What's the difference between SD and microSD?

They are the same memory card standard in two physical sizes. A full-size SD card is 32×24×2.1 mm; a microSD is 15×11×1 mm. Electrically they are identical, which is why a microSD dropped into a passive SD adapter works as a normal SD card — the adapter only changes the body shape, not the speed or capacity. The size you need is decided purely by the slot in your device: cameras and laptops usually take full-size SD, while phones, drones, dashcams and action cams take microSD.

What is CFexpress and do I need it?

CFexpress is a high-speed card format from the CompactFlash Association that runs the PCIe and NVMe protocols — the same interface as an internal SSD — instead of the older SD or CompactFlash buses. You need it only if your camera has a CFexpress slot, typically high-end mirrorless and cinema bodies shooting high-bitrate or RAW video. It comes in three sizes: Type A (one PCIe lane), Type B (two lanes), Type C (four lanes). Type A and Type B are not interchangeable, so buy the exact type your camera lists.

Will a UHS-II card work in a UHS-I device?

Yes. UHS-II and UHS-III cards add a second row of pins but keep the original single row, so they stay backward-compatible and simply fall back to the UHS-I bus (up to 104 MB/s) in an older slot. You lose the extra speed but not the card. The reverse logic matters too: putting a fast card in a slow slot wastes money, and putting any SD-family card in a CFexpress slot does not work at all — they are different physical and electrical standards.

Is CompactFlash still worth buying?

Only for legacy gear. CompactFlash is a 43×36 mm card with a 50-pin parallel-ATA connector, designed in the 1990s, and it has been superseded by CFexpress for new professional cameras. If you own an older DSLR or an industrial device with a CF slot, buy CF; for anything current, the slot will be SD, microSD or CFexpress instead.

SD vs microSD vs CompactFlash vs CFexpress — memory card formats explained

You buy a new camera, drone or dashcam, and the manual says it takes a "UHS-II microSDXC, V60 recommended" — or worse, a "CFexpress Type B." Five words, four of which look like passwords. The questions underneath are simple, though: which card physically fits my slot, and what do all these names actually mean? This article answers both, and the trick is to stop treating it as one question.

The one idea that untangles everything: shape vs bus

A memory card has two separate properties, and almost all the confusion comes from mixing them up:

The form factor is the physical shape — does it fit the slot? SD, microSD, CompactFlash, CFexpress Type A/B/C are all different shapes.
The bus (interface) is the electrical highway running underneath — how fast can data move? UHS-I, UHS-II, UHS-III, SD Express and PCIe/NVMe are buses, not shapes.

The shape decides if a card fits. The bus decides how fast it goes. A card label combines both — "UHS-II microSD" means microSD shape, UHS-II bus — and once you read it that way the rest is just a lookup.

The SD family: one standard, two body sizes

SD and microSD are not rivals. They are the same electrical standard in two physical sizes, defined by the SD Association [5]:

Form factor	Dimensions (mm)	Typical hosts
SD (full size)	32 × 24 × 2.1	cameras, laptops, card readers
miniSD (obsolete)	21.5 × 20 × 1.4	early phones; effectively dead
microSD	15 × 11 × 1	phones, drones, dashcams, action cams, handhelds

Because they are electrically identical, a microSD sitting in a passive SD adapter is a genuine SD card — the adapter changes the body, not the speed or capacity. So the size you buy is decided by exactly one thing: the slot in front of you. (Capacity tiers — SDHC, SDXC, SDUC — are a separate label again; the shape is unchanged.)

The SD buses: UHS-I → UHS-II → UHS-III → SD Express

Underneath the SD shape, the bus has been upgraded several times. The headline maximum bus speeds, straight from the SD Association [1]:

Bus	Max speed	How
Default Speed	12.5 MB/s	original single pin row
High Speed	25 MB/s	single pin row
UHS-I	104 MB/s	single pin row (SDR104)
UHS-II	312 MB/s	adds a second pin row
UHS-III	624 MB/s	second pin row
SD Express	up to 3940 MB/s	PCIe + NVMe (see below)

Two practical rules fall out of this table:

UHS-II/III are backward-compatible. They add a second row of pins but keep the first, so in a UHS-I-only slot they fall back to 104 MB/s. You never lose the card, only the extra speed.
The slot caps the speed. A UHS-II card in a UHS-I camera runs at UHS-I. Pay for UHS-II only when the host actually has the second-row contacts.

