Understanding Memory Cards

Getting yourself in a memory card muddle and not sure which card to buy? We look at memory card speeds and the fastest memory card on the market to help explain the differences so you can find out what's the best card for you.

Which memrory card should I buy?

Not sure what the difference is between a £50 Class 2 SD and a £450 Class 10 SDHC memory card? We've split them up into their categories and broken down the speed jargon by translating it into real speed ratings so you can decide if a certain memory card is worth the extra money.

The memory card market does sometimes seem overcomplicated, but if there's one thing you need to remember from this article then it's this: image quality is completely unaffected by your choice of memory card.

A £5 SD card from a supermarket will give you the same results as using the latest generation of card from Lexar, SanDisk or Samsung. The difference, however, is that the cheaper card may do it much more slowly, be less reliable, have fewer backup backup measures, different components, and, in terms of memory card data recovery, may not be such a wise choice if things go wrong and your images go missing.

Types of Memory Cards:

SD memory cardSD (Secure Digital) Memory Cards:

SD cards are by far the most common type of memory card. They are compatible with the majority of digital cameras.

SDHC (Secure Digital High Capacity) Memory Cards:

SDHC memory card

These are SD memory cards but with a higher capacity. Original SD cards only went up to 2GB, so SDHC was invented with a maximum capacity of 32GB. They are identical in shape and size, but they are different media types. Though your camera may fit a SDHC, be careful because if the camera was made before SDHC came along it may not recognise it.

SDXC (Secure Digital 'Xtra Capacity') Memory Cards:

SDXC memory card

These are SD cards but with a much higher capacity and faster processing speeds. These have a maximum capacity of 2TB (Terabytes). Similar to SDHC, in that an SDXC fits in a normal SD slot - but your camera may not be able to recognise this newer technology, so always check in advance. Computers also need to be able to read the exFAT filesystem to be compatible with SDXC. Currently Linux, Windows 7, Mac OSX (Snow Leopard) and some earlier versions of Microsoft Windows are compatible.

CompactFlash (CF) Memory Cards:

CompactFlash (CF) cards offer very high storage capacities and fast processing times. They were first introduced by Sandisk in 1994 and were widely used, but now they are usually only found in the most advanced DSLRs. Last year Canon chose CompactFlash as the recording media for use in its new lineup of professional high definiton (HD) video cameras.

Micro SD Memory cards:

Micro SD memory card

Micro SD cards were initially a popular method of storing images in mobile phones. In actual size they are the smallest commercially available memory card at 15×11×1mm but can store up to 2GB of information. The Micro SDHC versions are able to store much larger files from 4GB-32GB. Micro SD cards are now more commonly seen in GPS systems and MP3 players, however a small number of digital cameras (recent Samsung compact models) are also compatible with them.

xD Picture Memory Cards:XD Memory card

xD Picture cards (standing for 'eXtreme Digital') are a Fujifilm format used in some (older) Fuji and Olympus cameras, although these brands are now routinely compatible with more standard SD/SDHC technology.

Memory Stick DuoMemory Stick Duo Memory Cards:

Memory Stick Duo was launched, and is still used, by Sony digital cameras. Most Sony cameras are now also compatible with SD cards.

Multi Media Cards (MMC):

Multi Media Cards have the same physical appearance as Secure Digital Cards, but just without the access lock. They are used as an alternative to SD and will fit most compatible cameras, although transfer rates are lower.

UHS-I standard:

UHS-I standard SDHC cards were recently released by Sandisk and aim to offer quicker transfer rates, increasing write speeds up to 45MB/s or faster. The Sandisk Extreme Pro card matches up with the sheer amount of data streaming through the camera's buffer when shooting lots of Raw files or high quality HD movies. Prices range between around £50-£190 depending on the capacity (currently 8-64GB).

CFast 2.0 standard:

In 2012, the CompactFlash Association announced the CFast 2.0 Standard, promising read and write speeds of more than double what was then the current standard. In September 2013, SanDisk released the first CFast 2.0 card, billed as the world's fastest memory card, promising read speeds of up to 450MB/s and write speeds of up to 350MB/s.

