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    SD or microSD, XQD or CFexpress cards: better understand storage memory cards

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    While SD or microSD cards are well known, some features are sometimes obscure while other formats appear. This file reviews the main memory cards used on the market.


    Most of us have heard of if not used SD or microSD cards. However, we were also able to wonder about a new format or a characteristic with an unknown meaning. To better understand the storage of our cameras or that of our smartphones, let's review the main formats and essential characteristics of memory cards.

    Why so much complexity?

    But before starting this guide on memory cards, a question can quickly come to mind: "Why so many memory card formats with sometimes difficult to understand characteristics?" The answer takes a form that is both simple and heterogeneous. On the one hand, the different formats are pushed by various manufacturers or consortia. And financial and hegemonic battles can then appear.

    On the other hand, there have been many technological developments over the years. For example, the CompactFlash format developed by SanDisk appeared in 1994 with capacities planned to start at 2 MB! Responding to new needs complicates things when you don't have to wipe the slate clean. Generally, three issues motivate the appearance of new cards: increasing speed, increasing storage capacity or reducing the format of memory cards.

    Not all formats survived

    We are not here giving an exhaustive history or inventory of the memory card market through the ages. Only the main formats and main characteristics are covered in order to make the information as digestible as possible, because the subject is complex.

    We will therefore review six different formats that are more or less different or exotic:

    • CompactFlash card : This storage medium is the oldest in this folder. It arrived in 1994 thanks to SanDisk. The CompactFlash Association consortium was formed in 1995.
    • SD card et microSD : they are built around the Secure Digital standard introduced in 1999 by SanDisk, Panasonic and Toshiba. The three companies formed the SD Association in 2000.
    • Carte CFast : the faster variant of CompactFlash cards is presented in 2008.
    • XQD card : the format was introduced in 2010 by Sony, Nikon and SanDisk. The CompactFlash Association is then in charge of the development.
    • CFexpress card : this new standard was unveiled by the CompactFlash Association in 2016.

    Exit therefore the questions on MultiMedia cards, Smart Media or other Memory Stick which have now disappeared from circulation. Let's focus on the essentials.

    SD cards and microSD cards

    Say microSD card and not micro SD card

    Let's start our tour of memory cards with SD cards and microSD cards. These two formats are surely the two best known by the general public and also the most used. SD cards generally take place in more or less high-end cameras and have a dedicated slot in many computers.

    MicroSD cards, which are much smaller, are also used in entry-level or compact cameras, but also in action-cams, drones or even smartphones. Attention, the correct spelling of the format is microSD card and not micro SD card, contrary to what we can regularly read. We will ignore the little-known miniSD format.

    Although very different in appearance, the two types of cards have a lot in common. For more than 20 years, their characteristics have not ceased to evolve to the point of having made reading the technical data sheet complex. Supposed to provide clarity for consumers, the various certifications have not helped to simplify the whole.

    The capacities

    Let's start with the sinews of war, the storage capacity. There are four main categories that should be identified. You have to be careful to check the compatibility of your SD or microSD card with your device to avoid any disappointment. Indeed, the compatibility between the different capacities is not always appropriate.

    • SD or SDSC (Secure Digital Standard Capacity): up to 2 GB.
    • SDHC (Secure Digital High Capacity) : entre 2 Go et 32 Go.
    • S (Secure Digital eXtended Capacity)  : entre 32 Go et 2 To.
    • SDUC (Secure Digital Ultra Capacity) : entre 2 To et 128 To.

    On the left, the back of a UHS-I SD card, with a single row of connectors. On the right, a UHS-II SD card and its two rows.


    SD cards were originally designed to run at 12,5MB/s. Since then, speeds have improved with new technologies. By default, the builder usually displays the read speed. Most often, the write speed is also specified. The speed is always displayed for sequential transfers, i.e. on a continuous large file.

    For a transfer with many small files, the card is usually in more trouble. Our tests show disparities between the types of transfers, but also between the values ​​announced by the manufacturer and those actually available. To benefit from the best speeds (or the best prices), it is advisable to carefully check the bus used by the host device.

    The bus is the data transmission system. UHS-I and UHS-II buses are the most common. The old buses are no longer used while the following ones are still very rare or still in the development stage. We are thinking in particular of SD Express which will use the PCIe interface.

