Some of Canon’s EOS cameras employ CompactFlash (CF) memory cards, and some use Secure Digital (SD) cards. A number can use both. So, what’s the difference and why are both card formats used? The answer is a complex mix of history and technical requirements. To start, both CF and SD are storage cards containing “flash memory.” This means they contain small computer chips which can be erased and reused. The chips store data even when they’re not connected to a power source, so they’re sometimes called “non-volatile” memory. The differences between the two types are related to the physical size and shape of the cards and the electronics they contain. CompactFlash CF is the older standard. The cards are small rectangular devices, about the size of a matchbook. They come in two physical sizes – type I and type II; the latter being thicker than the former. However, type II cards are essentially obsolete today – they were used for tiny mechanical hard drives known as MicroDrives. All CF card