Understanding memory cards, their speed ratings, capacities and the types of cards available for digital cameras
The memory card market seems very complicated, and in some ways it is. For the new photographer working out which card to buy it can all feel a little overwhelming, as it can too for some who’ve been shooting a long time. It’s not so much the type of card needed that’s the issue, as that’s usually clear from the camera’s memory card slot, but knowing the speed rating you’ll require is another matter. Manufacturers don’t always make that information as clear as they could. In this article I’ll set out what the speed ratings mean, as well as how you know what speed you’ll need. And we’ll look at memory card types along the way, and some other ways of saving and storing the images and video from your camera.
What is a memory card?
When a camera takes a picture or records a clip of video it makes a new file to save that image and all the data that goes along with it – the lens used, exposure information, the date it was taken and so on. As most cameras don’t have much internal memory, that file needs to be shifted out of the system before another picture can be taken. Usually cameras will shift the new file to a small plastic or metal card that’s inserted into the memory card slot – the memory card. Inside the card is a solid-state memory chip to store the files sent to the card, along with a controller chip that directs where that file is saved and which helps organise the data so that we can retrieve the image later.
Using a memory card allows us to carry on taking pictures or recording video when the amount of data produced is too great for the camera to hold itself. It also allows us to take the card out of the camera so we can slot it into a computer to read, download and work on the images from there. Obviously the detailed mechanics of this process are a little more complicated than that, but a basic understanding is all most people actually need.
Which memory card do I need?
So long as you get the right shape of card and one that’s quite recent, most stills photographers will find it hard to buy a ‘wrong’ card. In the majority of cases, any card the stills photographers buys will be fast enough for most situations, so if you are buying a card for a holiday or for regular hobby photography you don’t really need to get too caught up in the speed ratings.
Where some people get caught out though is expecting the card they’ve used for years to work just as smoothly when they get a new camera that has twice the resolution, an ability to record high quality video and which can shoot a leaping squirrel at a rate of 30fps. That old card may find it can’t keep up, and the photographer won’t get the best from the new camera.
If you like to shoot bursts of pictures or make videos you do need to pay a bit more attention than the average snapper. If your camera creates image files or data more quickly than the card can save them the whole system will clog up and the camera will stop shooting until the backlog of images is cleared.
What memory card capacity do I need?
When people talk about the size of a memory card they aren’t usually referring to the physical dimensions of the card, but how much storage space it contains. Space is usually measured in MB (megabytes), GB (gigabytes) or TB (Terabytes) with MB being the smaller and TB being the larger.
When you put a fresh card into your camera the back screen will display how many pictures you have space to store or how many minutes of video you can record. If your camera tells you that a 16GB card can hold 200 pictures, it makes sense that a 32GB card will hold 400 and a 64GB card will hold 800 – so you can easily work out the ‘size’ of card you need for the number of pictures you intend to take.
Your camera’s handbook or the manufacturer’s website will tell you how big each picture is likely to be as well. This can also be a guide to how much space you require. If your camera creates JPEG files that are about 12MB you can work out reasonably easily that 100 of these will fill 1200MB (which is about 1.2GB) of memory card space, and 1000 will fill 12GB – approximately. You won’t work this out with a great deal of accuracy as JPEG picture files vary in size, but it will give you an idea.
If you prefer to shoot RAW files and those RAW files are about 30MB each, 100 of them will occupy 3GB and 1000 will fill 30GB of storage.
If you take about 100 pictures a day and you are on holiday for a week, you’ll shoot about 700 pictures and will need 8.4GB if you are shooting JPEGs or 21GB if you are shooting RAW files with the camera in the example above. Of course, if you go into a shop and ask for an 8.4GB card you’ll get a funny look, as cards come in a limited number of capacities. You may still be able to buy an 8GB card, but usually they start at 16GB, and then options double in size – so, 32GB, 64GB, 128GB, 256GB, 512GB, 1TB and so on.
