SD Memory Cards: Everything You’ve Ever Wanted to Know About SD Cards But Were Afraid to Ask

If your digital camera is fairly new, odds are it stores images on SD cards. SD stands for Secure Digital but it may seem more like Strange & Discombulationg because of the permutations that abound. This primer will school you on all you need to know about this efficient and progressive format, and help you make better purchasing decisions when it’s time to buy more cards.

Some older cameras, DSLRs in particular, use CF (Compact Flash), Memory Stick variants or combinations thereof. Some very old cameras use xD cards or even SSFDC. Perplexing? It can be like trying to remember which replacement bristles your electric toothbrush uses. A recent search on B&H’s website revealed 19 types of memory cards spanning 14 brands. The counts at Adorama were about the same. Five of them include the letters SD in their name.

Physical Characteristics
SD cards are non-volatile memory cards that are user-exchangeable. They measure 24 x 32 mm and are 2.1 mm thick. All cards have nine pins (electrical contacts) and a write-protect switch. All flavors operate on 2.7 to 3.6 volts. UHS-II and UHS-III cards (which we’ll get to in a minute) have slightly different specifications, including a second row of pin interfaces.  

For the purpose of this article we’re skipping the small-size versions of SD, namely MiniSD and MicroSD, except to say that they are physically smaller. The logic and electronics are the same as in SD memory cards. Commonly used in cell phone and other non-photo devices, they can sometimes be upscaled to the standard SD form factor via an adapter to facilitate download.

SD stands for “Secure Digital,” SDHC stands for “Secure Digital High Capacity” and SDXC stands for “Secure Digital eXtended Capacity.” They differ in a) maximum capacity, b) file system architecture and c) compatibility direction. SD cards use FAT16, SDHC use FAT32 and SDXC use exFAT. FAT is an acronym for File Allocation Table, the blueprint for data storage.

SD, SDHC and SDXC Capacity Limitations
SD cards, which first appeared nearly 20 years ago, have a maximum capacity of 2GB. Keep in mind that 20 years ago, 2GB seemed boundless. SDHC cards, which appeared almost 10 years ago, busted that upper limit with their ability to hold up to 32GB of data. SDXC can store between 64GB and 2TB. Someday soon, 2TB will seem inadequate.

Simple, once you study it. Cameras marked SDXC can use any full-size SD card, (SD, SDHC and SDXC). Cameras marked SDHC can use SDHC and SD cards only. And cameras marked SD can use SD (and only SD). (Once again, we’re ignoring MicroSD and MiniSD.)

Conversely, SD cards work in all cameras that accept full-size SD variants. SDHC cards work in all cameras marked SDHC and/or SDXC. And SDXC cards work in SDXC cameras only.

Most cameras manufactured within the last ten years or so can handle SDHC cards. It’s important to note that many older cameras originally designed for SD (or for SDHC) have been modernized to accept the newer SD flavors through camera firmware updates. If you have an older product, make sure the firmware is up-to-date and review the firmware version history to learn if any of this applies to you.

Speed Ratings
Let’s unpack the confusion surrounding Speed Class vs. Speed Ratings vs. X Speed vs. UHS Speed Class and the new Video Speed Class.

The Speed Rating indicates the maximum speed for reading and writing images to and from a memory card in megabytes per second. When you see a rated speed written as 15MB/sec or 90MB/sec, for example, that number indicates the maximum speed of the card—and the performance you can typically expect to experience in real-world usage of writing or reading files on the card.

X-Speed traces its roots back to the advent of the CD ROM player, where 1X indicated a transfer rate of 150 kilobytes per second, 4X equaled 150 x 4, or 600 KB/second and so forth. So when you see a number like 266X, multiply the numeral by 0.15MB/sec and the result then equates to the Speed Rating explained in the paragraph above. In this example, 266X equals 40MB/sec.

Speed Class, defined as Class 2, 4, 6 and Class 10, specifies the minimum sequential write speed also measured in megabytes per second. Class 4 cards, for example, perform at a rate of four (4) megabytes/second. Class 10, then are capable of 10 megabytes/second. These are worst case scenario performance standards and are therefore more trustworthy.

