The SD card association defines card speeds using two terms: Speed Class and UHS Speed Class.
Speed class is the original rating used and the number defines the card speed; 4 for 4MB/s, 6 for 6MB/s. etc.
UHS (Ultra High Speed) uses minimum write speeds to differentiate the cards; UHS-I Speed Class 1 has a minimum write speed of 10MB/s while UHS-I Speed Class 3 has a minimum write speed of 30MB/s.
What the logos mean
It’s easy to select a card that matches the interface and speed class required by the host device, such as a camera or camcorder.
When you need a faster card
Here are some signs you may need to upgrade your card:
Long buffering – when you press the shutter and the camera takes the shot but then it freezes until the data is fully written to the card.
Corrupted video – video cameras might produce corrupted video files if the card speed is too slow or the camera may downgrade the video to match the speed of the card. This could turn full 1080p HD video into a video with much lower resolution.
Shortened clips – when a camera has a higher writing speed than the Flash card, the camera will pause and the video clip will abruptly end once the speed limit of the card is reached.
Burst shooting – many cameras have a burst mode feature where one press produces a repeated series of photographs; this allows users to capture fast-moving events or subjects like pets and children. If the card writing speed is too slow, burst mode setting will not work.
UHS-I Speed Classes
Still images, photos and low-resolution video
Full HD videos
Cinema-quality full HD (1080p), 3D video
4K, full 1080p HD video and burst-mode action photography