Note that bus speed and speed class are different labels too. The bus (UHS-I/II/III) is the peak highway width; the speed class (Class 10, U1/U3, V6–V90) is the guaranteed minimum sustained write speed for video — V30 means 30 MB/s floor, V90 means 90 [2]. We unpack the V/U system in SD card speed classes explained.

SD Express: an SSD bus inside an SD card

SD Express is the big jump: it keeps the SD/microSD shape but throws away the SD bus and runs PCIe with the NVMe protocol — the exact interface an internal SSD uses. The SD Association rates it at 985 MB/s (PCIe Gen 3 ×1), 1970 MB/s (Gen 4 ×1 or Gen 3 ×2) and 3940 MB/s (Gen 4 ×2) [1]. It introduces its own speed classes — E150 through E600 — for guaranteed performance [2]. Host support is still rolling out, so an SD Express card falls back to UHS-I in an ordinary slot.

CompactFlash: the legacy heavyweight

Before SD took over, professional cameras ran CompactFlash (CF): a chunky 43 × 36 mm card (Type I is 3.3 mm thick, Type II is 5 mm) with a 50-pin parallel-ATA (PATA) connector [6]. That parallel interface is exactly why CF aged out — it could not scale to modern video bitrates the way a serial PCIe link can. CF still turns up in older DSLRs and some industrial gear, but for new professional bodies it has been replaced by CFexpress [6]. If your device has a CF slot, buy CF; otherwise you will never touch it.

CFexpress: PCIe/NVMe in three sizes

CFexpress is CompactFlash's successor from the same association, and like SD Express it runs PCIe + NVMe — think of it as a tiny removable SSD. It comes in three physical types that are not interchangeable [3][4]:

Type	Dimensions (mm)	PCIe lanes	Max speed (4.0)	Used in
Type A	20 × 28 × 2.8	1 lane	~2.0 GB/s	Sony α / FX bodies (Sony only)
Type B	38.5 × 29.8 × 3.8	2 lanes	~4.0 GB/s	Canon, Nikon, Panasonic, Fujifilm, RED
Type C	54 × 74 × 4.8	4 lanes	~8.0 GB/s	industrial; no cameras yet

The protocol has tracked the spec: CFexpress 1.0 used PCIe 3.0 with NVMe 1.2, 2.0 moved to NVMe 1.3, and 4.0 doubled bandwidth on PCIe 4.0 with NVMe 1.4c [4]. The key buying fact is brutally simple: Type A and Type B are different shapes and do not fit each other's slots. Sony picked the smaller Type A; almost everyone else picked Type B. Buy the exact type your camera lists — speed is irrelevant if it physically won't seat.

Which device uses which — a quick map

Phones, drones, action cams, dashcams, handheld consoles → microSD (UHS-I, sometimes UHS-II; pick the V/U class for your bitrate).
Mainstream and enthusiast cameras, laptops → full-size SD (UHS-I or UHS-II).
High-end mirrorless / cinema cameras → CFexpress (Type A for Sony, Type B for most others) for high-bitrate and RAW video.
Older DSLRs, legacy/industrial gear → CompactFlash.
Newest devices advertising SSD-class card speed → SD Express or CFexpress, both PCIe/NVMe.

For matching a microSD to a specific job rather than a format, see choosing the right microSD by use case. And if you are weighing a removable card against built-in storage entirely, eMMC, UFS or SD card for embedded storage covers that trade-off.

How to choose, in order

Read the slot, not the marketing. Find the exact format and bus your device lists — microSD UHS-I, SD UHS-II, CFexpress Type B, etc. That single line eliminates most options instantly.
Match the shape. A card that doesn't fit is a non-starter; a Type B won't enter a Type A slot, and no SD-family card enters a CFexpress slot.
Match the bus to the host. Buying UHS-II for a UHS-I body, or SD Express for a slot that lacks it, just pays for headroom that falls back to the lower bus.
Then pick the speed class for your workload — V30 for 4K, V60/V90 or CFexpress for high-bitrate and RAW. Endurance is yet another axis on top of speed; if the card records around the clock, weigh that separately.

Bottom line

Card naming looks like alphabet soup because it's two facts wearing one label: a shape and a bus. Separate them and every spec sheet reads cleanly — the shape says whether it fits, the bus says how fast, and the right card is simply the fastest one your slot was actually built for. As a source factory in Guangdong, we build across the SD and microSD range (UHS-I and UHS-II) — tell us the host device and the duty cycle and we'll spec the exact format, bus and class instead of selling you headroom you can't use.