SD, SDHC, SDXC and all their micro equivalents – whether you buy them branded by Jessops, SanDisk, Kingston, Trascend, Fujifilm, Lexar, Samsung; they’re all basically the same, right? Wrong, but you knew that. What you might not have been so clear about is just what the differences are.

That’s not the only jargon we’ll explain. What about UHS-I and UHS-II? Or U1 and U3. Even write speed versus read speed? And why won’t your camera record 4K video with your old card inserted in the camera? Yes SD memory cards can be confusing. 

In our guide to SD memory cards, 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.

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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 and SDXC varieties, as well as micro SD/ SDHC/ SDXC – we’ll explain the differences later. Ultimately, how much you’ll pay for a memory card reflects the different compatibility, speed, capacity and brand name. 

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 

The dimensions of an SD memory card are 24x32x2.1mm (WxHxD).

There is also the micro SD memory card variety, which is the most commonly used memory card in portable devices like smartphones. 

Most micro SD cards come with an adaptor for the SD card format, so they also can be used in a camera that accepts SD memory cards, without changing performance ratings. The dimensions of a micro SD card are 11x15x1.0mm.

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

Who produces SD cards?

There are numerous brands that distribute SD memory cards. The big, reputable names in Europe include San Disk, Lexar, Toshiba, Kingston, Samsung, Sony, Delkin and PNY. (Sorry if we have missed any out.) You’ll also find a host of other memory cards available.  

Eye-Fi is a brand of SD memory card whose cards feature built-in Wi-Fi. This feature was quite handy a while back, but most of the cameras today feature built-in Wi-Fi, effectively making these memory cards obsolete. In addition, the performance of these cards is no match for today’s high-speed cameras.

Avoiding fakes

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. 

SD card slots

Most cameras are limited to a single SD card slot, while select others feature twin memory cards slots. Those with twin slots may have twin SD card slots or mixed media e.g. one compact flash (CF) slot and one SD slot. 

Additionally, those with twin SD card slots may differ in card compatibility e.g. the primary slot may support a UHS-II card and the other one be limited to UHS-I (we’ll explain that terminology shortly).

SD v SDHCv SDXC

There are three ‘varieties’ of SD memory cards; SD, SDHC and SDXC. What are the real differences between these cards? 

Well, they all have the same physical dimensions, but differ in terms of compatibility, speed and capacity limitations.

You may find that very old digital cameras only accept SD memory cards, not the newer SDHC and SDXC types. It’s always worth checking if an old camera has the latest firmware upgrade, because compatibility with newer memory card types is often added. 

SDXC memory cards are the fastest of the three and boast the highest capacity. They are unlikely to be backwards compatible with older digital cameras. On the flip side, SD and SDHC memory cards are forwards compatible. 

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.

Capacity

When SD cards were first launched, the capacity of the cards supplied with cameras would barely hold a single photo from one of the high-resolution cameras of today. 

Fast forward almost 20 years and SDXC memory cards are available in capacities all the way up to a whopping 512GB.

The original SD memory card was eventually limited to 2GB capacity, while the SDHC memory card type (2.0 version) is limited to a maximum 32GB capacity. In general, card capacities double in size up to these values (e.g. 128MB, 256MB, 512MB, 1GB, 2GB, 4GB, 8GB, 16GB and 32GB).

SDXC memory cards on the other hand offer a capacity up to 2TB. Currently the largest SDXC card capacity is 512GB, but don’t be surprised if a 1TB card is launched soon.  

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. 

If however the speed of transferring images from card to a computer is also important to you, then look out for a solid read speed too. Generally, good read and write speeds go hand in hand. 

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. We’ll work in order of speed, from the oldest and slowest through to the cream of the crop.

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 (U1) or UHS (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. 

Confusingly, there is also UHS ‘Bus’ Speed: UHS-I and UHS-II, while UHS-III will be upon us soon – more on Bus Speed shortly. 

While a UHS-I (U3) provides a minimum sustained write speed of 30MB/s, 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.

