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In the world of digital photography, memory card choices abound; you can choose both a wide variety of sizes (both physical and electronic) and manufacturers. Sometimes your choice of camera will drive your memory card type, which is something you’ll want to consider if you’d like to keep using your memory cards for a long time. Most memory cards are based on “flash memory", so I’ll take a little time to explain that, then move into the card types. I haven't included pictures of the cards, but you can go to All Memory Cards for more detailed specifications, dimensions, and pictures.
What is flash memory?
Flash memory is another name for a specific kind of electronically-erasable, programmable read-only memory (or EEPROM), a solid-state memory device. Some of the virtues of ROM that this kind of memory possesses is non-volatility (losing power to your device doesn’t cause your data to be lost), durability, and high temperature & shock tolerance. One disadvantage is that it has a finite number of write-erase cycles, but so-called wear management techniques make that of little concern to the average user.
A venerable member of the memory card family, CompactFlash (or CF) was introduced in 1994 by one of the industry’s key players, SanDisk. CF comes in two types, I and II, with the second being marginally larger. Widely used in digital cameras, it’s the largest card you’ll run into. If you work with higher-performance digital cameras, there’s a good chance you’ll be working with these. Their size can be a weakness, but they have an internal IDE hard disk controller (for those of you who have worked inside a computer before, examine the socket end of the card- it should look familiar). This yields better potential speeds than the smaller cards, though file structure can affect the write time of your particular device.
Originated by IBM in 1999, Microdrive is precisely what its name suggests- a very small hard drive. As such, it has some of the disadvantages of hard drives (its response to jarring, abrupt motion, or extremes of temperature, for instance) but initially boasted a better maximum capacity than CF cards. Today, flash memory is generally superior, but microdrives still have an edge on a dollars-per-unit-memory basis. And unlike flash memory, they lack a write-erase limit.
Secure Digital (SD)/Multi Media Card (MMC)
MultiMedia Cards have been around since 1997 and are widely used in non-camera applications such as PDAs, GPS receivers, and cellular phones. Smaller versions of the MMC cards also exist (RS-MMC and MMCmicro; all are compatible with standard MMC applications with the use of a mechanical adapter. Secure Digital, a format based on the MMC card, was released in 2000 and is similar enough that MMC cards can generally be used in lieu of SD cards. (SD cards are thicker, so the substitution doesn’t work the other way.) SD, being the newer standard, has a number of advantages such as data transfer rate and a write protect switch. Similar to MMC, there are two smaller versions (miniSD and microSD) that are compatible with SD card slots with the use of an adapter. At this time, SD cards are likely the most widely used card format.
FujiFilm and Olympus cooperated on the xD memory card format, while the Memory Stick series (MS, MS Duo, and MS Pro) is Sony’s answer to your portable memory needs. If the camera you choose is manufactured by one of these companies, your memory card choice will be made for you. Some manufacturers occasionally do a dual-card format; for instance, the Sony DSC-828 and the Olympus Evolt series are capable of taking multiple card formats. In closing, considering your long-term budget during camera/memory card selection can be to your advantage, since any card choice will restrict your camera choices to a greater or lesser degree. If you like to swap memory cards between devices, SD cards are ideal- I can use mine in the Pentax Optio cameras we have at work, our Dell Axim handhelds, or flip the end to expose the USB connector and plug it into a computer directly. When you make your choice, keep in mind not only where you are now, but where you are likely to find yourself in the near future.