Check that the card type your device uses can cope with the capacity of the card you are considering.
The high-capacity SDHC cards, for example, use a different Card-Specific Data register from SD, which means that many earlier SD devices can’t read them. SDHC cards also come pre-formatted with the FAT32 file system, and SD cards don't.
The Memory Stick Pro is a higher-capacity version of Memory Stick and can have a maximum capacity of 32GB, the same as for SDHC.
Duo and Micro are smaller versions of Memory Stick, similar to Mini SD and Micro SD cards in application.
CompactFlash cards range from 1GB to 64GB, though the specification supports capacities up to 128GB.
You can pay over the odds if you buy a fast card for a slow device, but more awkward is buying a slow card that can’t store data fast enough. This may well lead to dropped frames in recorded video, for example.
Memory Stick Pro cards with capacities higher than 1GB support a high-speed mode, which is normally enough to record even HD video.
The Memory Stick Pro-HG Duo uses an 8-bit parallel interface and an increased clock rate, making this class of card up to three times faster than Memory Stick Pro Duo. This is a better bet for 1080p – aka full HD – recording.
Classes of SD card are more varied. There are currently four different classes: 2, 4, 6 and 10, which correspond to minimum write speeds of 2MB/s, 4MBps, 6MBps and 10MB/s.
You may also see SD cards rated in the older ‘x’ system and the speed classes correspond to 13x, 26x, 40x and 66x respectively.
Typical applications for the different SD classes are SD video for class 2, HD recording at 720p and 1080p for classes 4 and 6, and full HD video with consecutive stills for class 10.
CompactFlash cards also use an ‘x’ rating system, though confusingly, not the same one as SD cards did. CompactFlash cards are rated using the same scheme as for CD-ROMs, where the maximum read rate equals the ‘x’ rating times 150Kb/s. So, for instance, a 133x card has a notional read speed of 20Mb/s.
Just in case it crosses your mind, both SD and Memory Stick cards incorporate Digital Rights Management, using a built-in processor and a protected area of memory, so you can’t use them to circumvent copy protection on commercial videos. ®