Does your camera need a fast SD card?

Flash memory cards for digital cameras are now absurdly cheap. A 64GB SD card can be bought online for around £30. That’s enough space to store 5,000 raw files produced by a typical DSLR – or upwards of 30,000 JPEGs.

These cut-price cards come with a caveat, though. Their transfer rates are comparatively low, meaning that it can take several seconds to store an image once you’ve pressed the shutter. SD card manufacturers encourage serious photographers to pay more for faster cards to ensure they don’t miss a shot – and aren’t left waiting around when it’s time to transfer their pictures to a PC at the end of the day. But how significant is the real difference between a dirt-cheap card and a premium one? And is it worth the cost?

SD card speed ratings

The first challenge is understanding the relative speeds of different cards. Helpfully, all SD cards are rated with a “class”, which reflects their performance. There are four standard ratings, which you’ll see advertised as class 2, 4, 6 and 10; these respectively guarantee that the card can sustain a write speed of 2MB/sec, 4MB/sec, 6MB/sec or 10MB/sec. (We’ll discuss what this means in practical terms later on.)

The class system makes it easy to distinguish the slowest cards. When it comes to high-end cards, however, it’s useless, since a card that supports 40MB/sec will receive the same class 10 rating as a 12MB/sec card.

For this reason, manufacturers may supplement a card’s class rating with an explicit declaration of transfer speeds in megabytes per second. They may also give a speed rating as a multiplication factor, such as “100x” or “200x”. This reflects how much faster the card is than (believe it or not) a standard CD-ROM drive with a transfer speed of 150KB/sec; a rating of 66x or above would thus be equivalent to class 10. A 200x rating would imply a transfer rate of 30MB/sec.

Be warned that these ratings don’t have a standard meaning in the way that class ratings do. Unless the manufacturer explicitly asserts otherwise, the figures quoted on the packaging could reflect the card’s theoretical maximum read speed – rather than its minimum sustained write speed, which is the important factor for camera performance.

You may also see cards marked with a UHS-1 rating. This indicates compatibility with the relatively new Ultra-High Speed SD standard, which raises the theoretical maximum transfer speed from 104MB/sec to 312MB/sec. However, certification on its own doesn’t tell you anything about the write performance of the card – a UHS-1 certified card could be slower than an uncertified one.

How fast is fast enough?

The class rating system has its limitations, but it can be a handy guide to the practical capabilities of different cards. A class 2 rating means the card is guaranteed to be fast enough for standard-definition video recording, while classes 4 and 6 are fast enough for Full HD video (which one you need will depend on the bit rate of the video format you’re using).

The highest rating, class 10, is faster than required for any modern video standard: rather, it’s aimed at stills photographers. The idea is to minimise the time it takes to write a photograph to the card, so you can take multiple shots in rapid succession without having to wait around for each one to be stored.

It may seem counter-intuitive that capturing still images requires a faster card than shooting video, but Full HD footage isn’t as space-hungry as you might imagine. Despite the “high-definition” terminology, each HD frame has a comparatively low resolution of just over two megapixels. Plus, since consecutive frames of a video are often extremely similar, clever compression techniques can be used to store moving images efficiently. A data rate of 4-6MB/sec is ample for continuous shooting.

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.

To get a meaningful measure of the differences between SD cards, we therefore loaded a variety of cards into a DSLR and observed exactly how rapidly each one enabled the camera to shoot JPEG in continuous drive mode. (One of the cards was a microSD card used in an adapter – see SD flavours, below.)

We then switched to raw mode and repeated the test. The camera we used was a Sony SLT-A77, which produces raw files of approximately 24MB and JPEGs of around 5MB with standard settings. If your camera produces smaller files then you can expect a faster turnaround, but the relation between cards should remain the same.

The results

You can see the results of our tests in the graphs below. As we’d expect, continuous shooting performance was blazingly fast for the first few seconds of the test while the camera was able to make use of its buffer. Once the buffer filled up, the differing speeds of the various cards came into play. The shooting rate fell into a slower rhythm (with the odd hiccup), giving a clear indication of each card’s true data-writing performance.

It’s immediately obvious that our two top-end SD cards outshone the rest by far. In our JPEG test, once the camera’s buffer was full, class 4 cards dawdled for more than a second between shots – while the 64GB Kingston 233X and the SanDisk Extreme 45MB/sec each kept snapping away at more than three shots per second. Clearly, the faster cards offer you a much better chance of capturing the perfect moment.

