The basics, capacity and speed

Memory cards are easy to use: you simply place them in your camera, format them and you can start. Trying to understand their different figures and symbols, however, is a different story.

The situation has become more problematic in recent years because more advanced maps have been provided with new terminology to indicate certain aspects of their performance.

More base cards are fortunately free of many terms, but as cameras become more sophisticated, it becomes even more important to understand whether you are actually using the right card to do justice to its capabilities. Do not do this and you can eventually shorten the burst depth of your camera or interrupt your video recording, and hang around a lot while waiting for images to be recorded.

To clarify everything, we will go through all the symbols currently used on standard SD-type cards and explain what each card means

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  1. Brand
  2. Position in range
  3. Capacity
  4. Type
  5. Speed
  6. Speed class
  7. UHS Class
  8. UHS Bus IF product family
  9. Video Speed Class


This is the easy one: the manufacturer of the card. The most common names you'll see here are Sandisk and Lexar, although Kingston, Transcend, Samsung, Toshiba and others are also widely available. You can even have one from the same manufacturer as your camera.

Most people have a card from one of the first two brands because they are the most popular, but there are perfectly good cards from the others that are often cheaper. As with hard drives, memory cards are usually only made by a handful of companies and simply re-built by others.

Some brands are known to provide particularly good guarantees or image recovery software with their cards as standard, so you may want to include these issues in choosing between brands. Your best option is to check the manufacturer's website for complete information about what you get.

Position in range

This indicates where a manufacturer's card is placed in the range. Not all manufacturers have these different classes, but they do give you a quick idea of what kind of performance you should expect from a card.

For example, Sandisk currently has Ultra, Ultra Plus, Extreme, Extreme Plus, and Extreme Pro classes for its SD-type memory cards, as well as a more basic class that does not have a specific designation. As you choose a higher class, you will probably see a higher transfer speed (more on this later), and more advanced maps may also offer protection against water and freezing temperatures. This will of course be reflected in the asking price.


All memory cards have a capacity that must be clearly indicated on the card itself. This can be as little as 4 GB or 8 GB (and even less for older SD cards) or even 512 GB (at the time of writing). Larger 1TB and 2TB cards will also be available at some point, but frankly, even 512 GB is far above the needs of most people.

The larger the card, the more images and videos you can squeeze into it, although what ultimately ends up in practice depends on a number of factors. For example, you create JPEG images or RAW files, along with the compression level you use, whether you create high-resolution videos and how the camera records them, among other things.

Most people tend to go for a card that is around 16-64 GB in size and these are now very affordable. From the point of view of security, it is a good idea to have a number of smaller cards instead of one large one, but the ease of being able to fit weeks of photos or video images onto one high card makes it tempting.


At present all SD cards fall into one of three camps: SD, SDHC and SDXC. They all have the same shape and size, but the type is clearly indicated on the front.

Secure Digital (SD) cards still exist, but there is not much demand for them because they do not offer the types of capacities and transfer rates to do justice to today's cameras. Even if you manage to find one, you get a considerably better value for an SDHC or SDXC card, so they are best avoided.

You can still find SD cards, but you better opt for a newer SDHC or SDXC card

SDHC (Secure Digital High Capacity) cards are cards with a capacity between 4-32 GB (included). As they grow, they double in capacity: you can therefore opt for a 4 GB, 8 GB, 16 GB or 32 GB card. If you find an SDHC card with a different capacity - for example 21 GB - you should probably go shopping somewhere else.

SDXC cards (Secure Digital Extra Capacity) are cards that offer something else. These are currently 64 GB, 128 GB, 256 GB and 512 GB cards, but soon this will increase to even larger capacities.

Most contemporary cameras that use SD media support SD, SDHC, and SDXC cards, but older cameras that only support SD cards do not work with SDHC or SDXC cards and cameras that use SD only and SDHC types do not accept SDXC cards.


This is where things are starting to get a little confusing.

Most cards have one or more of their transfer rates written in MB / s (megabytes per second) or with an "x" to show this as a factor. This indicates how fast the card can work - more on this in a second.

The card below is one of a kind that has both. These numbers mean exactly the same thing, but it can be confusing when trying to compare cards that are not marked with both. This card is a good example of how you do that; a speed of 150MB / s is equal to 1000x, because a speed of 150kb is equal to 1x. A card with a rating of 45MB / s is therefore the same as 300x, 90MB / s is the same as 600x and so on.

Some cards show transfer rates in both MB / s (150 MB / s) and a factor (1000 x)

Often there is only one digit and in this case it refers to the reading speed. Reading speed is how fast information can be read from the card. This is different from the writing speed, which refers to how quickly information can be written. So when you take photos on your camera, they are written to the card. When you insert your card into your computer, images are read from it.

