Everything You Need to Know about SD Cards

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The phrase “SD cards” is a blanket term that covers SD, SDHC, and SDXC media. While SD cards aren’t the fastest cards available, they’re still pretty darn fast and are, by far, the most widely used of all the memory card types. Their versatility is a huge advantage, and they will often be seen in any grade camera, ranging from those used by beginners all the way up to flagship models used by the pros. But, beyond just knowing you need an “SD card,” which specific card is the right one for you? and what do all those numbers and letter designations on the front of an SD card mean? Also, what is the future of SD media, and for how much longer will they continue to be viable? By the time you’ve finished this article, you should have a pretty good idea as to all the answers.

SD: What Does It Mean?

Let’s start by breaking down the blanket term that is “SD,” short for Secure Digital, which is a memory card format developed by the SD Association. SD cards currently come in three tiers describing memory capacity ranges: Standard SD cards go up to 2GB; SDHC (High Capacity) cards range from 4GB to 32GB, and SDXC (Extended Capacity) cards go from 64GB up to 2TB.

SD Card Labeling and Specifications

One of the great things about SD cards is the amount of information that’s provided on the front of the card, so you can expect to find not only SD, SDHC, or SDXC, but also the card’s specific storage capacity. But what do all those other symbols, letters, and numbers mean—a number such as 4, 6, or 10 enclosed in a circle, or the number 1 or 3 inside the letter U?

  • The 4, 6, or 10 refers to the card’s minimum-rated sustained write speed: A Class 4, 6, or 10 card is rated to never write slower than 4, 6, or 10 MB/s, respectively
  • The number 1 or 3 within the letter U refers to the U1 or U3 speed-class rating. U1 is identical to Class 10 and means that a card is certified to write at a minimum of 10 MB/s; U3 cards are certified to never write slower than 30 MB/s
  • The difference between Class 10 and U1 (and U3, by association) is that U1/U3 designation indicates they employ the UHS-I or UHS-II bus
  • Non-UHS SD card read speeds max out at speeds of 25 MB/s, UHS-I cards max out at 104 MB/s, and UHS-II cards have a second row of pins on the back that helps to achieve speeds of up to 312 MB/s
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  • Often, the maximum achievable read speed will also be shown on the front. This speed indicates how well a card might perform during bursts, but in terms of reliability, a better metric to use for card comparing is related to sustainable speed

With all of this in mind, how do you go about selecting a memory card? Well, going with the fastest cards can never hurt, but you might be paying for more memory card than needed if you’re just casually snapping photos. Try to think about the type of shooting you do, and whether that includes video or fast continuous burst shooting or not. These two types of capture tend to be the most taxing on a card’s performance and are the ones that are benefitted most by UHS-II cards with high sustainable read/write speeds—4K video recording requires a lot from a card, as does shooting high-res photos at 20+ fps. If you’re strictly a stills photographer, content with a 24MP or so sensor, shooting one frame at a time, then most modern card types will be more than suitable for this slower working process.

Recommended Media

So, which cards do I recommend? In my Sony a7R IV, I use Sony SF-G Tough Series UHS-II cards, which are V90-rated and have max read/write speeds of 300 and 299 MB/s. Other worthy UHS-II cards include Sony’s V60-rated SF-M Tough cards, as well as SF-M and SF-E cards, plus SanDisk Extreme PRO, Lexar Professional 2000x, Delkin Devices POWER, ProGrade Digital, Angelbird AV Pro Mk2, Transcend 700S, and Kingston Canvas React Plus cards.

Sony 128GB SF-G Tough Series UHS-II SDXC Memory Card
Sony 128GB SF-G Tough Series UHS-II SDXC Memory Card

If you’re in the market for UHS-I media, take a look at SanDisk Extreme PRO, SanDisk Extreme, Sony SF-UY3, Lexar Professional 667x, Delkin Devices Advantage, Transcend 300S, and Kingston Canvas Go! Plus cards.

Last, but not least, are microSD cards. Spec-wise, they are the same as SD cards, aside from the smaller “micro” form factor. You can browse B&H’s selection of microSD cards here. MicroSD cards may also be used in full-size SD devices with an adapter.

Final Thoughts

When buying an SD card, it doesn’t come down to which card is the best, but which card is the best for what you’re doing. Unless you’re buying for the future, there’s not much point in buying a UHS-II/V90 card if all you are shooting is Full HD video and raw photos with a 20MP sensor. Furthermore, if you have a UHS-II card, but your camera or card reader doesn’t support UHS-II, then you won’t see the speed benefits provided by UHS-II. Class 10 and U1 cards are good for Full HD video and raw photos, but expect to see slower write speeds, especially with burst photos. U3 and V30 cards can handle 4K video and frame rates up to 60 fps, but if you’re looking to shoot HDR and HFR video of 120 fps or higher, or have a camera with an extremely high-res sensor, then I’d make the jump to a V60 or V90 card.

In conclusion, for those concerned that SD cards may be eclipsed by CFexpress, the future of SD media is very much alive. While not yet available just yet, UHS-III has been spec’d out with an impressive maximum read speed of 624 MB/s. There’s also SD Express, with support for PCIe 3.1 x1 (985 MB/s), PCIe 3.1 x2 and 4.0 x1 (1969 MB/s), and PCIe 4.0 x2 (3938 MB/s). What kind of cameras might feel this need for speed? I’m not sure, but I’m looking forward to finding out, and excited that SD media is up to the challenge.

Which do you think are the most practical SD cards for your needs? Share your thoughts in the Comments section!

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