Flash memory cards for digital cameras are now absurdly cheap. A 64GB SD card can be bought online for around $60. That’s enough space to store 5000 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 actual difference between a dirt-cheap card and a premium one? And more importantly, 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
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 bitrate 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 even 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 a 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 memory 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.