Reading speeds are usually slightly higher than writing speeds, so if you see only one number, this is this one. A higher figure seems to be more impressive. Some cards show both. Either way, you should be able to find out what both speeds are on the card manufacturer's website.

read write
Some cards have clearly marked both reading and writing speeds, but most have only the previous number

This is particularly useful to note when taking pictures with a modern camera with a high-resolution sensor, especially when taking burst pictures at one time. You may find with slower cards that you cannot make images sequentially for such a long period.

Speed class

For some time now, SDHC and SDXC cards have been marked with a figure in an almost complete circle. These numbers are either 2, 4, 6 or 10 and refer to the speed class of the card.

What this figure tells you is the minimum sustained write speed of the card in MB / sec. In other words, this is how quickly the card guarantees that information is written continuously. This is useful for capturing videos, where data must be stored on the card for an extended period of time without interruption.

This card has a speed class 10 rating

A Speed Class 2 card guarantees a minimum sustained writing speed of 2MB / s, a Speed Class 4 card guarantees a minimum sustained writing speed of 4MB / s, and so on. Keep in mind that this is the minimum guaranteed speed, not the set constant speed.

These figures do not sound very good compared to the aforementioned, but video is recorded on still images in a different way and the requirements are not entirely the same. But what do you need? The SD Association believes 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. However, this also depends on the frame rate, with higher frame rates that require faster cards. When you start recording 4K video, you need something better - more about this in the next section.

UHS Speed class

There are currently two UHS speed classes: UHS Speed Class 1 and UHS Speed Class 3. The way in which this is written on a card is with the number 1 or 3 within the letter U.

This is fairly easy to understand: UHS Speed Class 1 cards have a minimum write speed of 10MB / s, while UHS Speed Class 3 increases this to 30 MB / s. Again, this is one for those who record video, who need to know that their images are being recorded steadily and without problems.

These can only be found on SDHC and SDXC cards instead of older SD types. You can still use these cards in older cameras that do not support the UHS standard, but you will not achieve the same speed benefits.

UHS Bus IF product family

In order 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 map these are simply marked with a Roman numeral.

This figure refers to the "bus interface" of the card, which plays a crucial role in determining transfer rates. 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 double this to 624 MB / s, but they are not yet available.

Why is this important? A faster card helps your camera to have a longer burst depth and will write images in less time. As such, this factor is especially important for sports, action or nature photographers.

It also means that you can transfer images and videos from the card to a computer in a shorter time, provided you use a card reader that supports this technology. At the moment, it is a specific concern for those who record VR and 360-degree video, or for any other data-intensive recording.

Left: UHS-I Right: UHS-II

UHS-II and UHS-III cards are easily recognizable for their two rows of contacts on the back, while UHS-I cards have only one.

To ensure that you benefit from UHS-I, UHS-II or UHS-III cards, you must check the specification list of your cameras. In addition to the type of memory card supported by your camera, it is usually indicated if support is offered for one or more of the UHS formats. Please note that cameras designed with two card slots may not support UHS equally well everywhere. As a general rule, the primary slot is better, although some are now the same.

These cards are backwards compatible, meaning that UHS-III and UHS-II cards can be used in devices that only support UHS-I (or even do not support UHS at all). You don't get the same speed benefits from this.

Video Speed Class

There are currently five Video Speed Classes: V6, V10, V30, V60 and V90. Just like Speed Class described above, each digit corresponds to a minimum sustained write speed in MB / s. The V6 card therefore has a minimum sustained write speed of 6MB / s, the V10 has a speed of 10MB / s and so on.

This relatively recent designation is designed to meet the requirements of video recordings on modern cameras. Again, what you need depends on how exactly you make videos, 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 does not mean that you cannot use a V90 card for Full HD video, only that this is not mandatory. In essence, the rule is that cards with a higher rating are designed for video images with a higher resolution.

What about CompactFlash cards?

CompactFlash cards do not have the same UHS and video designations as SDHC and SDXC cards, but issues such as capacity and speed are usually marked in the same way. However, they sometimes have a number of icons that you will not find on SD-type media.

sandisk sdcfxps
UDMA and VPG markings (bottom left) can be found on CompactFlash cards but not on SDHC and SDXC cards

One is UDMA. This stands for Ultra Direct Memory Access, a technology that has been used by CompactFlash cards for some time. This usually has a number beside it and this provides information about the performance of the card. The most recent types offer UDMA mode 7, simply written as UDMA 7, with a rating of 166 MB / sec. The older UDMA mode 6 has a rating of 133 MB / s, although it is fairly common to only see UDMA with no figure beside it.

The other icon exclusive to CompactFlash cards is the speed of the video performance guarantee (VPG), which shows a number in a small clapperboard icon. Although this differs slightly from the Video Speed Class marking on SDXC cards, the principle is the same: the number simply indicates the minimum sustained write speed in MB / s.

The best you can do ...

... is to see what the manufacturer of your camera recommends for use with your camera, because he knows your specific model better than anyone else. This is stated in the manual, often detailed with exactly the same icons that you see on the card itself.

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