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Does your camera need a fast SD card?

Posted on 22 Feb 2013 at 17:47

Still photographs have a far higher resolution: a typical consumer DSLR may capture around 12 megapixels of detail, and high-end models often record more than 20 megapixels. Each scene may therefore contain ten times as much information as a comparable video frame – and because every image stands alone, compression options are more limited. Indeed, photographers wishing to capture the full tonal depth and quality of a scene will probably shoot in raw mode, with no compression applied. A single photograph captured in this way can easily require 16MB of storage or more.

The camera’s buffer

If your camera produces 16MB image files, a slow SD card will clearly be a drag. A class 2 model will take eight seconds to record a single image. Even a fast 30MB/sec SD card will still take a good half-second to store each photo.

Happily, this needn’t mean always waiting half a second between consecutive photographs. Camera manufacturers understand that when an unexpected photo opportunity arises, there’s a good chance you’ll want to capture as many exposures as possible.

For this reason, every digital camera has a buffer of very fast dynamic memory, in which pictures are initially stored when you press the shutter, before being written to the SD card at whichever speed the card supports. This means that even if you have a slow SD card, you can snap freely until the buffer fills up. Once it does, however, you won’t be able to shoot again until the camera frees up some space by moving images onto the SD card. The size of the buffer varies from camera to camera, but the principle is the same for all models.

The speed of your SD card, therefore, doesn’t affect how quickly you can fire off two photos. It comes into play only after you’ve shot sufficient images, in a short enough space of time, to fill the camera’s buffer.

You can confirm this by putting your camera into continuous drive mode and holding down the shutter so that it fires off a string of exposures. You’ll probably find that the first few shots trigger in quick-fire succession, but then the rate slows down, as the camera can now only take additional exposures as quickly as it can write images to the SD card.

Measuring the difference

We could test the read and write speeds of an SD card by connecting it to a PC via a USB card reader and running our standard storage benchmarks, which time how long it takes to copy files to and from a drive in Windows. However, this doesn’t accurately represent how SD cards are really used in cameras. For a start, our large-file test uses a huge 1.5GB data file, while the small-file test uses thousands of 100KB files. A card’s performance with files of such vastly different sizes doesn’t necessarily tell us how it will handle raw and JPEG photographs, which are closer to 16MB and 2MB in size respectively.

Second, the camera’s firmware may well write files to the SD card in a different way to Windows. Clearly, a camera will have less memory available than a full-fat Windows PC, and fewer resources for multitasking, so it’s likely to be less efficient at moving data onto a memory card than a PC. In short, knowing how a card performs in Windows doesn’t necessarily tell you how it will fare when it comes to shooting.