Why Your Camera Stalls Mid-Burst (Not a Full Card)

A 24-megapixel camera shooting RAW at 10 frames per second has to write roughly 250MB of data every single second to keep up with itself. Most SD cards cannot sustain that for more than a few seconds, which is why the first dance or the breakaway at midfield gets three sharp frames and then a stretch of nothing while the camera catches its breath. The card isn’t full. The buffer is, and the only thing that empties it is how fast the card can actually write.

It’s RAM, not storage, that’s the bottleneck

Every camera with a continuous shooting mode has a small bank of fast internal memory, the buffer, that frames land in the instant they’re captured. From there they get written out to the card in the background while you keep shooting. As long as the card can write frames out as fast as the shutter adds new ones in, the buffer stays mostly empty and the burst rate holds. The moment the card falls behind, the buffer starts filling, and once it’s full the camera has no choice but to slow down to the card’s actual write speed, frame by frame, until you let off the shutter and it catches up.

That’s the stall. It shows up as the frame rate visibly dropping, or the camera locking up for a second with a card-access light that won’t quit, and it has nothing to do with how many gigabytes are left on the card. A card at 80% capacity with a slow sustained write speed will stall sooner than a nearly full card with a fast one, because capacity and speed are two different specs doing two different jobs.

The number on the front of the card isn’t the number that matters

This is where most of the confusion starts. The big “up to 300MB/s” printed across a memory card’s packaging is almost always a read speed, measured off a card reader under ideal conditions. It tells you nothing about how fast that same card can sustain a write, which is the number your camera actually needs during a burst.

The spec that does describe sustained write performance is the small Video Speed Class symbol, a V followed by a number: V6, V10, V30, V60, or V90. Each one is a floor set by the SD Association, guaranteeing that many megabytes per second of sustained sequential write, no less, for the life of a full card. A V60 card is rated for at least 60MB/s sustained; a V90 card for at least 90MB/s. That guarantee is what actually governs buffer clearing, and it’s printed in a corner most people never look at twice.

There’s a second ceiling underneath the V-rating: the bus interface. UHS-I tops out around 104MB/s no matter what’s printed on the card, so a UHS-I card physically cannot deliver on a V90 rating even if one existed in that format. UHS-II adds a second row of pins and raises the ceiling to roughly 312MB/s, which is why every V90 card on the market is also UHS-II. In practice, real UHS-II V90 cards write in the 250-300MB/s range at their best, and V60 cards land closer to 150-180MB/s. Both numbers sit comfortably above their guaranteed floor, but the floor is the one you can rely on when the card is nearly full and running warm two hours into a reception.

CFexpress Type B cards, now standard in flagship bodies like the Nikon Z9 and Canon R3, skip the SD bus entirely and talk to the camera over PCIe using the same NVMe protocol as a laptop’s internal drive. Sustained write speeds on current CFexpress Type B cards run from around 800MB/s up past 1,700MB/s, roughly 15 to 20 times what the fastest SD card can hold. That’s not a nice-to-have for casual bursts; it’s the only way to keep up with 8K video or a 40fps electronic shutter, both of which can out-produce data faster than any SD card, UHS-II included, can carry it away.

A real buffer, measured

Canon’s own spec sheet for the EOS R6 Mark II shows exactly how this plays out on one body. On the mechanical shutter at 12fps, a fast card lets it hold that rate for 110 RAW frames (1,000+ if you shoot compressed C-RAW instead). Switch to the 40fps electronic shutter and the same card only sustains 75 RAW frames before the rate drops, because 40fps asks for more than three times the sustained write throughput that 12fps does, from a card that didn’t get any faster. Same camera, same card, same RAW format, two very different stalling points, entirely down to how much data per second that shooting mode demands against the card’s ceiling.

What to actually check before your next booking

Match the card to the mode you shoot most, not the mode printed on the box art. If you shoot bursts at 10-14fps in RAW on a 24-45MP body, a genuine UHS-II card rated V60 or V90 is the floor, not an upgrade, since that range is exactly where 250MB/s-plus sustained throughput starts to matter. If your body has a CFexpress slot and you shoot anything past 20fps electronic or high-bitrate 4K/8K video, the SD slot is the one for overflow and backup, not the one doing the real work. Either way, check the Video Speed Class symbol itself rather than the headline transfer number, and buy from the card’s actual spec sheet rather than the “up to” claim on the front of the pack.

Speed and capacity are both real budget lines, and they’re easy to conflate into one number when you’re planning what to bring to a shoot. The storage calculator handles the capacity side, how many gigabytes a given resolution and frame count actually produces, so you can size cards correctly before you’re also checking whether they’re fast enough to keep up with how you shoot. Capacity planning matters just as much once you’ve picked a fast enough card: the same per-wedding storage math that sizes your card order also sizes the backup drives waiting for it at home. And if a card’s write speed isn’t the only thing standing between you and a lost sequence, running two cards in backup mode is the other half of protecting a burst you can’t reshoot.

A camera that stalls mid-burst isn’t asking for a bigger card. It’s asking for a faster one, and now there’s a spec on the card itself that says which one that is.

If gear math like this is useful as an ongoing reference rather than something you re-Google before every upgrade, it turns up occasionally in our field-notes emails.