What the numbers mean

SD cards and Compact Flash memory cards carry a bewildering array of names, numbers and icons. Let us explain what they mean, and why they are important

1 ‘Extreme Pro’ is just one of SanDisk’s brand names. It’s not a defined standard for card performance. Most makers break up their memory cards into ‘families’.

2 This logo tells you whether it’s an SDHC card (which this one is), ordinary SD (rare these days) or SDXC. 3 If you see an additional ‘I’ here, it means the card conforms to the faster UHS-I standard. There’s an even faster UHS-II standard found on the most expensive memory cards.

4 This indicates the card capacity. Card capacity and speed don’t always go together, though the more expensive cards generally offer both.

5 A number in a circle tells you the card’s minimum speed rating. Class 10 cards are designed for shooting full HD video. Class 6 and Class 4 cards can do it in theory but may stutter or drop frames.

6 This ‘1 in a bucket’ symbol indicates that this card also conforms to the UHS-I standard. It’s broadly equivalent to Class 10 speed, but uses an intrinsically faster system. 7 This is the card’s maximum speed, but it will only achieve this under ideal conditions. It’s helpful for choosing a card for stills, but not for video, where you need a reliable minimum speed.

8 This lock switch is unique to SD/SDHC/SDXC cards – you don’t get it on Compact Flash cards. You can use it to prevent accidental deletion of the card contents. 1 The ‘Professional’ branding identifies this memory card within the Lexar range, but it doesn’t relate to any overall standard for Compact Flash memory cards.

2 The maximum speed of the memory card is sometimes quoted as a multiple of the old CDROM standard, which was 150KB/s. This card’s ‘1066x’ speed equates to 155MB/s. 3 Compact Flash cards don’t use SD card ‘Class’ ratings for minimum sustained speed, but some have a VGP (Video Performance Guarantee) logo. VPG 20 is 20MB/s, VPG 65 is 65MB/s. 4 Newer memory cards use the faster UDMA standard, and all recent Nikon D-SLRs support UDMA (see the table overleaf). UDMA 7 is the newest and fastest variant. 5 Compact Flash cards once came in Type I and thicker Type II variants. Type II is no longer made and not supported by today’s Nikon D-SLRs. All Compact Flash cards now are Type I.

6 Card capacity is quoted in GB, as it is with SD cards. This one has a capacity of 128Gb, but Lexar has recently announced it is making a 256GB Compact Flash card.