Burst mode is one of those features that looks impressive on paper and behaves strangely in practice. The spec sheet says 30 frames per second, the marketing photos show streams of action frozen across a sequence, and you imagine you can hold the shutter down indefinitely while the camera fires. Then you try it on real subjects and the camera stops shooting after a few seconds and refuses to keep up. The next dozen seconds are spent watching a busy indicator blink while the buffer drains to the card.
What the buffer actually is
A digital camera does not write each image directly to the SD card. The image first goes into a fast memory buffer on the camera body, and a background process moves it from the buffer to the card. This works because the buffer is fast, much faster than the card, and lets the camera continue accepting new images even while older ones are still being written out. As long as the buffer has space, the shutter fires at full rate.
When the buffer fills, the camera slows. It will not stop entirely, but the shutter rate drops to whatever speed the card can absorb, which on a fast card is typically a quarter to a half of the burst rate, and on a slow card is much less. The exact behaviour depends on the camera. Some cameras gracefully reduce rate. Others stop firing entirely until enough buffer space frees up. Either way, the full burst rate quoted on the spec sheet is the rate while the buffer has room, not the sustained rate.
The number of frames you get at full speed before the buffer fills depends on three things. The size of the buffer, which is fixed by the camera body. The size of each image, which depends on the file format and the sensor resolution. And the speed of the card, which determines how fast the buffer is draining at the same time.
Why the file format matters more than you think
Raw files are large, usually 25 to 80 megabytes per frame on a modern full-frame body. JPEGs are much smaller, often 5 to 15 megabytes. A buffer that holds 50 raw files might hold 200 JPEGs. The camera also has to compress JPEGs as it writes them, which adds processing load, but the file size advantage outweighs the processing cost in almost every case.
Some cameras offer raw plus JPEG modes, which write both formats simultaneously. This doubles the buffer usage and roughly doubles the time the card takes to clear the buffer, since two files are being written for every shutter release. Photographers who shoot heavy bursts and need to deliver previews quickly sometimes use this mode anyway, but the buffer cost is real. A more thorough overview of how memory card speed classes interact with buffer depth covers the side of the question that depends on what is in the card slot.
Card speeds and what the ratings actually mean
SD cards are marked with several speed ratings, and only some of them are relevant for burst photography. The Class number such as Class 10 and the UHS Speed Class such as U1 or U3 describe minimum sustained write speeds, which matter for video. The Video Speed Class, marked V30, V60, or V90, is a newer spec for the same purpose but with higher numbers.
For burst photography, the number that actually matters is the maximum sustained write speed, which is what determines how fast the buffer drains. This is sometimes quoted in MB/s on the card. A V60 card is rated for at least 60 MB/s. A V90 card is rated for at least 90 MB/s. Real-world speeds can be higher or lower depending on the card and the camera.
CFexpress cards, which are now standard on professional bodies, are much faster than SD. CFexpress Type B cards routinely sustain 800 MB/s or more, which is enough that the buffer rarely matters even with 100 MB raw files. The downside is cost. CFexpress cards are several times the price of SD cards at the same capacity, and the readers and cables to get the files off them are also more expensive. A clear comparison of CFexpress and SD performance for high frame rate work covers the sustained throughput tradeoff in detail.
The card-camera mismatch
A fast card in a slow camera is wasted money. If the camera’s card controller maxes out at 250 MB/s, a V90 card and a V60 card will perform identically in that camera. Buying a V90 card for a body that cannot use the speed is a common mistake.
The reverse is also true. A V30 card in a body that wants V60 or higher will cause buffer overruns much sooner than the same body with a faster card. The combination of fast camera and slow card produces the worst case: you can fire short bursts at the rated frame rate, but the buffer fills almost immediately and the camera spends most of its time waiting for the card.
Mechanical, electronic first curtain, electronic shutter
The shutter mode interacts with the burst rate, which our notes on the different shutter modes cover in more detail. The short version is that mechanical shutters have a maximum cycling rate set by the physical movement of the curtains, typically around 10 to 15 frames per second on a modern body. Electronic shutters have no such limit and can read the sensor much faster, often 20 to 30 frames per second on a stacked sensor, with some bodies reaching 120 frames per second for short bursts.
Rolling shutter and the panning problem
The electronic shutter has the downside of rolling shutter artefacts when the subject is moving fast, since the sensor reads top to bottom in a fraction of a second and anything that moves during the read appears distorted. For sport and wildlife the trade is usually worth it. For subjects with horizontal motion at high speed, like cars in panning shots, the distortion can be objectionable.
Pre-buffer features and what they change
Some modern cameras include a pre-buffer mode, where the body is continuously capturing frames into a circular buffer while you half-press the shutter. The full press records the most recent few seconds of frames, including images that were captured before you fully pressed the shutter. Sony, Canon, and OM System all have versions of this, marketed under different names.
The use case is unpredictable action. A bird taking off, a child smiling unexpectedly, a sport moment where the reaction time of your finger costs you the frame. With pre-buffer enabled, the camera fires backward in time as well as forward, and the pre-buffer frames are saved to the card alongside the post-press frames. The cost is some battery drain from the constant capture and a slightly different feel to the shutter timing.
The practical question
For most photographers, the burst rate quoted on the spec sheet is irrelevant. A wedding photographer rarely shoots faster than three or four frames per second sustained. A landscape photographer rarely shoots bursts at all. A portrait photographer occasionally bursts during expressive moments but is more often shooting single frames with deliberate timing.
The photographers who actually need high burst rates and deep buffers are a narrower category. Sports, action wildlife, and certain documentary situations where the decisive moment cannot be anticipated. For these uses, the combination of fast body, fast card, and adequate buffer is genuinely important, and the difference between a body that holds 50 raw frames at full speed and one that holds 200 is the difference between catching the peak action and missing it because the camera stopped firing.
The honest reading of a spec sheet starts with what you actually shoot. If your work does not require bursts, the burst spec is a number you can mostly ignore. If your work does, the spec to look for is sustained frames per second with a realistic card, not the headline maximum that the marketing prefers to quote.