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Choosing media cards can be a minefield - part 7 of 1 2 3 4 5 6 7 8

by Mike McNamee Published 01/12/2016

MicroSD, MicroSDHC and MicroSDXC

The microSD is the smallest version. You might have also seen it referred to as TransFlash or abbreviated as a TF card. MicroSD cards, as the name implies, are physically tiny and came about predominantly to be used in smartphones that would benefit from a much smaller card.

Like the full-size SD card, there are also microSDHC and microSDXC variants that expanded the storage beyond the initial limitations. Right now, the largest microSDXC available is 128GB, which isn’t as much as the largest SDXC card but then you’re limited by the physical size of the card. When buying a microSD, microSDHC or microSDXC card you’ll usually get a full-size SD card adaptor as well. Again, SDXC-compatible devices will be backwards compatible with microSDHC and microSD cards, and microSD cards can be used in microSDHC devices.

SD card speed classes

The next area of confusion is around speed classes. These are how different SD, miniSD and microSD cards are rated in terms of read and write speeds. These are important particularly when the cards are used in camcorders or action cameras as the speed of the card will limit the video resolution and bit rate you’re able to record. High-resolution and high bit rate video requires a lot of data to be written to the card very quickly. Stills cameras with high resolutions and fast burst shooting will also take modern cards to their limits.

The SD Association devised a way to standardise the speed ratings for different cards. These are defined as ‘Speed Class’ and refer to the absolute minimum sustained write speeds. Cards can be rated as Class 2 (minimum write speed of 2MB/s), Class 4 (4MB/s), Class 6 (6MB/s) or Class 10 (10MB/s).

It’s important to note that these are the minimum, so it’s entirely possible a card can achieve faster speeds but these give you an impression of the least you can expect.

Many SD card manufacturers will also list a specific speed alongside the Class rating. This means a card can be Class 10 but also be listed as ‘up to 80MB/s’. The wording is important, as that’s the best you can expect but not necessarily what you’ll always achieve. You might also see a description such as ‘533x’. This refers to a multiplication of the speed of an old CD-ROM (150KB/s). So in this case 533 x 0.15 = 80MB/s (as there are 1,000KB in a MB). On any type of SD card, the Class rating is denoted by a number inside of a C symbol.

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1st Published 01/12/2016
last update 21/07/2022 08:46:25

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