Compactflash
From ProPHOTO WIKI
Contents |
Introduction
One of the first popular storage formats used in digital cameras, Compactflash is a smaller version of the older PCMCIA standard. While compact cameras have generally moved to smaller formats, most high-end DSLRs still make use of this format.
Formats
Compactflash cards come in two basic varieties, type I and type II. These types are identical except for the thickness of the card. While type I cards (see fig 1) are only 3.3mm thick, type II cards (such as the Microdrive) are 5mm thick. These thicknesses corespond to type I and type II PCMCIA cards, allowing them to to fit into those slots via an adapter. Most Compactflash devices can take either type of card, however some smaller cameras are restricted to type I cards.
Revisions
Over the years there have been a number of revisions to the Compactflash specifications to enable higher transfer rates. The first revision, Compactflash 2.0 (CF+) increased data rates to 16MB/s and allowed memory cards larger than 4GB in size. The IBM Microdrive was one of the first cards to use this specification, however other cards followed shortly.
Recently, Compactflash 3.0 has been released enabling transfer rates up to 66MB/s. While support for this is still limited to the high-end, it is begining to trickle down to the rest of the market. In addition to accelerating transfer rates in camera, this technology provide significant acceleration when transfering images from the card to the computer.
Speed Ratings
While there is no formal rating mechanism, many vendors have informally standardized on using the "X" speed rating used for CD-ROM drives. That is, one "X" is 150KB/s - so a card capable of transfering data at 20MB/s would be rated at 133X (20MBps/150KBps = 133X).
Unfortunately, each vendor has their own way of measuring performance so these numbers aren't always the best way to compare performance. As such, it is strongly recommended that users look to an independent test when deciding which card to purchase.
Adapters
Passive Adapters
As these cards generally use the ATA mode allowed for in the Compactflash/PCMCIA specifications. This allows these cards to be attached to computers using simple passive adapters. That is, these adapters do not contain any electronics and simply reroute pins from the Compactflash card to the PCMCIA/PATA socket. Due to their simplicity, these adapters are typically quite innexpensive and relatively compact.
Unfortunately, due to limitations in the ageing PCMCIA interface simple passive adapters are often relatively slow. While they are capable of providing sufficient bandwidth, there are significant overheads that hurt their real-world performance.
Cardbus Adapters
While looking very similar to passive PCMCIA adapters, Cardbus adapters are active devices that work more like card readers than their less expensive bretheren. These adapters have their own microcontrollers that retreive data from the cards and then relay it via a faster 32-bit (132MB/s) interface. This bypasses many of the overheads of the traditional interface, and allow higher transfer rates.
USB 2.0/Firewire Readers
These external units connect to computers through a USB 2.0 or Firewire cable and relay images via an onboard microcontroller. As both interfaces are much faster than any existing Compactflash card, they will generally provide higher performance than the simple passive adapters listed above.
See Also
- Secure Digital - A smaller memory card format used in many compact digital cameras.
- PCMCIA - The larger card standard that Compactflash was derived from.
External Links
- Rob Galbraith's CF/SD Performance Database - Comprehensive database comparing the performance of many different memory cards in both cameras and computers.
