The binary system might be simple, but it needs a lot of space to store all the digits that define the tones and hues of a digital image. EOS digital cameras store this data on a CompactFlash card Type I or Type II, or a Microdrive (some models also use a SecureDigital card). With the Wireless File Transmitter WFT-E2, data can also be stored directly to external media, such as hard drives. The term flash has nothing to do with lighting − it refers to the method of holding the data.

CompactFlash (CF) and SecureDigital (SD) cards are available with different storage capacities. Low capacity cards are being phased out as the increasing pixel counts on cameras demand the larger capacity cards. A 1 GB (gigabyte) CompactFlash card is now considered normal, and larger capacity cards are readily available.

The capacity of a card has no bearing on its physical size. CompactFlash cards come in two types. The only difference between them is that Type II cards are slightly thicker than Type I cards. Only Type I cards will fit a Type I slot, but either card type can be used in a Type II slot.

Type II slots also accept MicroDrives. Unlike the solid state CF and SD cards, Microdrives are miniature hard disks similar to those used in computers. They have rotating platters that store the data. With their moving parts, Microdrives are more susceptible to damage if dropped. They were popular initially because their 1GB storage capacity was greater than that of early CF cards. They are less popular now that CF cards offer capacities greater than 1GB.

Shooting resolution

As digital cameras improve, the size of the image files increases. This means there is a need for larger and larger storage cards. A basic (RAW) file from the EOS-1Ds Mark II, for example, has a size of about 14.6MB. This means that you can only record about 25 of these images on a 512MB CompactFlash card. But do you need to record all this data every time you take a picture?

Much depends on what you want to do with the picture. If you only intend to make small prints, you can probably get away with a file half this size. And if you only want images to put on an internet website, a very small file will be more than adequate.

That is why most cameras give you the choice of shooting at different resolutions (see table). At these reduced resolutions, the camera records one value from a cluster of adjacent pixels, rather than a value from each individual pixel. This gives less data, reducing the size of the file, but also reducing the amount of detail in the image.

If in doubt, always shoot at the maximum resolution of the camera. You can reduce the resolution later, but you cannot increase the resolution of an image once it has been captured. (Actually, there is a technique, called ‘interpolation’ which tries to improve the resolution of a digital image. It does this by looking at two adjacent pixels and making an intelligent guess about the colour and tone of an extra pixel that it inserts between them. The results can be effective, but are never as good as shooting with the extra pixels in the first place.)