Everything You Need to Know about SD Cards

Everything You Need to Know about SD Cards

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 1TB (with the capacity to go up to 2TB eventually).

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 refer 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

  • 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

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.

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.

Lexar 128GB Professional 2000x UHS-II SDXC Memory Card

Recommended Media

So, what are some of our go-to cards? We like the Sony SF-G Tough Series UHS-II cards, which are V90-rated (indicating a minimum write-speed of 90 MB/s) and have maximum read/write speeds of 300 and 299 MB/s or the similarly specced Exascend Catalyst UHS-II cards, which also offer a V90 rating and transfer speeds up to 300 MB/s. Other notable UHS-II cards include Delkin's BLACK and POWER UHS-II V90 cards, SanDisk's Extreme PRO line, or Lexar Professional's 2000x1800x, and 1667x series cards, or even the Angelbird AV Pro MK 2. If you're in the market for UHS-I media, the SanDisk Extreme PROSanDisk Extreme, or Lexar Professional 1066x are some great cards with which to start your search.

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.

What kind of cameras might feel this need for speed? I’m definitely looking forward to finding out and am excited that SD media is up to the challenge. Let us know in the Comments section, below, which of these SD cards best suit your needs.


Thanks for your great article with more information provided than any supplier or manufacturer, but I am still at a loss I have two tablets with expansible memories and do not know what cards are compatible, no where whether in the manual of device does it it reference what to use and the companies hot line is as useful as it's on a bull. UMI DIGI should you ask.


Thanks Doug

Thanks for this. Can I still use a UHS-I SD card on a brand new camera rated for UHS-II? 

Yes, you can use in most cameras that are UHS II  a UHS I card. But it depends on the camera. And if you are shooting video that will be too slow. And for still shooting fast action a UHS I card in a UHS II camera can be to slow as well.

Is there a reliability difference (failure rate) using a full-size SDXC card vs a microSDXC card in, say, a Canon R5?

It is not recommended using a memory card adapter in a camera. 

It can slow the speed of the memory card. And that can produce glitches, especially while shooting video. 

Thank you for the insightful article, you've provided a valuable learning experience to me, an upcoming photojournalist. Much appreciated and greetings from Nairobi, Kenya!

Hi Patrick, thanks so much for your comment! We're glad you found the article useful.

I think  something to consider is that the manual of your camera tells you the SD suggested but I wounder if you use one of the latest SD very fast and your camera is 8 years old may not be worthy? 

If you do not know the brand/model of your camera, then it would be best to simply go by the recommendation in the camera's instruction manual.  Which card is compatible and will take full advantage of the camera's capabilities will depend on the brand and model of your particular camera.  The most I can say generally is if your instruction manual or your camera's specifications states it is compatible with SD cards, it may use cards up to 8GB in size.  If it states it is compatible with SD and SDHC cards, it may use cards up to 32GB in size.  If it states it uses SD, SDHC, and SDXC cards, it can use cards 64GB and larger.  However, if you have an entry-level camera, you may still want to check the instruction manual for card limits as there are a few older cameras that used SDXC memory cards, but could only use up to 128GB or 256GB, and would not work with the larger SDXC cards.  In terms of card speed, typically, you will not have an issue using a faster card in a slower camera, WITH ONE CAVEAT.  The caveat is concerning UHS-II memory cards.  I recommend ONLY using UHS-II SD/SDHC/SDXC memory cards in cameras that state they support UHS-II memory cards.  While a UHS-II memory card will fit in older SD memory card slots, UHS-II cards have an extra row of pins on the card designed for faster throughput of data between the camera and memory card, enabling the faster read/write speeds.  Cameras that do not have UHS-II support do not have the internal second row of pins to communicate with the UHS-II memory card.  As such, while the card is theoretically faster, you will actually get SLOWER transfer speeds when using a UHS-II memory card in a camera or device that does not offer UHS-II support.  This is one case where the faster card is not better.  If your camera does not offer UHS-II support, I recommend do not use a UHS-II memory card.  In all other cases, using a faster card should not have an effect on your camera's performance.

One more thing about the cards I would like to know...  How is it safe to label them.  Are there stickers that are safe to put on?  Is a fine-point Sharpie the way to go?  Do you just mark them A, B C... and create an index somewhere of what that means?

There are labels you can use on SD cards such as the Guetti Labels Memory Card Label Stickers (90-Count), BH # GUGMCL90 which you can write on using a fine-point Sharpie.