high-definition video capture, you need to have a Class-6 card minimum to ensure the card is fast enough to capture the video data without dropping anything.
But that’s how the SDHC class speed rating system works.
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