    • High Speed : up to 25 MB / s.
    • UHS-I (Ultra High Speed) : up to 104 MB / s.
    • UHS-II : up to 312 MB / s.
    • UHS III : up to 624 MB / s.
    • SD Express 3.1 : up to 985 MB / s.
    • SD Express 4.0 : up to 1969 MB / s.

    For SD Express cards, the speed can be doubled if two communication lines are used, bringing the theoretical maximum speed to almost 4 GB/s!

    On the top of the card are affixed the various pictograms which make it possible to identify the type of card.

    Speed ​​classes

    The SD Association has defined several speed standards called classes. They define a minimum speed that the card certifies to have, both in reading and in writing. We distinguish three of them. The oldest rated "C". Fast speeds rated "U". Finally, video classes rated "V". The corresponding letter is then associated with a number which corresponds to the minimum speed available.

    • Class C2, C4, C6, C10 : for respective minimum speeds of 2 MB / s, 4 MB / s, 6 MB / s, 10 MB / s.
    • Class U1, U3 : for respective minimum speeds of 10 MB/s and 30 MB/s.
    • Classes V6, V10, V30, V60, V90 : for respective minimum speeds of 6 MB / s, 10 MB / s, 30 MB / s, 60 MB / s, 90 MB / s.

    The U3 and V30 (or higher) classes are the most suitable for speed-intensive uses. We are of course thinking of 4K video recording which is now a standard. Also, if speed is not always an essential aspect, it can still be useful to choose a fast card in the case of large capacity SD or microSD cards. Indeed, the transfer can be very long in the opposite case.

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    A new class for applications

    Recently introduced for microSD cards, the Application Performance Class is dedicated to the certification of IOPS (input/output operations per second), or "input-output operations" in Spanish. Unlike sequential reading or writing which concern very large files, IOPS measures the performance of a storage system on the very small files necessary for the proper functioning of an operating system or an application. .

    Knowing the performance of the card on this point is less interesting when you want to store photos or videos. On the other hand, it can be useful if the microSD card is inserted in a smartphone or a video game console with applications installed on it. Theoretically, the higher the number of IOPS, the more responsive the application will be in its operation.

    • Class A1 : 1500 IOPS in reading and 500 IOPS in writing.
    • Class A2 : 4000 IOPS in reading and 2000 IOPS in writing.

    Once again, it will be necessary to check the compatibility of the device used with the class. Indeed, in some cases, Class A2 microSD cards may be less efficient than A1 microSD cards if the device is not compatible.

    The main pictograms to remember when choosing your SD or microSD memory card.

    The logos and actual performances

    To better differentiate capacities, speeds and different classes, the SD Association offers manufacturers several logos for each of the criteria. Problem, they can accumulate and do not always make easy the identification of the characteristics by the neophyte. However, it quickly becomes easy to identify a particular class to choose your memory card more efficiently.

    However, it should be kept in mind that the maximum performance promised remains theoretical and is often below the announcements in real conditions of use. This aspect is confirmed on the fastest cards, even if the differences fade a little in this case.

    Kingston MicroSD Canvas Go! 128 Go

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    How the SanDisk Extreme Pro 128GB SDXC UHS-II 300MB/s Price Chart Works

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    CFexpress cards

    One format to rule them all?

    Just introduced in 2016, the CFexpress memory card format is designed to meet the current and future needs of the industry, but also to standardize the memory card ecosystem. Of course, it must be convincing enough to replace all its predecessors. In any case, this is the promise that is made, and it is not really won with well-established SD cards.

    If the presentation took place in 2016, it was only in 2017 that the first CFexpress card made its appearance for a start of marketing in 2018. The very recent format therefore uses the PCIe bus (PCI Express) with the NVMe interface ( Non-Volatile Memory express). Characteristics close to SSDs for computers.

    The Sony A7S III offers two SDXC UHS-II card slots and two CFexpress Type A card slots. A maximum of two cards can be used simultaneously. © Sony

    We said "keep it simple"

    When you look at the characteristics of SD or microSD cards, there is enough to tear your hair out. The promise of simplicity with the new CFexpress single format is starting to fade. This is because CFexpress cards can use multiple communication lines to improve performance. Card types then come into play.

    Introduced with the type B format, CFexpress cards will continue to evolve. Two new formats of types A and C, respectively smaller and larger than type B, were presented at the beginning of 2019. It cannot be ruled out that a type D will appear, larger and even faster than the previous formats. .