Memory Card Readers:

memory card reader

You'll need a memory card reader to transfer photos to your computer if you don't fancy lugging around a USB cable for every one of your devices. You'll be able to get a card reader for each of the above types of memory cards and some come with built in memory and can also function as a USB flash drive. But check the device you're loading your photos to as some computers, printers and notebooks already come with built-in memory card slots. If you're using more than one memory card regularly it will probably be worth investing in a multi-card reader, which accept multiple types of memory cards and brands. Some even take as many as 35-in-1.

What to look for: 

Amateur Photographers:

If you're just starting out or just do photography as a part-time hobby then, generally speaking, the most important feature to look for when buying a card is the capacity. Most memory card manufacturers publish tables on their websites to show how many images you can save on the specific card. Different file types, compression and resolution all affect the size of each file, so the number of images you can put on one card from one camera to the next is never the same. Between 1GB and 8GB storage should be enough for an average beginner photographer using a compact camera and these won't break your bank either.

Which is the fastest memrory card?

Professionals or Semi-Professionals

When things get a bit more serious, enthusiasts and professionals need to look for the speed of a card, as most DSLRs can produce large Raw files, shoot HD video or capture multiple shots in a single burst, the data streaming through the camera's buffer will need to be met by a card at the end that can 'match up' to its specification to receive all the information. (See below for how to work out the speeds of a card.)
Professionals should also look at how reliable a card is as you can't take the risk of losing all your photos. This can be worked out by Mean Time Before Failure (MTBF). SanDisk claims a MTBF of over 1,000,000 hours for its memory cards - that's almost 115 years before the average card is expected to fail.


The speed of a memory card is important for two reasons - read and write speeds.

A card's read speed describes how fast data can be retrieved from a card. This performance is seen when transferring card contents to computers and printers for example. A faster read speed will transfer images to your computer more rapidly also (depending on how the SD card is wired up to the computer, as a direct connection vs USB 2 vs FireWire 800 vs USB 3 will make a significant difference also, as will, potentially, your hard disk or SSD storage memory speed).

The write speed describes how fast images can be saved onto a card, which is important when shooting bursts of images in continuous shooting mode, HD video or when using high resolution cameras that shoot particularly large files.
Therefore if you're doing sports photography, especially with a high continuous burst shooting mode, you will need a card with a fast writing speed.

memory card for continuous shooting mode

Or if you are shooting weddings and downloading a lot of Raw files to your computer then it would be worth investing in a card with a fast reading speed.

Two types of card speed:

You'll find an indication of a memory card's read or write speed from the various cryptic markings on it. But before you get out your school algebra book and attempt some mathematical calculations, remember read speed is faster than write speed.

Cards often have a multiplication factor written on them which usually represents read speed (such as 133x, 200x, 300x, etc). This is called the ‘Commercial x rating' with 1x being equivalent to the speed of the original CD-ROM of 150 KB/sec. This makes it easy to convert between the two by multiplying or dividing by 150. So, 200x will equate to 1 seconds to read a 29.5MB image file (200 x 150 = 30,000/1016 = 29.528).

A fairer and more recent system is the ‘class rating'. The SD Association created the speed class rating test which focuses on finding the absolute minimum data transfer rate of SD/SDHC/SDXC cards, as opposed to a sustainable rate.

memory card speed class ratings
A Class 2 card can handle sustained writing of data at a rate of 2MB/sec; a Class 4 card achieves 4MB/sec; a Class 8 card 8MB/sec, and so on. However, this is the minimum rate rather than the actual rate. It's entirely possible a Class 4 SDXC card will also brandish 15MB/s on its exterior - a claim that can only be made as a possible speed rather than a full-time sustainable one. 

memory card speed class ratings

The speed class rating was based on request from movie and video companies, as video recording in different formats and resolutions requires certain write speeds when recording to the card. As a result, the goal of the class ratings is to allow consumers to easily identify cards that meet the minimum level of required performance based on their use or application of the host device. Below is a listing of typical applications for each speed class.


 memory card class rating grid

 memory card speed commercial rating grid

*With a new card.

Thanks to Lexar and the SD Association for providing this information

For more information on the two speed types visit the SD Association.

What's the fastest memory card available now?

