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A beginner’s guide to memory cards

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Introduction to memory cards, and main types

Photograph by Terry Sullivan

When it comes to understanding how a digital image is created, the focus is most often placed on a camera’s lens and image sensor, which makes sense, since these two components have the greatest impact on image quality.

But once the image or video is captured, it will need to be stored somewhere on the device. Today, the location of where an image or video file is stored differs depending on the device. For instance, if you’ve captured a photo on a phone or tablet, you’ll store the media on built-in, non-removable flash storage. However, when it comes to capturing photos or videos on a stand-alone digital camera, whether it’s a point-and-shoot or an interchangeable-lens camera, like a mirrorless camera or digital single-lens reflex (DSLR) camera, you’ll be saving your photos and video on a little rectangular-shaped card, known as a memory card.

What’s a memory card, and how do you use it?

Memory cards are literally just miniature digital data storage devices. These cards house non-volatile flash storage within them, which simply means that they can retain stored information or data even after they’re unplugged – i.e., removed from a camera.

Memory cards are available in various capacities, just like computer hard drives and SSDs. Today, you can buy memory cards that range in capacity from 8GB up to 256GB, 512GB and even 1TB or 2TB capacities.

Memory cards are removable and reusable. So, after you’ve captured your images, you can remove the memory card from your camera, insert it into a card slot on your laptop, desktop, tablet or other device and transfer them to the computer’s storage. Memory cards are meant to be used as temporary storage for your images, but don’t worry – the data stored on them won’t degrade over time.

An important part of working with memory cards is formatting them. This deletes old files and gets the card ready for fresh data to be written, which helps prevent issues with corrupted or unreadable data. All digital cameras have a card formatting option, even if sometimes you have to go ‘menu diving’ to find it.

To reuse a memory card, you’ll first need to format the card. Here’s how: When you first put a memory card into your camera or when you’ve just transferred your photos from your camera over to your computer’s hard drive (and, of course, backed those images up, as well), you’ll want to make sure you format your memory card by finding a setting found in your digital camera’s set-up menu.

Formatting the memory card performs two tasks at once: It creates a directory on your card, and it deletes any existing photos or video clips that were on your memory card. (If you accidentally delete your photos or video clips before you’ve transferred them, you can use file recovery software to attempt to get them back, but it’s never a sure thing.)

There are several reasons why you want to format your memory card on your camera, as opposed to on a computer or a camera other than the one you intend to use it with. One of the most important is for compatibility: Formatting your memory card on your camera will allow most devices, including your computer or tablet, to read the card properly. If you don’t format it first – before you use it – you run the risk of not being able to access the photos or videos you captured when you want to download them from the card.

Types of Memory Cards

Photograph by Terry Sullivan

Memory cards have been on the market nearly as long as digital cameras have been available to consumers, but there are several different types of card available. The most important question a digital photographer needs to ask themselves is: “Is this memory card compatible with my digital camera?”

You should be able to find out which kind of memory card your camera is compatible with just by opening up the card door, but if you’re not sure, you can find the answer to that question either in the owner’s manual of your camera or find it on the manufacturer’s website.

If you’re not happy about needing to buy an expensive memory card for your pricey new camera, check out “Dealer’s Choice: It’s new card time. Is that such a bad thing?” by DPReview editor Richard Butler, who has some thoughts on just this issue.)

Capacity & price comparison… over time

The good news for photographers and videographers is that memory card manufacturers have continued to expand capacities and improve performance. What’s interesting to note is that today, SD memory cards, the most popular type on the market, offer one thousand times the storage capacity as an SD memory card from 15 years ago, and for far less money.

Here’s a list of current memory card types that are compatible with today’s digital cameras.


SD (Secure Digital)

Secure Digital – or SD – cards are among the most common types of memory cards available to photographers, and they come in a few different variants. SD is more of a family of memory cards than a single ‘type’, and the first SD cards were actually introduced way back in 1999 with a maximum capacity of 2GB. These were based on the even older MultiMediaCard (MMC) format.

All the full-size versions of SD (SD, SDHC, and SDXC) measure 24mm x 32mm x 2.1mm, while microSD cards (which come in SDHC and SDXC types) are smaller, and measure 11mm x 15mm x 2.1mm.

Here are the three current SD memory card versions, along with some information on each format, as well as a future specification (recently introduced by the Secure Digital Association):

  • SDXC: This is the most modern variant of Secure Digital cards, which has a theoretical maximum capacity of 2TB. Generally speaking, this is the version you’ll want to reach for if you’re shooting video, since these types of memory cards have the greatest capacities, which for now, are as large as 1TB. These cards are available in three memory-card speed designations – UHS-I, UHS-II and UHS-III. (Note: See page 2 for more on memory card speeds.)
  • SDHC: Older than the SDXC specification, SDHC cards have a maximum capacity of 32GB. They’re also available in just two memory card speeds—USH-I and UHS-II.
  • microSDHC & microSDXC: The smaller size of these two types of SD card formats make them ideal for mobile devices, such as phone, tablets and action cams. Like the full-size SD card versions, the microSDHC memory card format has a maximum capacity of 32GBs and the microSDXC format has a maximum capacity of 2 TBs. Currently, you can also order microSDXC cards that are as large as 1TB.
  • SD Express and SDUC memory cards (not yet available): In June of 2018, the SD Association also announced specifications for a new card format—the SD Express card format, which is part of the SDA’s SD 7.0 specification. What’s more is that in 2020, the SDA also introduced another SD specification update—SD 8.0 specification, which would allow even faster transfer speeds. However, none of these memory card types have been introduced into the market just yet.

