The original compact flash by IBM (left) and a current CF card sold by Calumet Photo
Compact flash card are miniature hard drives that have been optimized for use in today's high performance DSLRs. Currently CF cards are readily available with storage capacities from 1 megabyte to 64 gigabytes. (Because of the ever decreasing cost of these cards, CF cards lower than 1 gigabyte are more difficult to find and rapidly disappearing.)
Type II CF card (left) and Type I CF card (right)
CF cards come in two flavors: Type I and Type II. The difference is thickness. Type II cards are thicker than Type I. (5mm vs. 3.3mm respectively). Your camera manual will tell you if you are limited to Type I or Type II.
The original IBM compact flash card has a place in history because every subsequent advancement uses the writing speed of the original as its speed measurement base. (This is the same tactic taken by CD manufacturers when citing writing speed -- comparing the newest CD to the original Compact Disks.)
The speed of the original IBM CF Type II was 150,000 bytes per second. Today CF cards can be purchased with 305X write speeds. That's 305 times the original 150,000 bytes per second or 45 megabytes per second.
It's obvious that as the megapixel count on DSLR image sensors increase, the files stored on CF cards are becoming larger and larger. So higher storage capacity certainly makes sense -- unless you like changing CF cards or downloading images every few shots.
But what about the increases in write speed? Why is this important? Well, write speed of a CF card ultimately determines how quickly a modern DSLR can release an image from its internal memory. And this is very important when shooting in "continuous" mode (sometimes referred to as sports shooting mode). The number of images your camera can take in rapid succession is in large part determined by how fast the previous file(s) can be written to the CF card and eliminated from the camera's memory. With some high-end DSLR cameras having the capability of taking 10+ images per SECOND, the bottleneck to continuous shooting becomes the speed the CF card is able to accept and write image data.
For most mid-range DSLRs currently on the market, a speed rating of 133x and up should keep up with the data throughput demands of the camera in continuous shooting mode. Of course, this will change tomorrow as faster cameras are introduced.
Beware of CF card prices that seem too good to be true. They probably are. Double check the specifications of these bargain cards to ensure that they meet the parameters of this discussion. If you aren't provided specs, don't buy the card.
If you have questions or comments, please let me know.