PetaPixel

Back to Basics: The Difference Between SD SDHC & SDXC, and Which is Best for You

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I will start off by saying I am partial to SanDisk memory cards, but I recently found a great write up on their website that is pretty much universal, explaining the difference between SD/SDHC/SDXC memory cards. I wanted to share this information with everyone because sometimes it can be confusing trying to figure out which SD Card is best for you.

First off you might be asking yourself, ‘What is SD?’ SD stands for Secure Digital. SDHC? Secure Digital High Capacity. And SDXC? Secure Digital eXtended Capacity. Other formats include miniSD and microSD but your DSLR will not accept these. There is no quality/security difference between these three formats. Cards labeled SDHC usually have a capacity of 4GB — 32GB. SDXC formats can be very large and expensive. For the purpose of this guide, I am going to recommend that everyone stick with SDHC format. These cards have more than enough storage, even for the most trigger-happy shutterbug.

You will see a Class 2,4,6 or 10 on SD Cards. This is the speed rating which measures maximum transfer speed for reading and writing images to and from a memory card, expressed as megabytes per second. However, video doesn’t need as big a data pipe because the video format is a smaller “fixed stream” that uses only a portion of the data pipe.

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Unlike card write speeds that measure maximum performance, class ratings measure the minimum sustained speed required for recording an even rate of video onto the card. The class rating number corresponds to the transfer rate measured in megabytes per second. Class 2 cards are designed for a minimum sustained transfer rate of 2 megabytes per second (MB/s)1, while Class 10 cards are designed for a minimum sustained transfer rate of 10MB/s2.

So what does this difference mean for you? Rated Speed (e.g. 15MB/s, 30MB/s, etc.) is maximum speed of the card and also what you would expect to approximately see in typical usage of writing or reading files on the card. This measurement is pertinent to still photography, especially for taking pictures with high-resolution and/or saving in RAW format where the files created are very large. The faster the card, the faster it can save the file and be ready to take another picture. You can really notice speed differences with high-megapixel DSLR cameras when using multi-shot burst mode.

Still digital images shot on high-megapixel cameras should utilize fast data throughput (a large data pipe), higher speed cards for improved performance. Higher speed cards can also improve how fast you can transfer the files to and from the card and your computer.

Speed Class is a minimum speed based on a worst case scenario test. The Speed Class is important for video mode or camcorders, where the device is actually saving a steady stream of data. The resolution and format of the video determines the amount of steady stream data. This translates to a minimum speed you need to guarantee that the video captured on the cards is recorded at an even, sustained rate with no dropped frames (which would result in lost data and choppy playback).

Compared to high-megapixel photography, video doesn’t need as big a data pipe because the video format is a smaller “fixed stream” that uses only a portion of the data pipe. But you do need a minimum guaranteed speed for the SDHC card that satisfies the requirement of the data stream. Your camera’s specifications should state the minimum SDHC Class Rating required.

Using a card without the proper class rating on a more advanced camera, such as a high-definition (HD) camcorder or Digital Single Lens Reflex (DSLR) camera with HD video record settings is likely to result in an error message indicating that video can only be recorded at a lower definition setting.

The current SDHC specification defines Class 2, 4, 6, 8 and 10 as follows:

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UHS Speed Class was introduced in 2009 by the SD Association and is designed for SDHC and SDXC memory cards. UHS utilizes a new data bus that will not work in non-UHS host devices. If you use a UHS memory card in a non-UHS host, it will default to the standard data bus and use the “Speed Class” rating instead of the “UHS Speed Class” rating. UHS memory cards have a full higher potential of recording real time broadcasts, capturing large-size HD videos and extremely high quality professional HD.

So that’s it! I hope this helps bring some understanding and knowledge so you can make the right choice the next time you are on the hunt for some new SD cards for your digital camera.


*1 1 megabyte (MB) = 1 million bytes
*2 Based on SanDisk internal testing; performance may vary depending upon host device.


About the author: Martin Moore is a photographer, traveler, filmmaker and writer based out of Milwaukee, WI. You can see his work on his website, or by following him on Twitter, Facebook and Instagram. This article originally appeared here.


 
  • ContrailCowboy

    bogorad, i think you’re misrepresenting the most common scenarios for SD cards, which are sequential writes. there should be no difference between the way video and photos are recorded to FAT32 on an SD card, and in practise both are generally sequential. when writing any kind of file to the FAT32 file system the File System will allocate the next unused cluster sequentially on the disk until it has come to the ‘last’ cluster, and then it will start to ‘plug holes’ (which have ‘opened up’ from indidvidually deleted files). on a typical HDD on your computer, (or a micro SD card used in a computing device like a mobile phone) files are deleted all the time without erasing the whole drive. if there aren’t large enough blocks of contiguous clusters to store the file the file will get fragmented, that is to say, their clusters will not be allocated sequentially. this is true of both video and photos, as both are abstracted from the actual cluster writing by the File System. in other words, both videos AND photos can be subjected to file fragmentation and may not be performing sequential rights, in principle.

    however, on an SD card used in a camera or camcorder, the way most people use them is to transfer everything from the card to another device at once and then delete all files or reformat the card, instead of deleting some individual files and not others. because of this, writes (regardless of which file type is being written) on a SD card in a camera/camcorder will generally be sequential

  • http://profiles.google.com/bogorad bogorad

    If you really want to find out – do a simple test.

    1. Check the speed of your SD-card with the fastest SD-reader you can get (preferably PCI- or USB3-based) with CrystalDiskMark. Make a screenshot or print out the result.

    2. Put the card back into your camera.

    3. Set your camera to the maximum speed (“HI” or whatever, choose the fastest mode – e.g. my Sony a6000 does 11 fps.

    4. Start your stopwatch.

    5. Press the shutter and hold it. Do at least some 100 shots.

    6. Wait until ALL data is written to the card (the LED stops flashing).

    7. Check the stopwatch.

    8. Put the SD-card back into the computer and divide the sum of all file sizes by the number of seconds it took to complete steps 2-7.

    9. See for yourself it’s NOWHERE NEAR the ‘SEQ’ speed from step 1, it’s a lot closer to ‘512k’.

    But hey, you don’t have to ;-)

  • ContrailCowboy

    how do you know when the memory buffer of your sony starts writing to the SD card? you certainly don’t know for a fact that it starts when the shutter button is depressed (in fact it almost certainly doesn’t since the shutter needs to close, the sensor voltages need to be read, the RAW or JPEG processing needs to happen by the camera’s CPU, and then the image file needs to be written to the camera’s buffer before it even BEGINS to start writing to the SD card, and it may choose to wait on a timeout for the shutter to be released, since reading and writing from the buffer simultaneously is computationally expensive)

    in any event, this proves nothing about about reading or writing ‘sequentially’ to the card, because multiple file writes to any file system incur more overhead than writing a single large file, whether the file system is fragmented (random write scenario) or not (sequential writes).

  • http://profiles.google.com/bogorad bogorad

    Are you in fact following the thread?! I think not. Nice talking to you.

  • ContrailCowboy

    i have, in fact. you claimed that the class ratings on the cards are misleading, then you used technically incorrect assertions to bolster your initial opinions. finally, you proposed a timing test in which you can’t know when to begin the timer, which is therefore meaningless. your arguments aren’t looking very strong.