Mar 7, 2011
The two main memory card types used in cameras are CompactFlash (the big chunky one used in higher end Canon DSLRs and some Nikon DSLRs) and Secure Digital (the small fiddly one used in most other DSLRs). This information may also apply to other memory card types as well, but I’ve not had any experience with those.
The Mysterious Multiplier
CompactFlash cards (and less so, Secure Digital) always have a rating along the lines of 133x write speed or similar. This doesn’t actually represent any real-world value, since there isn’t a strict standard for how it’s measured and it’s likely to be the top speed of the memory card. To calculate its equivalent in megabytes per second (MB/s), multiply it by 0.15.
Secure Digital Class Wars
Secure Digital cards also carry a Class number that represents their minimum write speed of the card. So for a SanDisk card that states 30MB/s on the packaging and is Class 10, you’re actually going to get between 10MB/s and 30MB/s in reality.
When a GB isn’t really a GB
A common trick for storage is to use a non-standard method of calculating sizes. Storage manufacturers will take 1GB as being 1000*1000 bytes (1,000,000), whereas systems always calculate it as 1024*1024 bytes (1,048,576), so you’ll never really had that whole GB at all!
- SDHC (Secure Digital High Capacity) – Allows memory card sizes larger than 2GB, newer cameras generally support this (sometimes with a firmware upgrade)
- UDMA (Ultra Direct Memory Access) – Allows faster read and write speeds, but your camera needs to support this