CFexpress (Types B and A)

CFexpress is a relatively new format, but it’s quickly becoming more common – especially in higher-end cameras. The CFexpress format promises significant increases in write, read and transfer speeds compared to earlier card types. This is great news for photographers and videographers who may want to shoot everything from 8K-resolution video to slow-motion video to rapid-fire burst modes of still photos.

There are two types of CFexpress cards available – Type A and Type B. And since nothing is ever simple in the world of tech or digital cameras, we’re going to discuss Type B memory cards first, since, confusingly, they were the first ones introduced – they’re also far more common.

  • CFexpress Type B: When the CFexpress Type B memory cards (an example is pictured above) first came to market, many photographers were intrigued that they had the same dimensions and fit into the same card slots as cameras compatible with older XQD memory cards (see below for more on XQD cards). Many cameras that were designed to use XQD-type memory cards can accept CFexpress Type B memory cards, but this might require a firmware update.

    Type B CFexpress cards have a 2,000MB/s maximum theoretical transfer speed, although at the moment, 1,000 – 1,700MB/s is more common.

  • CFexpress Type A: In July 2020, when Sony debuted the Sony a7S III full-frame mirrorless camera, the company also introduced a new, second type of CFexpress memory card—Type A. Currently, most pro cameras from all brands other than Sony are opting to use the Type B CFexpress memory cards. At the time of writing, type A CFexpress cards are available in just two capacities: 80GB and 160GB.

The most obvious characteristic of Type A CFexpress memory cards is that they’re smaller than Type B. In fact, they measure 20mm x 28mm x 2.8mm, which is the same size as an SD memory card, although Type A CFe cards are a bit thicker. This is why, when Sony manufactured the Sony A7S III full-frame mirrorless camera, it built two memory-card slots that could accept either an SD memory card (UHS-II) or a CFexpress Type A memory card.

Type A memory cards do have shortcomings, however. A significant one is that they’re slower than Type B cards. However, they’re still faster than all current type of SD memory card formats.

The Future of “Express” Memory Cards:

Although for the moment, there are just two types of CFexpress Cards available – Type A and Type B – there are plans for introducing additional formats in the near future:

  • CFexpress Type C: In the CFA’s 2019 press release on the association’s most recent CFexpress 2.0 specification update, the association noted that in addition to Type A and Type B CFExpress cards, a third type of card, Type C, which would be more than twice the size and nearly twice the thickness of type A cards: CFExpress Type C cards will measure 54mm x 74mm x 4.8mm. Although like Type A and Type B, Type C will have NVM Express 1.3 protocols, it would also include 4 PCIe Interface lanes and have a maximum theoretical performance of 4000MB/s – four times that of Type A cards.

However, although there is information on the specifications of such new memory-card formats, neither CFexpress Type C or SD Express memory cards have hit the marketplace yet. In June of 2021, Lexar announced plans on eventually producing an SD Express memory card, for introduction next year.


CompactFlash (soon to become obsolete)

This type of memory card is one of the largest in physical size, and it’s also one of the oldest, outlasting many other memory card types. You’ll find two types available – CF Type II (5mm thick, which can fit only fit into Type II card slots) and Type I (3.3mm thick and can fit into both types of card slots). Unlike SDXC memory cards, which have a maximum capacity limit of 2TB, CF memory cards have a maximum capacity of just 512GB.

CF cards are still used by plenty of currently-available cameras (including many DSLRs), but they’re being phased-out in new models, to be replaced by SDXC and / or CFexpress.


XQD (obsolete)

XQD cards are still available, and several current Nikon DSLRs and mirrorless cameras can accept them, but like CompactFlash, XQD has effectively been replaced by CFexpress Type B. CFexpress Type B cards are the same size as XQD, and share the same interface, but they’re a lot faster. As mentioned, there are several Nikon DSLRs and full-frame Z-series mirrorless cameras on the market that can accept both XQD and CFexpress memory cards. (Nikon lists both on the NikonUSA website) but to make the most out of your camera’s performance, we’d recommend going with CFexpress Type B. Note that unlike Nikon, Canon cameras which accept CFexpress Type B cards will not work with XQD media.

Since XQD and CFexpress Type B cards tend to be priced similarly, so there’s no bargain in staying with the older format.


CFast (obsolete)

Like XQD, CFast card slots can still be found in some current cameras, but the format is obsolete. At the time CFast cards were introduced, in 2009 and then updated to 2.0 in 2012, CFast was seen as the successor to the older (and similar in appearance, if not technology) CompactFlash standard, but CFexpress Type B has emerged as a more promising format.

CFast memory cards are only compatible with a handful of last-generation (but possibly still current, and still available) Canon, BlackMagic Design, and ARRI cameras. Camera manufacturers are no longer launching new cameras that accept this type of memory card.


On the next page, we’ll be explaining what the speeds, codes, and ratings mean on your memory card.


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