Myth: Memory Cards Are All The Same
Most memory cards offer an X rating (as in "times") denoting the speed at which it will transfer. Based on CD-ROM, 1x equals 150 KB/s, or 0.15 MB/s, so a 100x card would transfer at 15 MB/s, while an 800x card offers 120 MB/s. Manufacturers still use the X rating with CompactFlash, but Speed Class Ratings have largely replaced this with SD cards. CompactFlash, at the largest memory card dimensions of 43x36mm, is still king of capacity, but by a slim margin, with new models available at up to 512 GB. First introduced in 1994, the solid-state tech is also very fast, with top speeds of up to 1066x at 160 MB/s (which will support 4K video and even 3D). Newer CF cards also offer compliance with the VPG 20 standard for HD or 4K-compatible VPG 65 standard, which promises sustained rates of writing without dropped frames. The Achilles’ heel with CompactFlash is that it uses a pin array to plug into a camera, and these pins can easily bend or break with a slightly misaligned card—a costly fix on your camera.
Introduced in 1999, SD (Secure Digital) cards are particularly popular with electronics makers, thanks to their size and affordability. While the thumb-sized cards are easily lost in camera bags and elsewhere, they give gadget makers more internal space for circuitry and electronics. Now available as SD (up to 4 GB), SDHC or SDXC (with microSD versions of these for mobile phones and tablets), the different nomenclatures refer to high capacity (HC, up to 32 GB) and extended capacity (XC, up to a theoretical 2 TB, though currently they max out in the real world at 256 GB). From Class 2 (slowest) to Class 10 (fastest), SD/SDHC/SDXC cards are classed by the minimum write speed, though speeds in each card are generally much higher than this minimum. These Speed Classes are based on the needs of video. At a minimum of 2 MB/s, Class 2 is capable of standard-definition video while 720p is possible with Class 4 at 4 MB/s. Class 6 at a minimum of 6 MB/s is generally required for 1080p/i while Class 10 with a high-speed data bus guarantees sustained video with a minimum write time of 10 MB/s and cards that currently offer up to 40 MB/s. Speed Classes are shown on cards as a "C" with the respective number inside.
To make things more confusing, higher speeds from SD cards also can be attained through professional models with the Ultra High Speed Bus (UHS) technology. UHS-I supports up to 104 MB/s (with UHS104/SDR104-compliant cards; other UHS-I cards top off at 50 MB/s), and a theoretical 312 MB/s is possible through the UHS-II bus standard, which holds two pin layouts rather than just one. Announced this last November, UHS Speed Class 3 cards are coming soon, with a minimum write speed of 30 MB/s, fast enough to support 4K-resolution video. (Panasonic teased a UHS-3 card at CES in January with their mock-up of the new 4K-capable GH4 camera.)
It’s easy to get UHS Speed Class and UHS-I or UHS-II confused. UHS-I/UHS-II symbols on a card indicate a bus interface for heightened speeds while UHS Speed Class symbols (U1 or U3; on the card, this is shown as a U with corresponding number inside) define the minimum write speed. Equipment needs to be UHS-compatible to achieve these speeds, though most pro cameras offer compatibility. SD cards also are interesting in that a few models from Eye-Fi, Toshiba and Transcend feature internal Wi-Fi hot spots. These cards are smaller in capacity, but you can transfer files wirelessly to free up space.