    To make matters worse, CFexpress cards take up one of the great faults of SD cards with variable speeds depending on capacity. For example, the 64GB SanDisk Extreme Pro CFexpress cards don't write fast enough to handle Raw 8K recording with an EOS R5. Certainly, the "problem" is quite specific, but it remains a shame.

    Sony CFexpress Type A 80 GB Memory Card

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    How the SanDisk Pricing Table Works Extreme Pro CFexpress Type B 64GB Memory Card

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    A price problem?

    At present, it is still too early to comment on the sustainability of the format. However, the industry seems to be getting started slowly, at least on the top of the range presented from 2020. There is, for example, a CFexpress type B slot at Canon, in particular on the EOS-1D X Mark III reflex. or the EOS R5 and R6 hybrids. At Nikon, the D6 SLR and the Z6 II and Z7 II hybrids benefit from the same format.

    At Sony, the Alpha 7S III (A7S III) is compatible with CFexpress type A and the only one to offer this format in 2020. Note that the memory slot can be used either with a CFexpress type A memory card or a more classic SD card on Sony's specialized video hybrid. The choice is therefore up to the user and does not confine it to a little-used format. It's a good idea from the builder.

    For the moment reserved for the most expensive devices, the price will perhaps ultimately remain the biggest obstacle to the adoption of the CFexpress format compared to the SD standard.

    CFexpress type A card

    • Dimensions : 20 x 28 x 2,8 mm.
    • Theoretical maximum speed : 1 Go/s.

    CFexpress type B card

    • Dimensions : 29,6 x 38,5 x 3,8 mm.
    • Theoretical maximum speed : 2 Go/s.

    CFexpress Type-C Card

    • Dimensions : 54,40 x 74,10 x 6 mm.
    • Theoretical maximum speed : 4 Go/s.

    From left to right, an XQD card, an SD card, a CFexpress type A card and a microSD card.

    XQD cards

    XQD cards are now mainly used at Nikon, which is part of the trio behind the format with Sony and SanDisk. Since then, the XQD format should be replaced by the ambitious CFexpress format. This latter standard thus shares the design of CFexpress type B cards with XQD cards. For its part, SanDisk has abandoned the format. XQD cards are therefore surely destined to gradually disappear.

    Since the form factor is the same, CFexpress type B cards can therefore be inserted into an XQD card slot without difficulty. On the other hand, software compatibility does not always exist. To read them, the device receiving the cards must be updated when possible, in order to provide compatibility. This was particularly the case with Nikon's full-frame Z6 and Z7 mirrorless cameras. The most recent Z6 II and Z7 II benefit directly from dual compatibility.


    • Dimensions : 38,5 x 29,8 x 3,8 mm.
    • Theoretical maximum capacity : 2 To.
    • Theoretical maximum speed : 1 Go/s.

    CompactFlash cards or CF cards

    As explained in the preamble, CompactFlash cards use a very old format that was introduced by SanDisk in 1994, an eternity in the computer world. The format has been improved several times and it is still present on some old devices still marketed, but it remains rare. It is now on the decline, supplanted by other faster and often more compact cards.

    Unlike the Secure Digital standard, it is relatively easy to decipher the characteristics of CompactFlash cards. The speed is expressed in "x" and refers to the transfer speed of the first Compact Discs. The higher the number of "x", the faster the card. To find out the actual theoretical speed announced by the manufacturer, multiply the value by 150 kb/s.


    • Dimensions : 43 × 36 × 3,3 mm (for the more common type I).
    • Theoretical maximum capacity : 128 petabytes.
    • Theoretical maximum speed : 167 Mo / s.

    CFast cards

    CFast cards are an evolution of CompactFlash cards. For the small technical side, the latter use the Parallel ATA/IDE (PATA) interface while the CFast take advantage of the faster Serial ATA (SATA) interface. Therefore, the two types of cards are not compatible.

    The transition from CompactFlash to CFast cards did not happen. Only a few high-end and professional products incorporate a CFast card storage slot. Among the best known are Canon's EOS-1D X Mark II SLR and several cameras dedicated to the advanced production of videographic content, notably from Blackmagic Design or Arri.


    • Dimensions : 36,4 x 42,8 x 3,6 mm.
    • Theoretical maximum capacity : 2 To.
    • Theoretical maximum speed : 600 Mo / s.
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