Why does a DSLR stop shooting continuously after a few seconds?

Question

In continuous shooting, all cameras I used shoot for a few seconds, then stop. For example, a Nikon D7000 with a 200x (30 MB/s) single SD card makes ten RAW photos (approx. 19 MB each), six shots per second, then stops for a few seconds when the buffer is full.

Obviously, the SD card is too slow. But is the SD card the only bottleneck, or does it stop shooting also because the camera cannot process more images?

In other words, if I buy a 600x (90 MB/s) SD card or an even faster one, how this will influence the number of shoots per burst?

Answer 1

The speed of the memory card is definitely one constraining factor but as you suspect there are other bottlenecks. First there is the internal memory buffer of the camera. Each camera only has so much RAM installed. When you shoot this buffer is filled first and the camera does what it can to quickly empty the buffer to allow for more shooting. The size of the buffer generally correlates to the price of the camera. A professional DSLR will have a larger buffer than a consumer DSLR.

The size of the file being recorded affects how many images can be in the buffer before you experience the slowdown. So capturing 14 bit raw means you hit the wall much faster than 8 bit size optimized jpeg.

So that leads to your main question: will a faster memory card help in burst mode. Yes, it helps. But only to a point. Keep in mind that writing to memory in the camera is orders of magnitude faster than writing to a flash memory card. So having a faster card gets you a little more burst but not as much as you'd get from capturing significantly smaller files.

I'm mobile right now and can't do the math but my experience with UDMA and slower cards suggest that the faster card helps but it isn't a night and day experience.

Answer 2

The camera's buffer is most likely the bottleneck. Using a 133x card the dpreview.com review achieved the same 10 RAW frames. From the review:

The D7000's buffer is smaller than the D300S as well. We found that even with a fast card, no more than 32 pictures can be taken in a burst at the maximum frame rate before the camera has to slow to clear the buffer

Answer 3

In addition to what has already been said, many (most? all?) camera raw formats are not truly raw in the sense of pure sensor data readouts written to storage, but are compressed and include some metadata. This means that the camera's processor will need to do quite a bit of number-crunching (particularly to compress the data) which further constrains throughput. Many raw formats also include a small JPEG version of the photo, for previewing purposes.

If you look at the exact file sizes of the raw images, and they vary by any significant amount based on image content, then whatever the exact reason, the camera needs to do some processing of the raw data before writing it to storage. In that case, no matter how fast the pipe from sensor to buffer to storage is, if the CPU can't keep up then you hit a bottleneck. And replacing the CPU in a digital camera is almost certainly a heck of a lot more involved than replacing the memory card with a faster one...

Answer 4

As others have mentioned, the internal processing and buffer is part of the issue. But to address your question of whether a 90MB/s card would help, based on what I've seen, only somewhat. There are two parts to the speed equation, how fast the card can accept data (300x, 600x, etc) and how fast the camera can write the data. Unfortunately they don't make the second component easy to find. What what I've seen online the D7000 maxes out around 45MB/s, so anything faster is just wasted performance.