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(Pocket-lint) - SD cards are all the same, right? Wrong! What you might not have been so clear about is just what the differences are.

Here we'll explain what the differences are in speed, including UHS-I, UHS-II, U1 and U3, and much more. Because, yes, SD memory cards can be confusing. 

In our guide we'll start with the basics but work our way through to the more advanced stuff. We'll unpack the jargon and explain the differences between cards so you can make an informed purchase.

What is an SD memory card?

SD, or Secure Digital, is a memory card conceived in 1999 by San Disk, Panasonic and Toshiba. It has gone on to become the industry standard memory card for digital cameras - no other card is used more widely. 

There are now SD, SDHC (high capacity) and SDXC (extended capacity) varieties. Ultimately, how much you'll pay for a memory card reflects the different compatibility, speed, capacity and, more than likely, brand name. 

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For instance, you'll pay more for a new SDXC memory card that supports 4K video and has a large capacity, especially if it wears one of the reputable brand names. Depending on your camera, you may not need to buy the most expensive card around. 

Picture quality is in no way affected by which memory card is used in the camera. However, if the card's write speed cannot match the camera's performance, you'll experience delays when shooting. 

Card Dimensions and security 

  • SD card: 24 x 32 x 2.1mm
  • microSD card: 11 x 15 x 1.0mm

In addition to the standard card size, a number of devices - phones being the most typical - also use as the smaller-scale microSD format. The sizes are significantly different, as outlined above, so make sure you buy the correct one for your device.

Data on the card can be secured as read-only by flicking the lock switch - a physical little slider to the side. With the lock on, the card will not record new images or allow edits to existing images. There is no such lock on a microSD card.

Who produces SD cards? Avoiding fakes

There are numerous brands that distribute SD memory cards. The big, reputable names in Europe include SanDisk, Lexar, Toshiba, Kingston, Samsung, Sony, Delkin and PNY. You'll also find a host of other memory cards available.  

However, there are counterfeit SD cards out there, which more often than not look genuine. You may find that the counterfeit still works, but its capacity and speed are inaccurate, plus the card proves less reliable. It's advisable to purchase a memory card from an official retailer to avoid getting a fake.

Other memory card formats

There are other memory card formats used in today's digital cameras, namely Compact Flash (CF) and XQD. Both of these cards types are physically bigger and typically used in large DSLR cameras. XQD cards in particular are very fast.  

Capacity: SD vs SDHC vs SDXC

There are three varieties of SD memory cards: SD, SDHC and SDXC. They all have the same physical dimensions, but differ in terms of compatibility, speed and, most prominently, capacity limitations.

SD and SDHC cards are much more limited than SDXC in what capacity they offer and their read/write speeds. Certainly, you won't be able to get the most out of today's high-performance digital cameras by using a 10-year-old SD memory card.

At launch you'd get 16MB SD cards, nowadays it's possible to buy up to 1TB SDXC cards. Typical card capacities double in size up to these values: 16GB, 32GB, 64GB, 128GB, 256GB, 512GB, 1TB, with 2TB cards expected to launch in the future.

Read vs write speed

A card's read and write speed values are indicated in megabytes per second (MB/s).

The write speed is how quick images can be saved to a card. Read speed is how quick the images can be transferred from card to computer. 

When thinking if the card can cope with your camera's high-speed and high-resolution shooting modes, consider the card's write speed. 

Speed Class rating

OK, this is a little complicated, but we'll try to make the jargon as clear as possible.

Currently there are three main class ratings: the original Speed Class, 'UHS' Speed Class, and 'Video' Speed Class.

For the original Speed Class there are four speeds: 2, 4, 6 and 10. Each respective number indicates the minimum sustained write speed, indicated in MB per second, e.g. Class 10 is 10MB/s as a minimum, though its maximum can be much more. All modern digital cameras could really do with a Class 10 memory card in order to get the best possible performance.

Then there are the UHS speed ratings. You'll find either a UHS-I (U1) or UHS-II (U3). Again, each indicates the minimum sustained write speed, this time by multiples of 10MB/s, e.g. U1 is at least 10MB/s write speed and U3 is at least 30MB/s. While a sustained write speed of 30MB/s is possible, the UHS-I card type can provide up to 104MB/s transfer speed. 

Thirdly, some cards also chuck in a Video Speed Class. Again, the numbers represent the minimum guaranteed sustained write speed, indicated in MB/s, between 6 and 90. So V30 means 30MB/s, which is fast enough for 4K video recording, while V90 (90MB/s) should cater for 8K video capture.

Class rating: Does speed matter?

Yes, speed matters. 

The speed of a card affects the camera's ability to process data and clear its buffer. For example, a camera may offer 10 full-resolution photos per second for a sustained length of time. Yet if the memory card used in the camera cannot match that performance, the sequence will stop early.

Likewise, you may try to record a 4K video, only for the camera to inform you that a U3 class card is required (because it needs a minimum guaranteed 30MB/s speed) to avoid frame loss. For example, the Sony A6600 shoots 4K video, but will only accept a minimum of UHS-I, Class 10, U3 memory card for 4K video. However, the same camera accepts a slower UHS-I, Class 10, U1 card for other functions like taking pictures, but not for 4K video.

Mbps vs MB/s

MB/s and Mbps are not the same thing, here's the difference:

  • MB/s = Megabytes per second
  • Mbps = Megabits per second
  • 1 byte = 8 bits

Memory card speeds are advertised in MB/s (Megabytes per second), whereas a camera or video bitrate specifies Mbps (Megabits per second) . It's confusing, but the latter is an eighth of the former.

Higher resolution videos have a higher bitrate (Mbps). 4K video recording has a higher bitrate than Full HD. Therefore, for 4K video recording, a high speed card is required in order for the card to keep up with the camera.

Bus Speed: UHS-I and UHS-II

Confusingly, there is also UHS Bus Speed: UHS-I and UHS-II, while UHS-III will be upon us soon.

Not strictly a speed class, the Bus Speed indicates a maximum possible speed, not a minimum guaranteed sustained write speed.

We mentioned how a UHS-I U3 card guarantees 30MB/s, but this card has the potential for up to 104MB/s. The MB/s written on a memory card is its claimed maximum performance, yet these often prove inaccurate.

A UHS-II card offers up to three times the speed of a UHS-I card. Although limited to 312MB/s, we have not yet seen a UHS-II card better around half of that potential performance in speed tests, with the fastest SD cards currently providing around 150MB/s transfer speed.

Jargon buster

  • Speed Class (C): Speed Class indicates the minimum guaranteed sustained write speed in MB/s
  • SD/ SDHC/ SDXC: Indicates the maximum possible capacity and speed rating as well as its compatibility with your camera
  • UHS speed: U1 guarantees a minimum sustained write speed of 10MB/s, while U3 guarantees 30MB/s
  • Bus Speed: UHS-I and UHS-II are the bus speed, which indicates the maximum possible read/ write speed of the card
  • Video class: Indicates the minimum guaranteed sustained write speed in MB/s
  • MB/s: This is the claimed maximum write/ read speed of the card, not sustained speed
  • Capacity: Indicated in GB, this is the maximum amount of data that can be stored on the memory card
  • Lock: A security lock that prevents read/write on the card

SD card maintenance

Formatting a memory card erases all the data on it. Therefore, before formatting a memory card, ensure any treasured images stored on it are backed up.

If the card is being used in multiple cameras, it is advisable to format the memory card before using it in another camera.

This article was originally published in August 2018 and has been updated to reflect new standards and information.

Writing by Mike Lowe. Editing by Dan Grabham. Originally published on 16 April 2013.