Currently we are at a maximum V30 (30MB/s) which is fast enough for 4k video recording, though in the future V60 and V90 will cater for 8k video recording. 

It’s all a lot to take in. We can see that different speed classes indicate a minimum guaranteed performance and the upper limit of the card type’s potential performance. But in reality what does this all mean?

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 (the is, being ready to continue full performance). 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). For example, the Sony a6500 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. Other cameras that shoot 4k video may accept a U1 card. This is partly down to the actual Megabits per second (Mbps) of the video recording – 4k video recording Mbps varies from camera to camera. (We’ll explain the difference between MB/s and Mbps later.)

Conversely, if your camera does not offer the latest and greatest video recording and high-speed shooting, then there is little point spending big on the fastest cards, when a cheaper/ slower card will suffice. Check what your camera can actually do and match your card purchase accordingly. 

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, whereas a camera specifies Mbps. It’s confusing.

Let’s put this simply. 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.

Camera and card manufacturers recommend a UHS-I Class 10 U3 card as minimum for 4k video recording. Some may function with a U1 card, but you may experience video frame losses. 

Bus Speed: UHS-I and UHS-II

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. This is where speed tests come in – just what is the card actually capable of?

Well, next up on the speed ranking is the UHS-II card, which 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, with the fastest SD cards currently providing around 150MB/s transfer speed.

In reality, UHS-II cards perform today at around double the speed of comparable UHS-I cards. 

Some top performing cameras are fast indeed. The Olympus OM-D E-M1 Mark II shoots 60fps. The Sony a6500 shoots 4k video at 100Mbps.

Can a card keep up? With the performance of cameras being improved every time, using a UHS-II memory card is becoming all the more important for the top end cameras. 

Although created more than 5 years ago, we are only starting to see UHS-II compatibility integrated in cameras more recently.

The Camera Memory Speed website. has a list of all the cameras that are UHS-II compatible

UHS-II cards are backwards compatible and use a different interface, as seen on the rear of the card.

Recently a UHS-III card type was announced. It features the same interface as a UHS-II card, yet will offer up to 624MB/s transfer rate.

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 and 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: A new class of 2016, video speed indicates the minimum guaranteed sustained write speed in MB/s
  • Capacity: Indicated in GB, this is the maximum amount of data that can be stored on the memory card
  • Lock: A security lock that when flicked down is on and will prevent the user taking more pictures on the card or editing the ones on there already
  • MB/s: This is the claimed maximum write/ read speed of the card. This information is usually an estimate and rarely spot on

Go big or go small?

Compatibility aside, consumers have two real approaches to SD memory cards. Go big or go small. One large capacity card, or several smaller capacity cards to the same capacity value. What is the best way? Well, there are plusses to either approaches.

The larger the capacity of the card, the more you will pay. e.g. A SanDisk 256GB Extreme SDXC card has a street price of £180, while the same card in 128GB capacity is £85.

Notice, it can often be cheaper to buy multiple lower capacity cards than one card that contains the same capacity as all the other cards combined. In the above example you’ll save £10 by buying two 128GB cards instead of the 256GB card.

Storing your data on multiple small capacity memory cards can be beneficial. If you lose one it is of less value and contains less of your precious images on it. 

You are also forced more often to process the images on a card if it fills up quicker, so your images see the light of day more often. 

However, large capacity cards have obvious plus sides too. You are less likely to be interrupted mid shoot (or even midway through a video recording) with a message from the camera saying card full. Replacing cards mid shoot is another thing to worry about. 

It’s also easier to keep track of what is on each card when there are less of them in your kit bag.

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SD card maintenance

So you’ve purchased the memory card, but a couple of housekeeping notes are useful to avoid any mishaps with the card. 

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

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

Some cards are advertised as shock proof/ freeze proof/ waterproof, while others not. The reality is that all cards feature proofing, it’s just not that all are advertised that way. Don’t pay more for a card based solely on how tough it is claimed to be. 

Ultimately, although SD cards are tough, it is advisable to store them in a SD card case when they are not in use and avoid mishandling