Everything you need to know about SD and CFexpress memory cards for digital cameras.

No matter what you shoot these days; from point-and-shoot cameras, through mirrorless masters, all the way up to cinema cameras, your media has to be saved onto some form of storage device. In most cases this will be a memory card. 

Knowing what type of memory card you need is the easy part, most cameras will only accept a certain type of card or, at most, two types of card and your camera manual will solve that problem for you. But that is often where the easy stuff ends and you now need to decide on the right set of card features for your camera and your intended use. Given the amount of features and specifications on modern memory cards, choosing the right card can be incredibly complicated. Good thing you found this article. We’ll explain what all the symbols and figures on memory cards mean, which will help you make a more informed choice. 

For the purposes of this article, we will be focussing on SD and CFexpress cards, as these are the most popular for use in current cameras, but we will also touch on MicroSD cards. There are many other memory cards, but the rules are mostly the same across the majority of all cards and providing details on every available card will require far more space than this article can provide.

The SD Card

Let’s kick it off with the most popular storage card in use today, the SD card. SD cards have become the standard card in use today. From professional video cameras all the way down to audio recorders, SD cards are used everywhere digital media needs a place to be saved on. And though most of us probably own a couple of SD cards, few know what all the designations on the cards mean and even fewer of us bought the right card for our camera the first time. So what do all the figures on that fancy new SD card mean? 

SD cards come in two dominant form factors: Standard SD cards and MicroSD cards. As the information contained below is the same of SD and MicroSD cards, assume that when I type SD card, I also refer to MicroSD cards. Within these form factors there are currently four capacity types and these are:

  • SD: Up to 2GB of memory
  • SDHC: 2GB –32GB of memory
  • SDXC: 32GB – 2TB of memory
  • SDUC: 2TB – 128TB of memory

This capacity type is often the starting point for choosing your SD card; does your digital device have any memory card size limitations? You might have a device that can only take up to a 32GB SD card (some audio recorders) and therefore a SDHC card will be perfect, but an SDXC or SDUC card will simply not work. 

Pretty simple so far, right? Well, now we are getting into the messy business of SD card speed indications. The speed at which your card can read and write digital information will be significant to your workflow and to your camera functioning at its maximum potential. You look at an SD card and you think: “The speed is obvious, it’s written right there. 250MB per second, obviously. Right?”

Yes and no. Mostly no, though. Looks at the Lexar 64GB Professional 1667x 250MB/s UHS-II SDXC Memory Card below. 

Yes, one of the speed indicators on this is card is indeed 250MB/s. However, this only means that a device that can read data from the card, can read at a maximum speed of 250MB/s. A good way to think about it is that information can be copied from the card at up to 250MB/s. While this is good, we can all agree that this speed is rather irrelevant to your camera recording information onto the card. What we need is the card’s write speed. How quickly can your camera write digital information onto the card. 

For this write speed specification, the SD Association grades cards based on their minimum write speed. They use the following ratings: Speed Class, UHS Speed Class, and Video Speed Class. As these ratings came into use, they progressively started to appear on SD cards. This is why, when you look at older SD cards, you simply won’t see some of these rating indicators. 

This is the perfect time to mention one additional metric for speed that is often listed on modern SD cards, but is not a speed grade related to the minimum write speed of a card, the bus speed. There are currently three bus interfaces and they are indicated with a UHS designation such as UHS-I. The bus speed represents a theoretical maximum amount of data that the card can transfer over the bus interface (literally the electronic contacts on the back of the card). UHS bus speeds are broken down as follows:

  • UHS-I – 104MB/s
  • UHS-II – 312MB/s
  • UHS-III – 624MB/s

In addition to these UHS bus speeds an even faster SD Express bus speed was also developed, offering theoretical speeds of up 985MB/s for SD cards. At the time of writing UHS-III and SD Express SD cards are not really available for easy purchase. There are some of these products on the market, but nothing near the theoretical indicated speeds yet and definitely not in any standard use yet. 

Let’s look at the SD Association’s standard speed grades listed on SD cards. The first of these that came into use was the Speed Class. These ranged from Class 2 up to Class 10 and is a measure of the card’s Minimum Sequential Write Speed. This class is represented by the value within the “C” on SD cards. These break down as follows: 

  • Class 2 – 2 MB/s
  • Class 4 – 4 MB/s
  • Class 6 – 6 MB/s
  • Class 10 – 10 MB/s

As the write speeds attainable on memory cards became progressively faster, the Class classification was superseded by the UHS Speed Class. This is the value within the “U” and again represents the Minimum Sequential Write Speed. UHS speed classes are as follows:

  • UHS Speed Class 1 – 10 MB/s
  • UHS Speed Class 3 – 30 MB/s

In recent years, the ability to record increasing quality video on pretty much all cameras have again pushed the minimum write speeds on cards further and, as you can probably guess, resulted in yet another speed classification listed on SD cards. This is the Video Speed Class and is represented by a “V” followed by a class value representing said minimum write speed: 

  • Video Speed Class 6 – V6 – 6 MB/s
  • Video Speed Class 10 – V10 – 10 MB/s
  • Video Speed Class 30 – V30 – 30 MB/s
  • Video Speed Class 60 – V60 – 60 MB/s
  • Video Speed Class 90 – V90 – 90 MB/s

Now let’s step back to UHS bus speeds; remember this is different from the UHS Speed Class and represents the theoretical maximum amount of data that can be transferred over the bus interface of the card. The UHS bus speed classification is represented by a Roman numeral on SD cards and in most readily available cards this will be a simple I or II.