With these installed, the camera was able to completely empty its buffer, ready to start shooting again, only six seconds after we took our finger off the shutter. Other cards were much slower: the class 4 SanDisk card took 25 seconds to catch up. Copying the shots off the card was faster too. There isn’t a perfect correlation between write and read speeds, but by using a USB 3 SD card reader, we were able to copy files from the Kingston 233X card at an average of 70MB/sec, while other cards offered less than a third of that read speed.

We saw similar results in raw mode – but the sheer volume of data caused even the fastest cards to feel sluggish. The fast Kingston and SanDisk cards achieved only 1.25 raw shots per second, with other cards taking between three and five seconds to capture each image.

Worth the price?

Our test camera had a large enough buffer to capture around 20 JPEGs or ten raw files at very high frame rates before the speed of the memory card became an issue.

If you use your camera mostly for stills and portraits, which you can shoot at a leisurely pace, it’s worth experimenting to see how many shots its internal buffer can handle. You may find your camera provides all the headroom you need, in which case there’s no point shelling out for a fast SD card.

If you’re likely to spend more time taking photos of wildlife or sporting events, however, our results show that a high-speed card will help you to squeeze in more shots per second, giving you a better chance of capturing the perfect moment. After all, having spent hundreds or even thousands of pounds on a DSLR, it makes little sense to baulk at paying £60 for a memory card that enables you to get the best from it.

A final point that our results neatly illustrate is the benefit of choosing JPEG mode, rather than raw, for continuous shooting. Although shooting in this mode captures less fine tonal detail, this is more than offset by the ability to fire off a sustained string of shots at three or even four times the rate available in raw mode.

SD flavours

When you’re shopping for a new SD card, you’ll probably see multiple sizes on offer – the regular size, measuring 24 x 32mm, plus microSD models measuring only 11 x 15mm. You may possibly also spot the odd miniSD card, measuring 20 x 21.5mm, although this format is comparatively rare.

These smaller cards are intended for use in small devices, such as smartphones, but with the right adapter – which is sometimes provided free in the packaging – you can use one in a full-sized SD slot. The adapter is simply a plastic sleeve with contacts; electronically, all three formats are identical.

You may also notice that some cards are described specifically as SDHC or SDXC media. This is because the original SD standard, devised in 1999, only set out electronic specifications for cards with capacities up to 2GB. As larger capacities became commonplace, new standards were needed. SDHC, introduced in 2005, raised the maximum capacity to 32GB; in 2009, SDXC raised the limit further to 2TB.

Happily, the later standards maintain backwards compatibility with the earlier ones, so a device that supports SDXC will also work with SDHC and SD cards. As such, there’s no need to worry about which type of card you buy – unless you plan to use a very large card with hardware over three years old, which may predate the SDXC standard.

As you’d expect, the HC and XC standards apply to small-format cards just as they do to full-sized ones, so high-capacity microSD cards are referred to as microSDHC and microSDXC media.

In case it isn’t obvious, the HC and XC suffixes stand for “high capacity” and “extended capacity”. But do you know what “SD” stands for? It’s short for “Secure Digital” media – a reference to the fact that the format has a built-in DRM system. This can be used to “lock” files on the card so that they can only be accessed with a specific key – making it possible, for example, to distribute movies and music on an SD card, which can be played by an authorised player.

In practice, though, SD has never been widely used as a distribution medium in this way, having instead established itself as a way of storing your own personal data.

Cards for tablets and smartphones

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.

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. The only advantage of a faster card for media is that syncing with your PC will be quicker. If you frequently transfer big movie files or folders full of music back and forth, an upgrade could be well worth the price.

It’s a similar story when it comes to apps. A fast microSD card will make games and utilities load more quickly, but in many cases the difference won’t be noticeable. Of the top ten Android apps in the Google Play store at the time of writing, the largest was Disney’s Where’s My Water, weighing in at 48MB (most are much smaller). Even the slowest SD card in our tests would be able to load this entire game into memory in around three seconds. Unless apps grow considerably in size in the near future, there isn’t much need for superfast media.

There’s one specific scenario in which we’d recommend looking for a fast card, and that’s if you’re using an SD card to expand the storage of an Ultrabook or MacBook Air. SD flash memory is a fraction of the speed of conventional hard disks and SSDs, so if you try to use a slow card for general-purpose data storage, you’re likely to find the experience infuriatingly sluggish.