Saturday, May 25, 2024

Which SD card?

SD memory cards are used for everything from mobile phones, field recorders, still cameras, and video cameras. The most common question is... which SD card should I buy? This article will decode all of the designations and provide simple answers.

Card type and capacity

First, decide on how much storage you need, based on what your device will support. This will dictate the Card Type, since each has a certain maximum capacity:

  • SDSC (1999) = 2 GB
  • SDHC (2006) = 32 GB
  • SDXC (2009) = 2 TB
  • SDUC (2018) = 128 TB

For example, I might want a 256 GB card for my video camera to enable long recording times at high bit rates. So I choose an SDXC. I check my camera manual (or online spec sheet) and sure enough SDXC is supported. I am good to go!

Bus speed

The earliest SDSC supported only Standard speed. Since then we've had a choice of buses, each of which indicates (but does not guarantee) maximum read/write speeds. 

  • Standard: 12.5 MB/s
  • High-speed: 25 MB/s
  • UHS-I: 50 MB/s or 104 MB/s half-duplex
  • UHS-II: 156 MB/s or 312 MB/s half-duplex
  • UHS-III: 312 MB/s or 624 MB/s full-duplex

You can often ignore these numbers because easier labels have been introduced. 

Speed Class Ratings

Speed Class Rating guarantees minimum sustained write speed. These replace the older "x" measures.

  • Class 2 = 2 MB/s
  • Class 4 = 4 MB/s
  • Class 6 = 6 MB/s
  • Class 10 = 10 MB/s

Video Speed Class similarly indicates minimum write performance, but is specifically aimed at video usage, as you might imagine from the name! There are 8 bits to a byte, but the numbers are rounded down for safety. V6 is the same as the old Class 6 and V10 is Class 10, but the rates go up from there. 

  • V6 = 6 MB/s (maximum bitrate 45 Mbit/s)
  • V10 = 10 MB/s (75 Mbit/s)
  • V30 = 30 MB/s (220 Mbit/s)
  • V60 = 60 MB/s (460 Mbit/s)
  • V90 = 90 MB/s (700 Mbit/s)

You might also see the older UHS Speed Class on labelling. It's less useful, and you can ignore it. But for completeness:

  • U1 is equivalent to V10
  • U3 is equivalent to V30

Though photography is not as demanding of storage media, you might need a faster card than you think. Burst shooting is sometimes accomplished at 30 frames per second, which is basically identical to shooting video. Note the write speed you'll need by reading your camera manual.

Videographers should note the highest resolution video mode you will record, along with the corresponding bit rate. If you are using a Lumix S, you can use my handy charts to look up these numbers. 

What brand?

Several brands tend to be recommended, but there's always the possibility of getting a lemon. Lexar Professional, Transcend, and SanDisk are popular choices. But be sure to buy from a popular retailer, since auction sites (including eBay) and grey market sources might well be pawning off counterfeit cards.

An example

Let's consider the card pictured at the top of this article. I purchased this in order to get the most from a Lumix S-5, which records video at up to 200 Mbit/s. Was this a good decision?

First, we can ignore the text that says "1800x" and "U3" since these are deprecated specifications. We can ignore SDXC once we have confirmed that our device supports that card type. 

What is most important is the V60 designation. This indicates a maximum video record rate of 460 Mbit/s which is more than enough. In fact we could have bought a V30 card... but at the time of purchase there was a sale that made the cost only incremental between the two speeds. 

As for the capacity, 256 GB is ridiculously large. This reduces the need to erase work regularly. Since my camera has two SD slots, I never erase the second card, effectively using this as a portable backup. 


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