- Make Sure it is a CompactFlash® card. CompactFlash® cards are larger and thicker than SD cards (below), and your device must have a CompactFlash® card slot.
- UDMA rating: Ultra Direct Access Mode is the transfer mode rating that defined the standard for maximum transfer rate among CompactFlash® cards. Standard UDMA started out with a 16.7 MB/s transfer speed; the newest protocol, UDMA 7, is ten times that speed, maxing out at 167 Mb/s.
- This is the write speed; the speed at which data is written to the card. This speed (represented in MB/s) is most important for photographers and videographers, since it is a truer determination of how fast you can get images or video on a card. For everyday use in other electronics, write speed does not make that much of a difference.
- The most important number: the capacity. CF cards range from 2GB to 512GB. Pick a card that can hold everything you need for a prolonged shoot—you won’t be misplacing these chunky memory cards easily.
- This is the read speed; how fast information is read from the card. This is the speed that will shave seconds from your downloading time. On some cards, this is represented as an “x” number (e.g., 1000x), but there may also be a MB/s number, as well. Don’t fret if it’s not displayed—you can calculate the MB/s speed easily by dividing the X number by 6.6666. In this case, 1000x/6.666 = 150 MB/s).
- Name: some card manufacturers identify their card by the targeted user; in this case, the Lexar Professional series is aimed at photographers and videographers.
- Read Speed: the speed at which data is read from the card.
- Type: as mentioned above, this will either be SD, SDHC or SDXC. Differences in capacity or speed are shown elsewhere on the card.
- Another speed rating, which can be calculated into MB/s with this formula: x rating/6.6666 = MB/s. 1000 ÷ 6.666 = 150 MB/s
- Class rating: this is the minimum sustained data writing speed, and class 10 is the highest right now, capable of 10 MB/s. The slowest rating is Class 2, which has sustained data writing speeds of 2 MB/s.
- UHS Rating: The UHS rating and the Class rating are closely intertwined. UHS rating of 1 means that the minimum sustained data writing speed is 10 MB/s, and UHS 3 is 30 MB/s. However, the UHS rating is more important for shooting 4K video, as the minimum data rate must be higher so frames are not dropped. For photographers, a UHS 1 and a Speed Class 10 is basically the same thing; however, the UHS cards are more effective in a device that supports UHS bus architecture, and not all devices do. So conceivably, you could have a Speed Class of 10 and a UHS 3 rating, but without a UHS device, it defaults to the speed class rating of 10.
- This number shows what UHS standard this card conforms to—I or II. There are four bus speeds for memory cards: Normal, High, UHS I (Ultra High Speed), and UHS II. This number shows what the bus architecture of the card is. These classifications were designed for theoretical jumps in speed, and added as the cards got bigger. There is no difference to the end user between a UHS I or UHS II card.
- The card capacity. Cards range from 2GB to 512GB, with 1TB cards on the horizon.
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