Camera Card Speeds: CFexpress, CFast, SD

Peak speed vs sustained write

A card label carries two kinds of speed. The large read and write figures are, in the CompactFlash Association's words, typically maximum values: what the card manages at its best. The small badge, VPG200 on a CFexpress card or V90 on an SD card, is a guaranteed minimum sustained write speed, tested under conditions the standard defines.

A camera recording video writes continuously for as long as you roll. If the card drops below the camera's data rate for long enough, the camera stops recording. So the badge, not the headline figure, is the number to compare with your codec.

Card interfaces and their ceilings

The interface sets how fast a card could possibly go. These are theoretical maximums of the bus, from the standards bodies; real cards are slower.

Type A and Type B are different sizes (20 × 28 mm and 29.8 × 38.5 mm) and do not fit each other's slots. Type B has the same size and connector as XQD. A Type C form factor exists in the standard, but the CFA says no cameras or cards use it yet.

VPG and Video Speed Class

CompactFlash: Video Performance Guarantee

SD: Video Speed Class

SD Express cards carry their own E150 to E600 classes, for 150 to 600 MB/s. Note the units: every class is in megabytes per second, and every codec rate is in megabits per second.

From bitrate to the card you need

The arithmetic tells you what is impossible, not what is enough. Camera makers set the card they support with margin built in, and their list is the one that counts. Sony's α1 help guide, for example, asks for an SDXC V90 card (90 MB/s) or a CFexpress Type A VPG200 card for XAVC S-I 4K at 600 Mbps, which is 75 MB/s, and for V60 (60 MB/s) for XAVC S 4K at 280 Mbps, which is 35 MB/s.

Record time per card

The free bitrate calculator gives the record time for any card size and data rate in your browser. For the GB-per-hour view of the same numbers and a whole-shoot budget, see storage planning; for why a 1 TB card can show as 931 GB on a computer, see TB vs TiB.

Offload time

Offloading is limited by the slowest part of the chain: the card, the reader, the cable and the destination. A fast card copied to a single hard drive moves at the hard drive's speed.

Verified offloads read the data back to compare checksums, which adds time on top. Plan enough cards that one can be offloaded, checked and returned while the camera keeps rolling on the next. The LTO archiving guide covers checksums through to the archive.

Recording to SSDs

Some cameras and external recorders write to SSDs. Portable SSDs carry no VPG or Video Speed Class rating; the box gives “up to” peak figures. A drive that copies files quickly on a computer may still slow down during a long continuous write. Use the drives the camera or recorder maker has tested, and test any new drive with a full-length take at your highest data rate before the shoot.

Next step: open CineMath's Data & Storage calculator in Runtime mode, enter the card size and the camera's data rate, and it gives the record time, plus the time at half and double the rate.