Each SD or Micro SD card has a speed rating, called a Class. Larger Class numbers correspond to a faster level of writing/recording (minimum performance) allowing files to be written to the card or recorded at a higher speed or definition (HD/4K).
The Class rating system rating corresponds to the minimum read speeds in MB/s as below
· Class 2 – 2MB/s (minimum recording speed),
· Class 4 – 4MB/s,
· Class 6 – 6MB/s,
· Class 10 – 10MB/s.
As an example for demostration purposes, Class 2 is sufficient for SD video recording, whilst Class 4 and Class 6 support HD recording. The Class rating is displayed on the card, by a number nested inside a circle.
After the Class 10 classification, it becomes a little more complicated. An additional standard called UHS was introduced, which allows cards to reach higher speeds. Here we see a similar system – UHS Class 1 (or just U1 or U-I), which has a minimum performance rating of 10MB/s writing speed, while UHS Class 3 (or U3 / UHS-III) has a rating of 30MB/s. UHS Class 3/III supports 4K video recording. UHS speed classes are shown on the card with a number inside a U.
Confusingly, UHS Class 1 and Class 10 refer to the same 10MB/s speed, so sometimes you’ll see cards that are labelled both as Class 10 and UHS-1.
If you have an Action Camera (Such as GoPro or EvoDX) that records HD at either 1080p or 4K you really need to be thinking about purchasing a UHS-3/III card, for the best performance and writing speed. Some cheaper action cameras will buffer the recording, but this is not practical in the slower card speed classes, and may cause juddering or stuttering of the recorded footage, as well as other issues. Similarly with the latest Digital SLR Cameras (especially in modes such as sport, multi-frame or burst-mode) where multiple images are taken, having a faster card will allow you to review the images much sooner (less waiting for the images to write to the card).
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