What do the 'X' figures on Compact Flash cards mean?

The speed rating system used with Compact Flash cards is the same as that used in the CD-ROM industry. A data transfer rate of 1X means that the card can transfer data at 150 kilobytes (KB) per second.

If you consider the speed of the card alone and not the capabilities of the input device (e.g. a digital camera) nor the output device (e.g. a card reader) then theoretically, the higher the speed rating, the faster your Compact Flash card can store images.

For example, if you take 30 high-resolution images and each one is 3.5 megabytes (MB) in size, the theoretical speed at which the various Compact Flash cards read and write the data are indicated below.
 

Speed Rating Transfer time of 30 high-resolution pictures @ 3.5MB each 8X 40 seconds 12X 26.7 seconds 40X 8 seconds Actual speeds may vary based on the quality of the flash card, size of the file and device used.

Not everyone will benefit from high-speed Compact Flash cards. The needs of someone who only takes a few digital photos are different to those of a professional photographer who will use expensive, specialized products designed to take advantage of fast cards. In fact, many people won't notice much of a difference as the actual performance of a Compact Flash card can have more to do with the limitations of your device than the flash card itself.

Not only do Compact Flash cards have a maximum speed rating, but your camera (or other device) also has its own speed rating. When these products are combined, they'll work to the slowest common denominator. For example, if you use a 12X flash card in a camera designed to work at 8X speed, you'll be transferring data at the slower 8X speed.

Determining the real speed of your camera can be difficult. Most camera manuals won't tell you what speed of flash card you should buy, just that you should only buy their brand, which isn't much help. A general rule of thumb: If your camera has a pixel count (resolution) of less than five megapixels, the speed rating of the flash card doesn't matter much. The improvement you're likely to see in a higher rated card will only be fractions of a second, virtually undetectable to all but the most discerning user.