Memory Card Speed

Cards with fast write speeds work best when shooting rapid bursts (10 fps or more) and recording video. They also provide time savings when transferring files to computers for storage and/or post-production editing.

Unfortunately, regardless of how fast a memory card is, the camera’s write speed will limit the maximum speed at which it can shoot while the memory buffer is filling. If the camera can write out data to the card fast enough, the available buffer capacity can be fully utilised.

Card speed also affects the time taken to review images on the monitor, which influences how fast you can shoot. Typically, read speeds are a little faster than write speeds.

In the past, CF card manufacturers defined the speed of their products as a multiplier, such as 133x, 300x or 1000x. In contrast, SD cards were listed by ‘speed class’ ratings, based upon the minimum sustained speed required for recording an even rate of video onto the card.

Both have been replaced by a more precise speed rating in megabytes per second (MB/s), which is based upon the maximum transfer speed for reading and writing images to and from a memory card.

These are the fastest speeds at which the card can operate under ideal conditions with the fastest possible interface. If a camera can only output data at 10MB/s, having a card that could accept data at 150 MB/s won’t provide faster data transfer than a card that accepts data at 20 MB/s. To complicate matters, the efficiency of the interaction between the camera’s software and the card’s onboard controller must also be factored in.

If we examine the typical performance of some DSLR cameras with fast and slow CF cards when writing RAW files, the influence of the camera becomes clear. The figures in the table here come from Rob Galbraith’s CF/SD/XQD Performance Database (www.robgalbraith.com).

	1000x	600x	300x	133x
Canon EOS 5D MkIII	80 MB/s	57 MB/s	39 MB/s	18 MB/s
Nikon D800/D800E	68 MB/s	55 MB/s	35 MB/s	19 MB/s
Canon EOS 7D	57 MB/s	53 MB/s	35 MB/s	17 MB/s
Nikon D700	41.9 MB/s	38 MB/s	26.5 MB/s	14.2 MB/s

As you can see, the fastest camera (Canon EOS 5D MkIII) gains more from a 1000x card than the others, though the Nikon D800 pair are fast enough to still gain some advantage from the fastest card. But with slower cards the difference between the cameras is reduced significantly, although the fastest camera retains a slight advantage.

Now let’s look at some figures for SDXC cards in the two cameras that include SD card slots:

	Class 10 UHS-1 95 MB/s	Class 10 UHS-1 45 MB/s	Class 10 30 MB/s	Class 4
Canon EOS 5D MkIII	19.3 MB/s	17.1 MB/s	10.4 MB/s	4.1 MB/s
Nikon D800/D800E	42 MB/s	30 MB/s	25.6 MB/s	5.6 MB/s

There’s a big difference in performance between these two cameras that doesn’t reflect their performance with CF cards. It appears Canon decided to equip the EOS 5D MkIII with a CF slot that supports the newer UDMA7 protocol but a standard SD card slot which doesn’t support the UHS high speed standard. The Nikon camera, in contrast, has two UHS card slots.

See Memory Card feature in Photo Review May issue Mag app and Jun-Aug 2014 print edition.