If you shoot video you may not be able to calculate the space you need in the same way. Putting a card into your camera might show how many minutes you can record on to it, but the camera’s display may only show you the number of minutes the camera is limited to per clip-length. Some cameras can only record 30 minutes at a time, for example, so will show that limit instead of how much video you can actually fit on the card. To help get an idea, you might record a one-minute clip and load the card to your computer to see what size that clip is. Then you can work out roughly how many one-minutes you can fit into 32GB for example. We need to be aware though that many cameras have a wide range of choices when it comes to video quality settings and resolutions, so record your one-minute clip using the resolution and quality settings you intend to set when making your film. And again, don’t expect this to be accurate – it will just give you a guide.
Memory card speed ratings
Amoung the numerous other details card manufacturers print on their card labels will be information about the read and write speeds of the card. This will indicate how quickly data can be written to the card by the camera, and how quickly we can copy that data off the card to a computer, so this is important information to which we need to pay attention.
We’ll look at Write Speeds first. If your camera creates JPEG files that are 12MB, and it can shoot 10 pictures in a second it will generate 120MB in one second. To be able to get all this data out of the camera’s system we need a card that can accept that volume of data in that space of time – so a card that can write data at a rate of 120MB per second. This is expressed as 120MB/s. If your card is rated at 250MB/s, for example, you’ll have no problems and the camera will be able to shoot unhindered until the card is full.
If though you are shooting RAW files that are 30MB each, 10 of them shot in a second will generate data at a rate of 300MB/s. If your card can write only at 250MB/s it will record the first 250MB and then the camera will have to stop shooting. You’ll only get 8 pictures before the system becomes full, so ideally you need a card that can write at 300MB/s.
The reality is that cameras have ‘buffer’ memory which can hold some of the data the camera can’t write to the card immediately, so the camera won’t just stop at 8 pictures, but it will slow down and then take some extra time to send all the pictures to the card after you’ve stopped shooting. The camera’s buffer makes a massive difference, and will allow you to shoot far more images than this basic calculation suggests, but this gives you an idea of how things work.
However, the Write speed quoted on most cards is the peak write speed – the maximum the card can manage. Some of the time we don’t get the speed that’s quoted, so if you shoot high speed bursts it pays to get the fastest card you can, and to prioritise speed over capacity, though you will use a lot of space too quite likely. Some cards will have a ‘constant’ or ‘Sustained’ write speed, which means you will always get that speed, and you’ll pay a bit more for that.
If you shoot video your camera may well tell you how much data the settings you have chosen are likely to create. When picking the resolution and codecs the menu might give you a Mb/s figure, so you’ll know what speed of card you’ll need to record those quality settings. Mb/s and MB/s are not the same thing, as there are 8Mb in one MB, so a little conversion is required to determine how the data rate your camera quotes relates to the speed of the card. Your camera may tell you the video settings you have chosen produce 150Mb/s, so if you divide 150 by 8 to get 18.75 you’ll see you need a card that can write at 18.75MB/s. A camera data rate of 1.5Gbps will need a card that can write at a constant rate of 187.5MB/s. Of course its simple to find a card that can write at 187.5MB/s, but we need one that can keep that pace up for the whole recording time. That’s why we usually need cards with a much higher quoted ‘peak’ speed than we actually.
As video creates a lot more data than stills do, you’ll find the camera’s buffer much less forgiving, and the camera will stop quite quickly if the card isn’t up to the job. You may even be warned in the rear screen that video can’t be recorded to certain cards or card slots.
How quickly the card can give up its data to be copied to your computer might seem much less important, but when you are sitting staring at your screen while 128GB of pictures are copied to your hard drive Read Speed becomes a much more urgent issue. Most cards are able to be read from more quickly than they can be written to, so ensure you pay attention to the difference between the Read Speed and the Write Speed. Some manufactures only quote one speed, and that will be the more favourable of the two – always assume that is the Read Speed.
Memory card speed classes and minimum speeds
Naturally card manufacturers want to show off the better attributes of their products, so they tend to quote the fastest read/write speeds on the card’s label and in advertising. A somewhat more crucial measure though for users is the minimum write speed – the slowest rate at which it is measured to write data. This will be referenced on the card, but in a slightly more abstract way than the top MB/s speeds we can expect.
Rather than expressly telling us what these minimum speeds are, we get told instead what ‘class’ a card is in. It is these classes that give away the slowest speeds we might expect from a card.
