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Media cards

A CompactFlash card is the most common form of memory storage device used in Canon EOS digital cameras at the moment. Its specification was formalised in 1994 by the SanDisk Corporation. It consists of flash memory contained in a standardised enclosure, which comes in two types. These are conveniently named Type I and Type II. The types refer to the two thicknesses of cards. Type I are 3.3mm thick, while Type II are 5mm thick. Either type can be used with all EOS digital cameras. Most of the commonly available cards these days are the thinner Type I cards.


CompactFlash cards are used by most of the current EOS digital cameras.

What is flash memory?

Flash memory is a type of ‘non-volatile read-write memory’. What this means is that data can be written to and read from the memory chip and no power is needed to retain the information on the chip. It gives faster ‘read and write’ access and is more shock resistant than hard disks. It is a lot like the RAM (Random Access Memory) in your computer, but being non-volatile, data is not lost when your take the power away.

What is the speed rating of a card?

Just as CDs are rated by the speed they can be read from or written to, CompactFlash cards have fallen into the same industry standard system.

A card with a rating of 1x (or 1 Speed) means it can read and write data at 150kb/second. So a card with an 80x rating can read and write data at 150 x 80kb/second, which is 12MB/second.

The speed of the card is determined not just by the memory chip inside, but also by the camera’s memory controller. Flash memory in some cards is much faster than any camera can handle, and it is often the camera memory controller that restricts the speed the card can receive the data.

Whether you need a faster card is down to your budget, how you work and what you shoot. As a rule, faster cards are more expensive, but unless you are using one of the latest very high-resolution cameras, or you shoot continuous action photography where you regularly fill the memory buffer in the camera, you are unlikely to notice even the very fastest cards making much of a difference in camera.

Where you will notice the difference is when downloading from the CompactFlash card to the computer. Faster cards are able to copy data through a card reader more quickly than slower cards.

Not all cameras interact with the flash memory in the same way. While one camera will be able to save data to the card at a rate of 100 megabytes per second, another may only manage 50 megabytes per second with the same card. Selecting a fast memory card simply ensures that you get the best experience possible because it allows your camera to run at its maximum capability.


UDMA, or Ultra Direct Memory Access, is a method of data access that allows data to be written or read directly from storage media, without having to transition the data through the central processing unit, or CPU. Removing a step in the chain takes load away from the CPU and allows a faster read/write cycle, which in photographic terms means quicker image storage onto a memory card for improved burst shooting capability.

How tough are CompactFlash cards?

Despite their diminutive size CompactFlash cards are remarkably resilient. This is helped by the absence of any moving parts that can be broken. There have been stories of forgetful photographers accidentally leaving CompactFlash cards in an item of clothing that was then put through a washing machine, and they were still able to retrieve images and carry on using the card afterwards.

Part of the specification for CompactFlash cards, as laid down by SanDisk in 1984, states that they should be able to withstand a drop from 10 feet (three metres) without breaking, corrupting or losing data. This is the equivalent of a 2000G shock.

The connectors on the CompactFlash card should withstand being put in and taken out of the camera or card reader at least 10,000 times without degrading and losing connection.

The memory itself is also very durable. All memory segments, the individual blocks of memory that make up a memory chip, are designed to withstand a certain number of write/erase cycles. This can be up to one million in some cases. The current specifications suggest that with typical usage, the life cycle should be in excess of 100 years.

This is partly because of some very clever technology built-in to the card that balances the load on each memory segment. The FAT file system used by the cards is actually not very well suited to flash memory because it writes and rewrites its file structure in the same place. This wears out some segments quicker than others. The memory cards, however, know this and they evenly distribute the data among the file segments to ensure that no segment is used more than any others on the chip.

What are SD cards?

Secure Digital (SD) cards are a lot like CompactFlash (CF) cards, but come in an even smaller package. Like CF cards, they use flash memory, which does not require power to retain the stored information.

SD cards are a newer technology than CompactFlash cards, borne out of the demand for ever-smaller products. Being newer technology, and despite often having the same speed rating, they usually exhibit slightly faster transfers rates. The transfer rate on SD cards is often measured in two ways – both in MB/sec, like CF cards, and also as a Class rating. The difference is quite simple – the MB/sec is a burst transfer writing speed and is related to stills shooting where data arrives in small bursts of information. The Class rating is related to video shooting. The Class indicates the minimum sustained write speed for a continuous stream of data as found in movie shooting. The common classes are Class 2, 4, 6 and 10, with the number indicating the sustained MB/sec speed, so a Class 10 card has a minimum sustained write speed of 10MB/sec.

