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.

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