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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.

Do you need a fast SD card?

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User comments


I'm not a photographer but I've always wanted to know the differences between all the SD card types, speeds etc and this article explains in really well. Thanks very much

By Chatan on 24 Feb 2013


Thanks for a good article with very useful info.

By cja4sun on 24 Feb 2013

MJPEG video

My wife uses a Panasonic FZ50 camera for her part-time videography. The camera records in MJPEG format in a QuickTime wrapper.

Class 4 cards are adequate for situations where there's little on-screen motion, but it stutters when she must pan. Class 10 cards eliminate that.

By RonJohn on 25 Feb 2013

Tablet/phone cards used for more than just media

"Many tablets and smartphones feature a microSD slot, enabling you to expand the internal storage – perhaps to accommodate a personal media library, or simply to download and install more apps.

Do you need a fast SD card? In this case, there’s often no need to splash out on fast media. A regular class 4 or 6 card that’s capable of recording HD video will also be fast enough to play it back."

This is fine if you're just using it for media files. However these days, many people supplement their tablet or phone's internal application memory flash drive space with external SD or microSD cards in order to install more applications into this space. In this case, using a slower card will delay the launch of any applications installed on that card. A faster card will give you faster app launch times. So depending on what you put on your external card, it can be worthwhile to opt for the faster card.

By storm311 on 25 Feb 2013

USB 3 Card Reader

What is the min. speed of the SD cards that will benefit from using an USB 3 reader (vs USB 2 reader)?

By Albert on 27 Feb 2013

how about MLC vs SLC?

I must say that your review, despite being 5 pages long misses the most important property of flash memory: namely, whether its NAND cells are single or multi-level cells.
Multi Level Cells are cheaper, but will severely (10-fold, give or take) reduce reliability of flash memory. Moreover the MLC’s have higher power consumption and smaller operating temperature range.
Most manufacturers somehow “forget” to mention whether their product is MLC or SLC. The only one indicator is the speed. The MLC NAND cells are always slower than SLC’s.
So, purchasing the SD cards with fastest transfer rates, will guarantee you the highest reliability as it will be manufactured using SLC technology.
For camera´s, the reliability does not matter so much, at worst, one will lose a picture or two, but if the purpose of SD card is storage expansion, then Single Level Cells are way to go.

By stasi47 on 28 Feb 2013

SD card size and speed querry

I have a 4gb class 2 SD card in my lumix camera and I recently purchased an 8gb class 10.
Why would the 4gb class 2 be faster? It is very noticeable when editing the pictures while still on the card.

By iggysmith on 16 May 2013

SD card size and speed querry

I have a 4gb class 2 SD card in my lumix camera and I recently purchased an 8gb class 10.
Why would the 4gb class 2 be faster? It is very noticeable when editing the pictures while still on the card.

By iggysmith on 16 May 2013

I have a Sony HX300 camera for which I purchased a Sandisk Extreme Pro 8GB 95MB/S CD card in order to speed up write times. After taking 10 frames the write time is the same as aslow class 2 card. Will you explain why please.

By HillClose on 25 Oct 2013

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