Memory cards are as ubiquitous as memory itself—almost every mobile device you own has some type of built-in storage, from cameras to iPods to tablets, and almost every device has a slot for adding additional storage space. Storage on a device can range from a couple of gigabytes of memory to dozens of gigabytes. You’ll soon find, though, that it’s never enough.

Photographers Love CF

Photographers are a rare breed. From amateur to professional, photographers depend on gear to make a difference in their art, and some of them produce astounding works of art. Like painters, photographers don’t skimp on materials—they need memory that matches their instincts when shooting—fast, in the moment, and reliable. Photographers like to have their pictures as uncompressed as possible, and to view those pictures in brilliant colors and high resolution. A memory card to match that would also have to perform in the field, away from the studio. And that’s where CompactFlash® came in.

CompactFlash® used to be the standard for high-end DSLRs, some years ago. During the infancy of SD, SD cards could not handle the speeds or capacity needed to store RAW files. Many also believed that the substantial weight and bulk of CF cards made them tougher and more resilient, and you will find CF cards that have higher temperature resistances. But all things change, and today the CompactFlash association backs XQD cards as the official replacement for CompactFlash cards.

As such, the speed of CompactFlash cards has been stagnant for some time, and while they still are very fast cards, they aren’t the fastest around anymore. Still, many camera manufacturers continue to include CompactFlash card slots in their equipment because they are good enough, and CFast and XQD cards are still competing for dominance in post-CompactFlash-card world. UDMA 7 CompactFlash cards are the fastest, and have maximum read speeds of 167 MB/s. This is more than fast enough for most photography and videography uses. Also, because UDMA 7 has been around for a while, pretty much all CompactFlash cameras and card readers support it, which isn’t the case with the fastest SD cards.

UDMA rating The UDMA rating determines the maximum bus speed at which a card can read, assuming the memory in the card is fast enough to match it. UDMA 1 supports speeds up to 16.7 MB/s. UDMA 7 is ten times that speed, maxing out at 167 MB/s. This is the minimum sustained write speed—the slowest the card will write. This speed (represented in MB/s) is most important for videographers, since sudden drops in writing speed can cause dropped frames. This is a rather outdated way of expressing the max read speed. It is based on the read speed of audio CDs at 150 KB/s. You can figure out how fast an 800x card is in KB/s by multiplying 150 by 800 and convert KB/s to MB/s by dividing by 1,000 (the answer is 120 MB/s) you could also just go by a card’s stated 120 MB/s speed in figure 5. Max Read Speed this is the maximum read speed of the card usually given in Megabytes per second (MB/s). Note that cards rarely are able to sustain these speeds for long periods of time. Capacity CF cards range from 2GB to 512GB. Pick a card that can hold everything you need for a prolonged shoot; you won’t be misplacing these chunky memory cards easily.

The Rise of SD Cards

Next on the block were SD cards. SD stands for secure digital, and these cards quickly rose to the top of the memory heap due to their high capacities and improved speeds. In the beginning, however, a standard SD card could only handle a capacity of 128MB to 2GB. Standard SD cards are a rare breed these days, as the industry has moved on to SDHC and SDXC cards. The file format for SD cards was FAT16, originally. They can be used in any device that has an SD card slot.

So what are SDHC and SDXC? SDHC cards use a different file system, FAT32, which can handle capacities between 4GB and 32GB. SDXC cards use exFAT, and support capacities between 64GB and 2TB, though 2TB cards aren’t on the market yet. Hardware-wise, there are no differences between SDHC and SDXC cards; however, older SDHC card readers that aren’t able to understand the newer exFAT file format won’t read SDXC cards. SDXC-compatible card readers and cameras will always be backward compatible with SDHC and normal SD cards reader, though.

Traditionally, SD cards were always a few steps behind CompactFlash cards when it came to performance, but since CompactFlash cards haven’t been updated in a while, SD cards have recently passed them with UHS-II cards. UHS-II SD cards offer read speeds up to 312 MB/s, almost twice the speed of UDMA 7 CompactFlash cards. However, because they are so new, not many cameras and card readers support UHS-II cards, and while UHS-II cards are backward compatible, they won’t work at full speed with older card readers. In fact, most high-end DSLR cameras are still unable to write at UHS-II speeds.

microSD cards are essentially just miniaturized versions of full-size SD cards, and share all the same classifications. Like their full-size brethren, there are microSDHC and microSDXC cards; there are even UHS-II microSD cards, and they have the same speed-class ratings. However, due to their small size, they typically are a little slower and available in lower capacities than the top-of-the-line, full-size SD cards.

But do you need that much storage?

It depends. A 4GB SD card can hold about 280 RAW images and 1,500 high-quality JPEGs. A 128GB card would be a nightmare to manage, with almost 9,000 RAW images and 48,000 high-quality JPEGs. Many photographers prefer smaller-capacity cards that can be easily labeled and managed.

For non-photographers, adding SD memory can be a blessing, especially if your device is low on internal memory. Many tablets, for instance, ship with low internal memory (and is mostly used by the system software and internal processes) and having an extra slot to increase memory could mean the difference between watching a lot of HD movies and being stuck with low-res home video of your sister’s Sweet 16 birthday party.

Maximum Read Speed This is the maximum read speed of the card usually given in Megabytes per second (MB/s). Note that cards rarely are able to sustain these speeds for long periods of time. This is another (rather outdated) way of expressing the max read speed. It is based on the read speed of audio CDs at 150 KB/s. You can figure out how fast a 1000x card is in KB/s by multiplying 150 by 1,000 and converting KB/s to MB/s by dividing by 1,000 (the answer is 150 MB/s). You could also just go by a card’s stated 150 MB/s speed, in figure 2. Type This is the type of card; different card types use different file formats and newer cards won’t work in older card readers. UHS Speed Class Rating This is the minimum sustained writing speed of the card; important for video recording. UHS Speed class 3 cards will never write slower than 30 MB/s, UHS Speed class 1 cards never slower than 10 MB/s. Speed-Class Rating This is an older speed-class rating. It is redundant of the UHS speed class, but many card manufacturers include it, as well, since many consumer products still recommend products based on the old standard. A class 10 is the fastet of the old speed class ratings and a class 10 card is verified to never write slower than 10 MB/s, class 4 would be never slower than 4 MB/s. UHS Rating The UHS rating of a card determines the maximum bus speed at which a card can read, assuming the memory in the card is fast enough to match it. Non-UHS cards max out at 25 MB/s, while UHS-I cards support up to 104 MB/s, and UHS-II cards support up to 312 MB/s. Both the card reader and card must support the same standard to benefit from the increased speeds, but UHS cards are backward compatible with older readers—they just won’t be as fast in them. Capacity This is the card's capacity: SD cards range up to 2GB, SDHC cards range from 2GB to 32GB, and SDXC cards range from 32GB to 2TB.

So that is everything you need to know—for now—about the memory cards that you use every day. A couple of minor notes: SD cards are easier to lose, especially microSD cards, so get yourself a memory card wallet to hold your cards. Also, SD cards have a unique physical lock on the front of the card that prevents accidental erasure of its contents. Go forth and spread the news; memory cards are as intrinsic a part of your tech life as anything else, and shopping smartly can shave dollars from your tech budget.

Editor's note: This article has been revised as of September 1, 2015.