Even compared to the rest of the tech world, memory and storage evolve quickly. Once-pricey USB drives are now given away like candy, SATA III hard drives and SSDs are faster than ever and camera memory cards are providing 32 and 64 gigabytes of storage for hundreds of high resolution images. Even within the SD, SDHC and SDXC card types we covered last year, there are important speed classes that determine what SD memory cards are capable of.
The confusing old "6x, 10x, 13x" etc. ratings have gone out of style compared to the common sense Class 2, Class 4, Class 6 and Class 10 ratings. Casual photographers won't even need to pay attention to the ratings of their SD cards, but pros shooting at high frames per second or capturing HD video absolutely have to pick the right card for the job. Here's how to figure out which memory card class is right for you.
As we covered last time, companies like SanDisk like to attach flashy buzzwords like "Ultra" and "Extreme" to their memory cards. These don't really mean anything--SanDisk's Extreme Pro series may be faster than its Ultra II series, but those aren't official qualifications. One company's extreme may be much faster than another's.
Thankfully, we have the SD Association's real speed classes to consult. Here's a handy chart that provides an at-a-glance breakdown of what each class offers:
|Speed Class||Minimum Write Speed||Suitable For|
|2||2 MB/s||SD video|
|4||4 MB/s||Some video, stills|
|6||6 MB/s||HD video, HD stills|
|10||10 MB/s||1080p HD video, burst shooting mode|
In addition to those regular speed classes, the SD Association has created the Ultra High Speed (UHS) speed class that supports a faster bus interface than its regular cards. While you could use a slower class SDHC/SDXC card in a device that supports UHS, those cards couldn't take advantage of the full speed potential of the device. UHS-I is currently rated at a maximum of 50 MB/s, and will eventually support up to 104 MB/s.
There's almost no reason to buy a Class 2 memory card because Class 4 cards are already dirt cheap.
Now let's dig into those speeds. The truth is, "minimum write speed" is pretty vague: the practical uses of each speed category are much more helpful. There's almost no reason to buy a Class 2 memory card because Class 4 cards are already dirt cheap. Newegg carries a mere 21 Class 2 cards and over 100 Class 4 cards. Similarly, there's almost no market left for plain SD cards, which support only 2GB of storage. SDHC cards hold up to 32GB; SDXC cards are rated for up to 2TB, but few cards even offer 64GB of storage.
Newer devices will support SDXC and SDHC, but if you're using a camera that's several years old it may only support the SDHC standard. Knowing that limitation is key to buying the right card--figure that out before you start worrying about read and write capabilities. A quick read-through of the manual or a Google search should get that job done.
The proper speed isn't quite so easy to determine. The kind of shooting you plan to do matters, but so does the quality and performance of the camera. The more capable the camera, the faster it can write data to an SD card. That's when SD write performance can become a bottleneck.
With DSLRs, the camera's buffer, where photos are temporarily stored for processing before, can be the transfer bottleneck.
With DSLRs, the camera's buffer can also be a bottleneck. That's where photos are temporarily stored for processing before they're transferred to the SD card. Continuous/burst shooting mode can fill up that buffer quickly, depending on how big it is (the better the camera, the larger the buffer likely is). This is your RAM, essentially. Thanks to the buffer, the speed of your SD card actually isn't all that important for simple photography. If you're snapping off a picture every few minutes--or even every few seconds--you're not going to overtax a Class 4 card.
For serious burst shooting or HD video recording, Class 6's guaranteed minimum write speeds of 6 MB/s are generally fast enough. So why do we have Class 10 cards? As DSLRs become more popular tools for shooting HD video, bitrates grow and threaten to outperform the memory speeds of cheaper cards. For example, the special Magic Lantern codec for the Canon 5D Mark II ups the video bitrate to 76Mbps, or 9.5 MB/s. To keep pace, you'd need the speed of a Class 10 SD card (with SD-to-CF adapter, since the 5D Mark II takes CompactFlash).
And it never hurts to future-proof: if you're buying a large card now, 32GB or 64GB, make sure you'll still want to use it two years down the road. A Class 6 card might be too slow a few years from now, while a Class 10 card is more likely to last.
Remember that the different classes are for minimum continuous speeds. Don't be confused if you see a Class 6 card advertising 25 MB/s performance. That's possible--it means the card can burst write that quickly, but won't keep that speed up at all times.
Class 10 and UHS-I cards are especially useful for dedicated video cameras, while cheap point-and-shoots obviously don't need anything above a Class 4. CompactFlash cards still offer some of the fastest transfer speeds on the market, but are generally more expensive than SD and not as widely supported.