What card speed rating actually matters?

The V class on an SD card and the VPG rating on a CFexpress card, because those are the only figures guaranteeing a sustained minimum write speed. The large numbers on the front of the packaging are usually peak read, occasionally peak write, and both are burst figures into a cache that say nothing about whether a recording mode will hold.

What is the difference between CFexpress Type A and Type B?

Type A is physically smaller and uses one PCIe lane; Type B is larger and uses two, roughly doubling the bandwidth. They are different cards in different slots with no adapter between them, and your camera decides which you need. Sony cine line bodies use Type A, while Canon, Nikon, Panasonic and most cinema cameras use Type B.

Why is CFexpress Type A more expensive?

Because the market is small and Sony cine line bodies are effectively the only customer, so the volume that drives down Type B pricing does not exist. It is also slower, using one PCIe lane instead of two. That premium appears on every card for the life of the camera and is a real ownership cost worth counting when comparing bodies.

Why does a card slow down when it gets hot?

CFexpress cards are essentially NVMe drives in a card body, and when they reach a thermal limit they reduce write speed to protect themselves. In a warm camera in a cage in direct sun, a card that sustains 400 MB/s on a bench can drop below the requirement for your mode, and it does so when the bitrate spikes on a complex shot.

Should I buy one large card or two smaller ones?

Two 512 GB cards rather than one 1 TB card. They usually cost less per gigabyte, and a failure or a loss then costs part of a shoot day rather than the whole thing. Five hundred and twelve gigabytes is roughly 4.7 hours at 240 Mbps, which comfortably covers a single-camera day at the codecs most people use.

CFexpress and SD Card Speed Chart: Formats, Ratings and Real Requirements

Short answer

The number that matters is sustained write speed, not the peak figure on the packaging. SD V60 guarantees 60 MB/s and V90 guarantees 90 MB/s. CFexpress Type A sustains a few hundred megabytes per second and Type B roughly double that, which is what unlocks high bitrate intraframe and RAW modes.

Card packaging quotes peak read speed, which is the least useful number on the box. What determines whether a card can hold a recording mode is sustained write, and that is often in small print or absent. This chart puts every rating alongside what it actually guarantees, and pairs it with the modes each supports on real cards like the Delkin 512GB CFexpress Type B 4.0.

The rating systems, decoded

Rating	What it guarantees	Where it appears
V30, V60, V90	Minimum sustained write of 30, 60 or 90 MB/s	SD and microSD, the useful rating
U1, U3	Minimum 10 or 30 MB/s sustained	SD, superseded by the V classes
Class 10	Minimum 10 MB/s sustained	Obsolete for video
UHS-I, UHS-II	Bus type, not a speed guarantee	SD, indicates the contact rows
MB/s on the front	Peak read, usually. Sometimes peak write	Marketing, largely meaningless
VPG 200, VPG 400	Guaranteed sustained write of 200 or 400 MB/s	CFexpress, the useful rating
CFexpress 2.0 or 4.0	Interface generation and maximum bandwidth	CFexpress, sets the ceiling

Two rows are the ones to read on a package. The V class on an SD card and the VPG rating on a CFexpress card are the only figures that guarantee a sustained minimum, and sustained minimum is exactly what determines whether a recording mode works. Everything else is bus capability or marketing.

What each format sustains

Format	Typical sustained write	Maximum bitrate supported	Cameras
SD UHS-I V30	30 MB/s	About 240 Mbps	Consumer and older bodies
SD UHS-II V60	60 MB/s	About 480 Mbps	Most current hybrid bodies
SD UHS-II V90	90 MB/s	About 700 Mbps	Most current hybrid bodies
CFexpress Type A 2.0	400 to 700 MB/s	Several thousand Mbps	Sony cine line
CFexpress Type A 4.0	700 to 900 MB/s	Several thousand Mbps	Sony cine line
CFexpress Type B 2.0	400 to 1,400 MB/s	Effectively unlimited for video	Canon, Nikon, Panasonic, cine
CFexpress Type B 4.0	1,400 to 2,800 MB/s	Effectively unlimited for video	Current high end bodies
CFast 2.0	400 to 500 MB/s	Several thousand Mbps	Older Blackmagic and cine bodies
External SSD over USB-C	400 to 1,000 MB/s	Several thousand Mbps	Blackmagic, external recorders

Notice that V90 SD covers modes up to roughly 700 Mbps, which includes ordinary 4K, most all-intra recording and BRAW at moderate compression. A large fraction of real work never exceeds that, which is why starting on fast SD in a dual-format camera and adding CFexpress when a specific mode requires it is a sensible strategy rather than a compromise.

Type A and Type B

CFexpress Type A is physically smaller and uses a single PCIe lane. Type B is larger and uses two lanes, so it reaches roughly twice the bandwidth. They are different cards in different slots and no adapter bridges them.

Which you need is decided by your camera. Sony cine line bodies including the Sony FX3, Sony FX30 and Sony FX6 use Type A in dual slots that also take SD. Canon, Nikon, Panasonic and most cinema cameras use Type B.

