Compact flash memory card in three size classes (SD up to 2GB, SDHC up to 32GB, SDXC up to 2TB) with Speed Classes from 2–90 MB/s for video recording.
Technical Details
SD cards are divided into three size classes: Standard-SD (up to 2 GB), SDHC (4-32 GB), and SDXC (up to 2 TB). Transfer speed is defined by Speed Classes: Class 2 (2 MB/s), Class 4 (4 MB/s), Class 6 (6 MB/s), and Class 10 (10 MB/s). For video recording, UHS Speed Classes exist: U1 (10 MB/s), U3 (30 MB/s), and Video Speed Classes V30 (30 MB/s), V60 (60 MB/s), V90 (90 MB/s). The file system varies between FAT16 (SD), FAT32 (SDHC), and exFAT (SDXC). The internal controller manages Wear Leveling and Error Correction Code (ECC).
History & Development
In 1999, Panasonic, SanDisk, and Toshiba jointly developed the SD standard as the successor to the MultiMediaCard (MMC). In 2006, SDHC followed for capacities over 2 GB, and in 2009, SDXC for up to 2 TB. The SD Association introduced UHS-I (Ultra High Speed) in 2009 with up to 104 MB/s, UHS-II in 2011 with 312 MB/s, and UHS-III in 2017 with 624 MB/s. Since 2018, the SD Express standard has enabled speeds up to 985 MB/s through a PCIe interface.
Practical Use in Film
Digital cinema cameras like the RED One initially used CF cards, while DSLR cameras like the Canon 5D Mark II established SD cards in feature films ("Act of Valor," 2012). Documentary filmmakers appreciate the compact form factor for unobtrusive shooting. 4K recording requires at least U3 cards (30 MB/s); 8K footage necessitates V90 cards (90 MB/s). Backup workflows utilize dual-slot cameras for simultaneous recording onto two cards. The limited number of write cycles (3,000-10,000 P/E cycles) requires regular card replacement with intensive use.
Comparison & Alternatives
CF cards (CompactFlash) offer higher capacities and speeds but are larger and more expensive. CFexpress cards achieve up to 1,700 MB/s but cost ten times as much. SSD recorders like the Atomos Ninja replace memory cards with 2.5-inch SSDs in high-end productions. P2 cards (Panasonic) and XQD cards (Sony/Nikon) serve as professional alternatives with higher reliability. MicroSD cards are used in action cameras and drones but offer lower speeds.