UHS Speed Class is indicated by the numeral 1, 2 or 3 positioned inside a larger letter “U.” UHS Speed Class 1 corresponds to a minimum sequential write speed of 10 megabytes/second. UHS 3 equates to 30 MB/sec transfer speed.

SD Card Speed Class Chart ©SDA and used with permission of the SD Card Association. Visit for more information.

The newer Video Speed Class was defined to support high resolution and 4K8K video recording, and to support future generation flash memory. The Video Speed Class increments are V6, 10, 30, 60 and V90. The numbers correspond to the data transfer rate; e.g., V30 clips along at 30 MB/second.

Card Readers Matter
Needless to say, to make the most of fast SD cards you need a fast card reader connected by the latest, fastest transfer protocol like USB 3.0. Otherwise you’re connecting a fire hydrant to a garden hose.

The speedy USB 3.0 Delkin Devices DDREADER-46 accepts Class 10, UHS-I and UHS-II formatted SD, SDHC, SDXC, microSD, microSDHC and microSDXC cards and costs about $8. Fits in a shirt pocket, too.

What Capacity is best?
Are higher density cards less reliable? Are you safer using multiples of small capacity cards, or one big fat one? Based on extensive but woefully unscientific research I conducted with engineers and product managers from all of the major card manufacturers, the consensus is that one high capacity is just as safe and sensible as a bunch of smaller cards. In fact, in one sense it’s better to have one ginormous card which stays in the camera instead of a handful of little ones that could possibly become lost.

What capacity SD card do you really need? For some photographers, the size of their SD card is their badge of competence. Others buy whatever is least expensive. If you shoot video, then by all means, buy the largest and fastest cards you can afford.

If you mostly shoot stills, you can probably get along fine with a lower capacity card, provided you have good work habits and frequently download and reformat. On a smallish 8GB SDHC card, a Nikon Df shot at full-frame (FX) can store about 729 JPEG Fine images, or about 216 uncompressed 14-bit NEF files. When used with crop-frame (DX) lenses, those two figures jump to 1,500 and 484.

Images on SD cards can become fuzzy if the card is dropped or shaken briskly. No—just kidding. SD cards are quite durable. However, the electrical contacts and the lattice that surrounds them can be damaged if handled carelessly. Don’t carry them loose in a pocket; use the original plastic box or an aftermarket case.

Recovering Formatted or Deleted Images
If you accidentally delete an image file, stop shooting. Similarly, if you inadvertently format a card that contains images you forgot to download, set the card aside. Images are never deleted per se. The space that an image file occupies on a memory card is marked as available when an image is deleted. If you continue shooting you run a high risk of overwriting that space and obliterating the older image. Tip: never sell or lend a memory card unless you are willing to share its lifetime contents to the new user. Fortunately, card recovery software usually includes a utility that can overwrite an entire card with 1s and 0s thereby really deleting old images.

For more info on image recovery, read my article from last year or even this old one I wrote on these pages in 2007.

—Jon Sienkiewicz


etudiant's picture

How long can we expect SD cards to retain their data? Can it be 20-30 years?
I keep my old SD cards for archival storage of my trip photos after I've downloaded a copy to my regular computer.
I think of it as a cheap and convenient way to ensure a separate backup, but wonder whether those images will vanish. My oldest cards were filled 10 years ago, they still seem ok, but I can't tell whether the computer is struggling to recover the images or whether all is still normal.

Greybeard's picture

For clarification:
- UHS card format UHS-I, UHS-II and UHS-III are completely different concepts from UHS speed class U1 and U3. The article doesn't make this clear and implies they are somehow the same thing.
- UHS-II and UHS-III have the second row of pins (not UHS-I).
- You could point out its important to check your camera specifications to see which format (especially UHS format) and card size they will support (not just SD vs SDHC vs SDXC).
- The same is true for the card reader - its not just USB 3.0 but also UHS card format compatibility that's important.
- Its also worth pointing out that the headline speed on the card applies to read and that write will be slower (for example the Extreme 32GB cards you picture have 90MB/sec read but 40MB/sec write).