CF cards are popular with professional photographers and high end equipment, mainly because they are considered more rugged and are capable of withstanding more physical damage than other types of storage cards. Due to their comparatively large size, these cards are typically not used is slimmer consumer devices.

There are two main types of Compact Flash (CF) cards, Type I which is 3.3 mm thick and the thicker Type II cards  which are 5 mm thick.

Speed Ratings

Unlike the SD and SDHC card standards that rate card speed according to a guaranteed transfer rate, CF card speeds refer to the maximum transfer rate of the card and are usually quoted as in "x" ratings, e.g. 66x, 133x, etc., which equate directly to CD-ROM speed ratings where 1x is the transfer speed of an audio CD (about 527 MB/hour). The base rate rate for calculating performance is 150kB/s, therefore a 133x card gives a transfer speed of 133 x 150kB / 1024 = 19.5 MB/s.

It is important to stress that these write speeds cannot be directly compared with SD/SDHC Class ratings as manufactures typically do not publish the minimum performance speeds of their devices.  That said, a 133x CF card usually performs equivalently to a Class 6 SDHC card.