If you are an SD card user you may see a number within an unclosed circle, a number within a U shape or a V with a number next to it. Each of these represents the speed class of the card, indicating the minimum write speed we should expect. Some cards have more than one of these speed classes, indicating performance with cameras capable of writing data at different speeds. Some cameras have one UHS-l memory card slot and one UHS-ll slot, for example, so the same card will feature different minimum, and maximum, write speeds depending on which slot it is in. Many cameras will have the type of the card slot marked next to where the card goes into the camera.
SD card capacity ratings
SD and MicroSD cards have an additional naming convention to indicate the memory capacity of the card, and camera manufacturers will state which capacities a camera is compatible with. Most cameras have some sort of upper limit on the size of card they can deal with. In the SD family we have SD, SDHC (for High Capacity), SDXC (for eXtra Capacity) and SDUC (for Ultra Capacity).
| Label | Capacity range |
|---|---|
| SD | 2GB or less |
| SDHC | Over 2GB to 32GB |
| SDXC | Over 32GB to 2TB |
| SDUC | Over 2TB to 128TB |
Memory card types
SD Cards
SD stands for Secure Digital, and is one of the longest serving forms of digital memory card. It is also currently the most popular with camera manufacturers as the standard offers enough speed and capacity for the majority of today’s cameras. The cards are compact enough that they don’t take up too much space in the camera, and they are large enough to handle easily when users put them in and out of the camera. The actual memory chip, processor and controller inside the card take up very little space, so most of the card is empty. The shape and size are designed to be convenient to work with.
MicroSD
These are the same as standard SD cards, just a whole lot smaller. They are used where space is premium, and generally where the card won’t be removed and inserted quite so often as in a larger camera. Some compact cameras use them, as well as PTZ cameras, and they are a favourite with smartphone manufacturers.
These cards are often supplied with an adapter so they can be used in the larger SD card slots in card readers, laptops and in larger cameras. Although they are smaller than regular SD cards, they can have the same capacities and read/write speeds.
You are likely to find MicroSD cards in drones, action cams, dashcams, CCTV systems, miniature gimbal cameras and gaming handsets such as the Nintendo Switch.
CFexpress
SD card write speeds currently top out at about 300MB/s, and although faster cards are theoretically possible they aren’t available to buy. When a camera needs to send higher data rates the manufacturer may choose CFexpress as the memory card format. These cards are physically larger than SD cards, but they can cope with greater storage and much faster transfer speeds.
Theoretically CFexpress cards can work at up to 8GB/s, but in reality the current crop of cards can manage 3.5GB write and 3.7GB/s read speeds. They come in capacities of up to 4TB, but not all cameras that accept CFexpress can handle that much capacity – so check the specification of your camera before buying.
CFast
CFast cards are physically large by modern standards and tend to be used in equally large cameras that produce large amounts of data. They are favoured in some high end video cameras, and have not been seen in recent times in a stills camera. They can support write speeds of up to 600MB/s, but in reality those available tend towards 500MB/s. While they look very similar to the CompactFlash card, they are not interchangeable, but they are a part of the CompactFlash family. The top spec cards can hold up to 1TB and write at about 500MB/s.
XQD
While still in use, the XQD card format has rather gone out of fashion for camera manufacturers, as read/write speeds are considerably better in CFexpress cards – which share the same form-factor and which can be used in many cameras designed for XQD. Technically they can read and write at up to 500MB/s, so they are still a formable and useful card in high end photography and videography. However, most available cards write at no more than 400MB/s.
Compact Flash
I’ve included CompactFlash here as it was an extremely popular card format not long ago, and if you are looking to buy a secondhand DSLR camera there’s a good chance it will use CompactFlash. We liked CompactFlash cards because they are big enough to handle easily, they don’t get lost and they seemed very reliable. At the time too, they had ground-breaking capacities.
The format is over thirty years old now, but you can still buy new cards for old cameras. If you are buying one try to get the latest version, so v6.0 or v5.0, as these will be the newer cards and will have the better read/write speeds. The write speeds will probably keep up with your camera, but a large capacity card will take ages to copy over if the read speed is low. These version 5/6 cards should have read/write speeds of around 133-167MB/s.