SD cards are most commonly used in pocket computers and compact cameras because of their small size. Several Canon EOS digital cameras can use CompactFlash and SD cards: these include the EOS-1Ds Mark II, EOS-1Ds Mark III, EOS-1D Mark II, EOS-1D Mark II N, EOS-1D Mark III, EOs-1D Mark IV and EOS 5D Mark III. The EOS 450D was the first EOS digital camera to accept only SD cards but now there are several models in the range, including the EOS 60D – the first EOS camera in the mid-range to only use SD cards.

SDHC and SDXC cards

The development of SD cards continues rapidly. While CF cards are still used in professional products, the gap in capacity and speed is decreasing with Secure Digital High Capacity (SDHC) and Secure Digital Extended Capacity (SDXC) cards. In fact SDXC cards can now reach a maximum capacity of 2TB according to the latest standards. SD cards, in all their forms, are also more durable in some areas - due to the design of the SD card, they can easily be made waterproof, unlike CF cards which can only be made water resistant due to the holes required for the contact pins.

Memory cards and HD Movies

When shooting HD movies with a DSLR, the camera produces a large amount of data in a continuous stream, rather than as small bursts. This puts a different requirement on memory cards than that for stills shooting, as they need to be able to continuously write data. For a good HD Movie shooting experience, you should use a CF card that has a minimum continuous write speed no slower than 8MB/sec. For SD cards, while a Class 4 card will work, a Class 6 or Class 10 card is a better choice. If you use a memory card that is too slow, a symbol will appear on the rear LCD screen of the camera showing the buffer is filling up. If the buffer fills, the camera will stop recording.

Being physically smaller, you do need to take more care with SD cards than with CompactFlash cards – smaller objects are easier to lose. Also, unlike CompactFlash cards, the connectors on SD cards are visible and you can touch them. This is not generally a problem as you are unlikely to damage them, but because they are exposed they are more at risk from being scratched by mishandling.


SD cards (left) are much smaller than CF cards (right).

Card readers

Although it is possible to download your images from memory cards via your camera and a cable, a card reader is often more convenient. It downloads the image faster, saving you time in front of the computer, and it doesn’t use up the camera battery. There are many card readers available. To minimise the time it takes, make sure you find one with a USB2 interface. Whether you buy a CompactFlash only or a multi-card format reader will depend on the type of cards you use.


Card readers offer a convenient, fast method of downloading image files to a computer.

What about Microdrives?

The Microdrive was invented by IBM. It is a mini hard disk inside the casing of a CompactFlash card. Being a proper hard disk with spinning data platters and a moving head to read and write the data, they are usually produced in the slightly larger Type II card format. Hitachi is the current manufacturer, having bought IBM’s hard drive division in December 2002.

Microdrives were very common a few years ago as they were available with 1GB of storage at a time when CompactFlash cards could only manage 512MB. However, although you can now buy 4GB Microdrives, CompactFlash cards are available with storage up to 8GB, so the capacity advantage of the Microdrive has been lost, and it has become much less popular.

Another disadvantage of a Microdrive is that, being a proper disk with moving components, it is relatively fragile. If you drop it or bang it on a table, it can quite easily be damaged. CompactFlash cards, being solid-state (no moving parts) are less susceptible to physical damage.

Although they are still available for cameras, Microdrives are now more commonly found in other electronic equipment, such as digital video camcorders, MP3 players and mobile phones.

File corruption

Occasionally, the data on a media card will become corrupt and unreadable. However, it is often possible to get your images back.

There are specialist software programmes that you can buy to help recover any type of digital information from a memory card. There are others that are devoted to image retrieval. If you type ‘media recovery software’ into an internet search engine, you will find many different programmes available for download.

Most manufacturers offer a free trial so, before you commit, try out several types and see which works best. We’ve found that some software will recover images where another has failed and vice versa, so it is often good policy to have several programs to hand.

FAT format

Even if you can afford one of the high-capacity CompactFlash cards, you might not be able to use it. Early EOS digital cameras use the FAT 16 file format, which only handles cards up to 2GB. Later EOS digital models use the FAT 32 file format that will handle larger capacity CompactFlash cards. The CompactFlash card specification allows for capacities up to 137GB, but there is still some way to go to attain this.

Cameras that only handle card capacities up to 2GB are the EOS-1D, D30 and D60. All later models handle card capacities greater than 2GB.