What is the difference between CF and SD cards?

SD cards use their own protocol that will expand up to 32GB over 2GB with the release of SDHC (there were several 4GB SD cards, but not very compatible) and then support up to 2TB with the release of SDXC.

If you remember, the transition from SD to SDHC was particularly painful as most other devices (Readers, Picture Frames, Card Readers, Notebooks, etc.) took years to capture. CF cards use IDE protocol which can index large volumes using dummy head, part, sector coordinates. Although FAT32 support is used above 2 GB, they continue to work as capacity increases. This makes the protocol more stable and extensible although the next revision is CFast (Compact-Fast) based on SATA protocol. The larger physical size of Compact Flash also provides an advantage in capacity and has a hint in terms of maximum speed. This was historically significant, but the gap is so narrow it's mostly a legacy case. In terms of camera ratings, each memory type has high quality models. The Pentax 645D Digital Medium Format camera uses SDXC cards, Canon's top models accept both CF and SD. This allows only Nikon to use CF cards in its high-end models.

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