Type A carries a substantial price premium per gigabyte because the market is small and Sony is effectively its only customer. That premium appears on every card you buy for the life of the camera, and it is a genuine ownership cost worth counting when comparing bodies. A Lexar 256GB CFexpress Type A Silver 4.0 at around $194 is the value entry into the format.

CFexpress Type B at 512 GB

$280 to $400. The working capacity, from three manufacturers with different endurance ratings and thermal designs.

Best value $279.99

Delkin Devices

Delkin 512GB CFexpress Type B 4.0

512 GB of CFexpress 4.0 at the lowest sane price per gigabyte, which matters because RAW eats cards faster than any other format decision.

Capacity: 512 GB
Spec: CFexpress 4.0 Type B

The value card for most cine bodies.

Check price

Safe default $399.99

Sandisk

SanDisk Extreme PRO 512GB CFexpress Type B

The card most camera makers test against, which is worth something when a body is fussy about sustained write speed.

Capacity: 512 GB

The pick if your camera is picky.

Check price

Sustained write $399.99

Angelbird

Angelbird AV PRO SE CFexpress B v4 512GB

Specified for sustained rather than burst write, which is exactly the number that decides whether a long RAW take drops frames.

Focus: Sustained write

For long-take RAW recording.

Check price

Heat and throttling

CFexpress cards run hot because they are essentially NVMe drives in a card body. When a card reaches its thermal limit it reduces write speed to protect itself, which can drop it below the requirement for the mode you are recording.

The conditions that cause this are cumulative: a camera that already runs warm, a hot exterior, a cage that traps heat, and a long take at a high bitrate. It fails in exactly the mode that needed the speed, and it fails when the bitrate spikes, which is when something complex happens in frame.

It usually appears as a recording that stops rather than a corrupted file. Cards with metal shells and better thermal design hold their rate longer, and on a body known to run warm that difference is worth paying for rather than economising on.

Choosing a card, in order

Check which format your camera takes. This is not a choice, it is a fact about the body.
Find the highest bitrate mode you genuinely intend to use, and divide by 8 for the sustained write requirement.
Buy a card whose guaranteed sustained figure exceeds that with real headroom, not one rated exactly at it.
Choose 512 GB unless a specific reason says otherwise, and buy two rather than one 1 TB card.
Prefer a card with a metal shell and a stated thermal design if your body runs warm.
Buy a fast reader at the same time, because offload time is a real part of every shoot day.

The second step is where most money is wasted. People buy for an 8K RAW mode they will shoot twice, then pay a premium per gigabyte on every card for the life of the camera. If your actual work is 4K 10-bit, a V90 SD card is genuinely sufficient in a dual-format body.

Next, from here

Bitrate and card speed calculator
Your mode against a card class.
Best CFexpress cards
Both types, compared.
CFexpress versus SD UHS-II
When the cheaper card is enough.
Codec bitrate chart
What each mode demands.
Storage per hour calculator
How many cards a day needs.
Media workflow and backup
What happens after the card fills.

Common questions

What card speed rating actually matters?: The V class on an SD card and the VPG rating on a CFexpress card, because those are the only figures guaranteeing a sustained minimum write speed. The large numbers on the front of the packaging are usually peak read, occasionally peak write, and both are burst figures into a cache that say nothing about whether a recording mode will hold.
Is a V90 SD card fast enough for 4K video?: Yes, and for a good deal more. V90 guarantees 90 MB/s sustained, which supports modes up to roughly 700 Mbps including ordinary 4K, most all-intra recording and BRAW at moderate compression. What it cannot hold is high bitrate intraframe above about 700 Mbps and RAW formats, which is what CFexpress unlocks.
What is the difference between CFexpress Type A and Type B?: Type A is physically smaller and uses one PCIe lane; Type B is larger and uses two, roughly doubling the bandwidth. They are different cards in different slots with no adapter between them, and your camera decides which you need. Sony cine line bodies use Type A, while Canon, Nikon, Panasonic and most cinema cameras use Type B.
Why is CFexpress Type A more expensive?: Because the market is small and Sony cine line bodies are effectively the only customer, so the volume that drives down Type B pricing does not exist. It is also slower, using one PCIe lane instead of two. That premium appears on every card for the life of the camera and is a real ownership cost worth counting when comparing bodies.
Why does a card slow down when it gets hot?: CFexpress cards are essentially NVMe drives in a card body, and when they reach a thermal limit they reduce write speed to protect themselves. In a warm camera in a cage in direct sun, a card that sustains 400 MB/s on a bench can drop below the requirement for your mode, and it does so when the bitrate spikes on a complex shot.
Should I buy one large card or two smaller ones?: Two 512 GB cards rather than one 1 TB card. They usually cost less per gigabyte, and a failure or a loss then costs part of a shoot day rather than the whole thing. Five hundred and twelve gigabytes is roughly 4.7 hours at 240 Mbps, which comfortably covers a single-camera day at the codecs most people use.

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