SD cards are confusing. We all have SD cards in our digital cameras but what kind do we need? There are different suffixes and different speed ratings and different buses and even more different kinds of speed ratings. What do they all mean? Let’s sort out this mess.

There are essentially three things about an SD card that need to be defined: their storage capacities, their speeds and their types of connection. I won’t be talking about microSD cards here, just plain old SD cards (and the newer versions thereof). Nor will I be talking about SDIO, a rarely used variant on the SD card in which things like WiFi or Bluetooth are built in.

Size

The Secure Digital Card was developed as a better version of the Multimedia Card. It’s 50% thicker (so it won’t fit in an MMC slot) and has nine electrical contacts instead of seven. And it was introduced way back in 1999. At that time the thought of two gigabytes of storage on a solid state memory card was ridiculous. So the SD card was designed with that as the hard upper limit.

But, as we know, they got to 2GB soon enough and people began clamouring for more. So in 2006 a new higher standard with sixteen times the storage – 32GB – was introduced: SDHC. That stands for Secure Digital High Capacity. There were no physical changes, but their internal logic meant that these were not compatible with the SD standard. All SDHC card readers would work with SD cards. But SD card readers would not work with SDHC cards.

A mere sixteen-fold increase? Ridiculous. SDHC reeks of being a stop gap. Only three years later a new standard was released: Secure Digital eXtended Capacity. Otherwise known as SDXC. That pushed maximum capacity to two terabytes. That’s another factor of 60-ish increase. We’re still a little way from there, with SDXC cards just now beginning to crack 1TB at outlandish prices (I’d guess north of a thousand dollars).

Again, physically compatible but with internal differences. Again, an SDXC card required an SDXC compatible interface, but the interface works with SD and SDHC.

MMC SD SDHC UHS-II card primer

MMC card left, SD card centre, SDHC UHS-II card right. Above, a 32GB Lexar UHS-II SDHC card capable of 270MB/s write speed, 300MB/s read speed.

In addition, SDHC comes formatted with the FAT32 file system. SDXC comes formatted with the exFAT system. The exFAT format is supported by Windows (since Vista SP1 in 2008) and OS X (since Snow Leopard in 2009), but only by some Android phones. So if you’re going to access photos from your SDXC card on your Android phone, check for compatibility or stick with SDHC. Nearly everything supports FAT32.

Remember, those three sizes – a 2GB, 32GB and 2TB – are the maximums. There are smaller capacity cards in each category, and at least in theory a 1GB card could be SDXC internally, although that would never happen in practice.

Speed

All that’s pretty straightforward. What’s more complicated are the speed ratings. Complicated even further by the fact that in almost all cases, flash memory (ie. the storage in SD cards) can be read more quickly than it can be written to. The most important measure is write speed, because that determines what your camera can do.

Originally it was not super important. A digital still camera takes its picture and stores it in fast internal memory briefly while it does its image processing. Then it saves it to the SD card. Slow SD card writing would just mean it would take longer before you could take the next photo (assuming the internal memory couldn’t hold more than one picture).

But it matters much more now because cameras take video and that has to be piped pretty much straight to the storage card. Furthermore, 4K video is almost becoming routine, along with higher frame rates (60fps or even 120fps). So fast writing is important.

There are two different speed ratings with a third coming fast.

speed class c10 uhs u3 uhs II Lexar UHS-II SDHC card primer

The Speed Class (C10), UHS Speed Class (U3), and UHS (II) markings on a Lexar UHS-II 32GB SDHC card

First there’s Speed Class. This is marked by a number in a circle with a gap on the right hand side, like a stylised capital “C”. This one’s pretty straightforward. The normal Classes are 2, 4, 6 and 10, and in each case this matches the number of megabytes per second for write speed. That is, a Class 6 card can be written to at 6MB/s. That is sufficiently fast for recording 720p and 1080i video at typical bitrates.