Portable SSD
Some cameras have the ability to record stills and video to an external SSD (Solid State Drive), partly as a means of attaching greater capacity storage to the camera but mainly to cope with the data rates high quality video can produce. Cameras like the Lumix S5ll X, for example, can shoot Apple ProRes 422 HQ in 5.8K, which creates 1.6Gb/s of data and will need a card that can maintain a write speed of 200MB/s. Many current memory cards can’t reliably cope with that sort of data rate, but most portable SSD devices can and offer lots of space to store the vast files this sort of video creates. For this reason the camera won’t allow us to attempt to record these files to an SD card – insisting that a portable SSD is used instead. Some PSSDs can read/write at up to 2GB/s, so can easily accept the highest quality video, and the 2TB capacities generally compatible with cameras that shoot this way allows lots of video to be stored in one place – rather than across multiple smaller cards.
Portable SSDs don’t have to be for these dramatic video files though, as the cameras that can record to SSD via the USB-C port will allow lower resolution video too, and of course still images.
In some cases shooting directly to a portable and external SSD device is more convenient than taking a memory card in and out of the camera, and if you like to shoot long high-speed bursts, for sport or wildlife for example, you’ll find an PSSD allows your buffer to clear more quickly and you won’t run out of space so soon.
As the SSD will be cabled to the camera via USB it is useful to find a clamp that will hold it, and often a cage for the camera that the clamp can be attached to.
Memory Card Hygiene
When you delete an image or video clip from a memory card the information remains in the memory, but the card marks the memory cells the data is stored in as ‘available’ for new data. When the next pictures are taken some of those cells will be overwritten and some won’t, so some stray bits of data will be left behind. If you load that card to a different camera that camera will create its own filing system alongside the one that already exists and things can begin to get a little bit untidy. When the camera sends a new image to the card, the card’s controller may have to start searching for spaces to store the new data among the bits of old data. That can slow the process and you’ll lose performance.
Imagine an empty multi-storey carpark at which 50 cars arrive at the same time. They all enter the ground floor and the first 25 park in the closest spaces, and the next 25 park in the closest spaces on the first floor. When new cars arrive we know the ground and first floors are full, so they should proceed directly to the second floor, where they can also park in sequence. That is a quick and efficient process. However, when some of the people return to their cars on the lower floors and drive away new arrivals don’t go directly to the empty third floor but dither around looking for spaces on the partly full floors, squeezing into spaces, holding up the traffic and driving slowly to see if that’s a space or just a small car parked right up to the wall. Some cars may have broken down, there maybe a shopping trolley left in a space that means we can’t park there, and perhaps no one wants to park under the beam where all the pigeons roost. Things suddenly begin to take a lot longer and a queue forms down the street.
What needs to happen in this situation is for someone to drive all the cars out of the carpark, get the whole place cleaned out and re-park the cars in sequence with no gaps. Of course that never happens. But in memory card land that can happen – we just reformat the card. It really pays off make a habit of formatting a card in its camera every so often. Some people do it every time they empty a card which is fine but you don’t need to do it that often. My cards go into multiple brands of camera and they get filled with the extra junk each system leaves behind, so I try to have a fresh start with each new brand. It always doesn’t happen that way, but I rarely let two cameras share one card for very long.
Memory Card Readers
In order to get your pictures or video clips off a memory card and into your computer you will need a memory card reader. While almost all cameras will allow you to download the content of the card by connecting the camera to the computer via USB, many people prefer not to wear-out the USB port or risk it being damaged should the camera fall and land on the cable. As unlikely as that is, card readers are usually more convenient too. Some card readers can handle only one type of memory card, while others can deal with multiple types at the same time – you may find your laptop has its own card reader built-in.
As we spend time checking the speed of our cards it is also important to check the speed of any reader you put the card into – it’s no good using a card capable to being read at 300MB/s if the card reader can only manage 150MB/s. The cable you use to connect the card reader is also important, as is the speed of the USB port in the computer.
Memory Lane
For no other reason than a bit of interest, here are some cards that we used to use in the earlier days of digital cameras.
You can find out a lot more about SD memory cards and the specifications of the format at the SD Association. If it’s CompactFlash, CFast or CFexpress you are interested in, visit the CompactFlash Association.