Speed ratings explained

When it comes to data transfer speeds, manufacturers often give an ‘x-factor’ rating. This is specified as a multiple of 150 kilobytes per second, the basic read speed of data from a Compact Disc. There are 1,024K in a megabyte so, a memory card with a 600x speed rating would have a transfer speed of 600 x 150 / 1,024, or around 88MBps.

That’s not the whole story, however: the quoted figure is often for read speed, and the write speed can be significantly slower. A clearer indication is given by a rating that simply states the transfer speeds, ideally for both reading and writing.

Decent sustained write speeds are essential for recording 1080p and 4K video. However, they can also be important for stills. If you’re capturing long sequences of images in continuous drive mode at raw quality settings, you can fill your camera’s memory buffer in seconds. Your camera will then slow down or stop, preventing you from taking further shots before the buffer has cleared to the memory card. The difference between fast and slow cards can result in the wait being just a few seconds or up to a minute or so.

Before you just go and buy the fastest, most expensive cards around, there’s another factor to consider. The write speed quoted is the theoretical maximum: in practice, the capture, image processing and data writing for any camera can present a bottleneck. For example, even using the very fastest cards in our test group, popular cameras like Canon’s EOS 6D Mark II and 7D Mark II topped out at 71MBps and 61.6MBps. Buying the fastest cards can be poor value.