Let’s look at all these speed indicators on the same Lexar 64GB Professional 1667x 250MB/s UHS-II SDXC Memory Card as above. 

  1. Maximum Read Speed – In this example the card is capable of a maximum read speed of 250MB/s
  2. UHS Speed Class – This is the UHS Speed Class 3 card.
  3. SD card Speed Class – The Lexar is a Class 10 card.
  4. Video Speed Class -Yes, a Video Speed Class 60 V60 card so we know that the card has a sequential minimum write speed of 60 MB/s.
  5. UHS Bus Speed – As this is a UHS-II card it has a theoretical maximum data transfer speed of 312 MB/s

As we can see from the above example, many of these speed classifications have become rather redundant as they all supersede each other, but are still listed on most SD cards. This all makes sense? 

The CFexpress Card

Shop your way up the digital camera hierarchy and you will undoubtedly run into cameras using CFexpress cards. CFexpress is the latest generational evolution of XQD and previous CompactFlash cards and they are simply ridiculously fast and, yes, I realise that this comment will age very poorly as soon as the inevitable shiny new type of memory card is invented, but right now this statement stands.

CFexpress cards use the PCIe 3.0 and NVMe 1.3 interface protocols for data transfer. This is technology that was previously only found in computer hardware and Solid-State Drives (SSD). These protocols get much more technical than we can explain, but the fact is that it allows CFexpress cards to attain some amazing speeds. CFexpress Type-C (currently the fastest available) can attain a theoretical performance of 4 000 MB/s. Yes, that is multiples of the fastest speed attainable by any SD card. 

CFexpress Types

Much like SD cards that are available in different form factors, CFexpress are available in three form factors or “Types”. Unlike SD cards these Types have significant differences within the data transfer architecture used and this results in rather significant performance differences between the CFexpress Type cards. These Types are currently Type A, B and C and each Type is a different physical size and also uses a different number PCIe data transfer lanes, resulting in different transfer speeds.

CFexpress Type A cards utilise just one PCIe lane, Type B features two lanes and Type C uses four lanes for data transfer. This results in the three Types offering different transfer speeds – Type A Cards have a maximum theoretical transfer speed of 1 000 MB/s, Type B cards can theoretically reach 2 000 MB/s and Type C cards can reach transfer speeds of up to 4 000 MB/s.

Well, then the answer should be simple. Everybody should use CFexpress Type C cards, get all the speed and be happy. Not so fast though. Due to the different architectures used within the three Types of CFexpress cards, they are completely different sizes. See this handy little graph below: 

Because of these size differences only Type A and Type B have found use in digital cameras, while Type C is mostly used in SSDs for computers and gaming consoles and might in future be used in Cine cameras looking towards 12K or high speed 8K recording.  In addition, the transfer speeds attainable by Type B cards is more than sufficient for even the most demanding video formats in use within regular digital cameras. 

Unfortunately, CFexpress manufacturers are just as guilty as SD cards manufacturers when it comes to not making actual sustained speeds easily visible on the cards themselves. Unfortunately, given that CFexpress cards are still relatively new some manufacturers list the bare minimum specifications and testing standards on their cards, but this will only improve. Let’s take a look at Sony’s 80GB CFexpress Type A TOUGH Memory Card. 

At first glance it is easy enough to see that this is an 80GB capacity card and that it is a Type A card. The information then lists two speed specifications, identifies as “R” and “W” respectively. This is the cards Read and Write speeds and on some CFexpress cards it is simply indicated as “Read” and “Write” with a transfer speed value. But if looking at SD cards taught us anything, it is that these values might not be the entire truth or be particularly valuable to the user. 

This Sony card lists a Read speed of 800 MB/s and a 700 MB/s write speed, but these are the maximum speeds attainable by the specific card. Ideally we want to know what the sustained writing speed is of the card in question. Thankfully, there is a testing standard for the sustained write speed – the VPG standard. On CFexpress cards this testing is performed by the CompactFlash Association and is indicated by a value within a movie clapper board symbol. The card in question lists 400 within the clapper board symbol and therefore has a minimum sustained write speed of 400 MB/s. 

The Future

Looking at the current state of the camera industry, it is clear that SD and CFexpress cards are not going anywhere and the technology of both will keep advancing, delivering faster and faster and more reliable cards. 

CFexpress cards are, however, the format with the biggest potential for exponential gains. The CFexpress format will be compatible with the upcoming PCIe 4.0 protocol specification, which will lead to dramatically increased speeds. Having a faster storage medium available will play a large part in allowing camera manufacturers to keep increasing the size of video, think 12K and up, and stills files and increase the recording speeds, allowing for faster and faster burst shooting speeds and ever faster video frame rates or RAW formats.

Exciting times…