What you need to know when buying SD camera cards

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(Source: Narong Jongsirikul via 123RF) (Source: Narong Jongsirikul via 123RF)
(Source: SDCard.org) (Source: SDCard.org)

Q: Can you help me understand all the numbers on SD camera cards?

A: Today’s memory cards are a confusing batch of geek speak but understanding some of the basics can make it easier to decipher.

Physical size
Most of today’s devices use either SD (24 mm x 32 mm) or microSD (15 mm x 11 mm) cards, which you can determine simply by looking at the card slot on your camera.

Storage capacity
The easiest specification to digest is the storage capacity, which is represented in gigabytes (GB). It’s easy to assume that a bigger number is better. But in reality, this is not necessarily the case.

Not only can it be cheaper to use multiple smaller cards, you won’t live and die with just one card because they degrade, get lost or experience data corruption.

SD vs. SDHC vs. SDXC
A memory card’s capacity is determined by the file system being used to store data designated by the type of card.

SD (Secure Digital) cards are the oldest, least used and limited to 2GB of storage.

SDHC (High Capacity) cards can store up to 32 GB of data, while SDXC (eXtended Capacity) cards can store up to 2 terabytes (2,000 GB).

Older devices may not be able to use the SDXC format, so make sure your device supports the larger cards before buying one.

[MORE: Data Doctors]

Rated speed
This is where things can get a bit confusing, as there can be various designations referring to the card’s speed. It’s represented in MB/s (megabytes per second) or a large number followed by an "x" or sometimes both.

The "x" designation is a marketing term --1x represents 150 kb/s, so 600x is the same as a 90 MB/s rating (600 x 150 = 90,000). This number typically represent the "read" speed, which is generally higher than the "write" speed.

Speed class
This specification is represented on the card as a number inside of the letter "C" to represent the minimum write speed. This number is most important to those shooting video or very large images in succession (burst mode).

There are four classes -- 2, 4, 6 and 10 -- representing the minimum sustained megabytes per second (MB/s) write speed. The higher the number, the faster the sustained speed.

UHS speed class
Faster SDXC memory cards will have the UHS or ultra high-speed rating represented by a number inside the letter "U." U1 means it’s 10 MB/s while U3 means it’s rated at 30 MB/s.

UHS bus class
Different from the UHS speed class, this rating refers to the bus interface and is represented by a roman numeral. Think of the bus interface as the number of lanes on a freeway versus the UHS speed class, which represents the speed of individual cars.

Video speed class
If you’re trying to shoot extremely high-resolution video (4K or 8K), this class of cards -- designated by the letter V and a number that ranges from 6 to 90 --  offers the fastest sustained write speeds.

[IMAGE: Write speed and speed class chart]

Overkill vs. Future-proofing
If you have older equipment that you plan on upgrading soon, getting a faster card than you currently need may make sense. But don’t spend more money than you need to as there’s no performance benefit.

To help you figure out what you actually need, check out SanDisk’s web tool. Once you figure out the specs for your desired card, you can easily comparison shop around the internet.

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