6. Speed Class
For some time now, SDHC and SDXC cards have been marked with a figure inside an almost-complete circle. These figures are either 2, 4, 6, or 10, and they refer to the Speed Class of the card.
What this figure tells you is the minimum sustained write speed of the card in MB/s. In other words, this is how quickly the card guarantees to have information written to it continuously. This is useful for those capturing videos, where data needs to be recorded to the card without any interruption for prolonged periods of time.
A Speed Class 2 card guarantees a minimum sustained write speed of 2MB/s, a Speed Class 4 card guarantees a minimum sustained write speed of 4MB/s, and so on. Bear in mind that this is the minimum guaranteed speed, not the set constant speed.
These figures don't sound very good in comparison with those mentioned earlier, but video is recorded in a different way to still images and the demands are not quite the same. But which do you need? The SD Association reckons that a card with a Class 4 rating is good enough for Full HD video, but that you should ideally opt for a Class 6 or Class 10 card. This does also depends on frame rate, however, with higher frame rates requiring faster cards. When you start to shoot 4K video you need something more capable – more on this in a second.
7. UHS speed class
There are currently two UHS speed classes: UHS Speed Class 1 and UHS Speed Class 3. The way this is written on a card is with the number 1 or 3 inside the letter U.
This one is fairly easy to understand: UHS Speed Class 1 cards have a minimum write speed of 10MB/s, while UHS Speed Class 3 ups this to 30MB/s. Again, this is one for those capturing video, who need to know their footage will be recorded steadily and without issues.
These are only found on SDHC and SDXC cards, rather than older SD types. You can still use these cards in older cameras that don't support the UHS standard, but you won't realise the same speed benefits.
The smaller micro SDHC and micro SDXC cards also bear many of these markings, so all of this applies equally, although they are not designed with the same write-protect tab on their side as full-size SD cards.
8. UHS Bus IF product family
Not to be confused with the U1 and U3 markings described above, there are currently three UHS Bus IF categories: UHS-I, UHS-II and UHS-III. On the card, these are simply marked with a Roman numeral.
This figure relates to the card's 'bus interface', which plays a crucial role in determining transfer speeds. UHS-I cards have a maximum bus speed of 104MB/s, while UHS-II cards have a maximum bus speed of 312MB/s. UHS-III cards, meanwhile, double this to 624MB/s, but they are not available yet.
Why is this important? A faster card will help your camera to have a longer burst depth and will write images in less time. As such, this factor is particularly important for sports, action or wildlife photographers.
It will also mean you can transfer images and videos from the card to a computer in less time, providing you're using a card reader that supports this technology. Right now, it's a particular concern to those shooting VR and 360degree footage, or for any other data-intensive recording.
UHS-II and UHS-III cards are easily recognisable for their two rows of contacts on the rear side, whereas UHS-I cards only have one. Read more about this here
To make sure you will benefit from UHS-I, UHS-II or UHS-III cards, you should check your cameras specification list. Next to the type of memory card your camera supports, it will usually state whether support is provided for one or more of the UHS formats. Bear in mind that cameras designed with two card slots may not support UHS equally in each. As a general rule, the primary slot will be the more capable one, although some are now equally matched.
These cards are backwards compatible, which means that UHS-III and UHS-II cards can be used in devices that only support UHS-I (or don't even support UHS at all). You just won't get the same speed benefits of them in these.
9. Video Speed Class
Right now, there are five Video Speed Classes: V6, V10, V30, V60 and V90. Much like Speed Class described above, each figure corresponds with a minimum sustained write speed in MB/s. So, the V6 card has a minimum sustained write speed of 6MB/s, the V10 has a 10MB/s speed and so on.
This relatively recently designation was designed to keep up with the demands of video capture on modern cameras. Again, which one you need depends on exactly how it is you're shooting video, but the SD Association recommends V6, V10 and V30 cards for Full HD video; V30 and V60 for 4K video; and V60 and V90 for 8K video. That's not to say you can't use a V90 card for Full HD video, just that it's not required to do so. Essentially, the rule is that higher-rated cards are designed for higher-resolution video footage.
What about CompactFlash cards?
CompactFlash cards don't have the same UHS and video designations as SDHC and SDXC cards, but things like capacity and speed are typically marked in the same way. They do, however, sometimes have a couple of icons that you won't find on SD-type media.
One of these is UDMA. This stands for Ultra Direct Memory Access, a technology that has been used by CompactFlash cards for some time now. This tends to have a number next to it, and this guides you on the performance of the card. The most recent types offer UDMA mode 7, simply written as UDMA 7, which has a rating of 166MB/s. The older UDMA mode 6 has a rating of 133 MB/s, although it's quite common to just see UDMA with no figure next to it.
The other icon exclusive to CompactFlash cards is the Video Performance Guarantee (VPG) speed, which shows a number inside a small clapper board icon. Although this appears slightly different to the Video Speed Class marking on SDXC cards, the principle is the same: the number simply tells you the minimum sustained write speed in MB/s.
The best thing to do ...
... is to see what your camera's manufacturer recommends you use with your camera, as it knows your specific model better than anyone else. This will be in the manual, often detailed with the exactly same icons that you see on the card itself.
Read more: Why do some SD cards have two rows of pins?