Which is the fastest memory card seems to change from week to week and several companies claim that they have the "fastest", but UDMA (Ultra Direct Memory Access) CF cards are supposed to be the quickest to date that are widely available for most cameras (CFast 2.0 cards are yet to be available for more than a narrow selection of cameras).

The fastest CompactFlash cards on the market have a speed rating of 150MB/sec (1000x), while SDHC cards currently tend to be around 20-30MB/sec (133-200x). However super-fast SD UHS-I Class 10 cards are due with potential write speeds of 80MB/s.

But before you take things into 6th gear; is your camera capable of the fastest speed out there? Probably not. The turbo speeds out there (such as Class 10) are usually aimed at video cameras producing movies which need to write as much data as possible every second. You need to make sure your camera can utilise all the speed your card can deliver, if not it goes to waste and so will your money. Consult your instruction manual or search the manufacturer's website for the fastest card speed supported.

Increasing speed:

From time to time it is considered good housekeeping to format your card and this can help increase its write speed. In most digital cameras you are able to format your card in the menu. This wipes all the images on the card, freeing up storage and clearing minor problems that may have developed on the card. Just make sure you have your images saved elsewhere before formatting!

You can sometimes help increase the read speedof your card to your computer if you are using a USB 2 or FireWire accessory such as the Lexar UDMA Dual Slot (CF and SD) model or the SanDisk ImageMate Multi-Card USB 2.0 Reader.

Top Memory cards on the market now...

Under £10

Kingston SDHC memory cardKingston SDHC 8 GB, Class 4

At under £10 this offers a Class 4 speed (4MB/s minimum) and more than enough storage for some holiday snaps with some videos as well. Kingston say all its cards are 100% tested and are backed by a lifetime warranty.

Best for: compact camera users and holiday snappers

Find the best deals for the Kingston SDHC

Under £20

Lexar SDHC 8GB, Class 6

For a few extra pounds you can upgrade to Class 6 speed and get Lexar's built-in erasure prevention, which it says will make sure you don't accidentally delete or overwrite your images

Best for: compact users and amateur DSLR users

Find the best deals for the Lexar SDHC

Under £50

Lexar Premium SDHC CardLexar Premium SDHC Card
32GB 100X, Class 6

From Lexar's premium series, this card offers a high speed (Class 6 or 100x) with writing capability of 15MB/s and large storage space. At 32GB you can save loads of photos and HD videos, plus use it as storage to transfer files to different devices. With a mid-range price this ticks all the boxes.

Best for: A bit of everything.

Find the best deals for the Lexar Premium SDHC

Under £100

Transcend SDXC Flash memory card
128 GB, Class 10
Around £80-£100

For an extra £50 or so, SDXC boasts faster processing speeds than SDHC cards and this Transcend is capable of write speeds over 30MB/s. With a built-in Error Correcting Code (ECC) to detect/correct malfunctions, it is a reliable component to have with you for professional shoots. With the exFAT file system, it can support files larger than 4GB, so is great for HD video. It's worth shopping around for the best deal on this one.

Best for: Enthusiasts

Find the best deals for the Transcend SDHC

Under £200

Sandisk UHS-I SDXC card class 10
64GB, 95MB/s.

Fast shot-to-shot performance for use with burst mode, even in extreme heat or freezing conditions. Makes large files more rapid to save, increasing speeds up to a potential 95MB/s.

Best For: Professionals shooting Raw files and HD movies

Find the best deals for the Sandisk UHS-I SDXC

Over £200

Sandisk Extreme Professional Compact Flash Card
64GB, 160MB/s.

SanDisk has likely had the last word in terms of speed for the Compactflash standard, with this 160MB/s monster. Sandisk claims it's almost 'unbreakable' with tests taking it to -25C and up to 85C.

Best for: Professional DSLR and HD video users

Find the best deals for the Sandisk Extreme Professional

Over £300

Sandisk Extreme Professional Compact Flash Card
128GB, 160MB/s.

The next step up in the Extreme Pro series, this is likely the definitive card of the CompactFlash standard. If you've got the cash to splash out on one of these you'll enjoy the same 160MB/s speed as the 64GB version, but with a whopping 128GB of storage.

Best for: Professional DSLR and HD video after the very best in perfomance

Find the best deals for the Sandisk Extreme Professional 

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