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Gear & Apps

Why You Should NOT Use an SD Card in the Canon 5D mkIII

By fotosiamo on January 3rd 2013

Canon 5D mkIII SD Card

One of the improvements of the ever-popular Canon 5D Mark III over the Canon 5D Mark II is the inclusion of both a CF card slot and an SD card slot. It allows the photographer the flexibility of recording to either an Secure Digital or a CompactFlash card, or both.

Well, according to photographer Jeff Cable (as reported by it turns out that using the SD card can cripple the mkIII’s buffer and write speed. The 5D mkIII utilizes the faster UDMA7 CompactFlash protocol that allows you to attain up to 90MB/sec of write speed with high performance CF cards such as the SanDisk 16GB Extreme Pro CF UDMA 90MB/s and the Lexar Professional 1000x 16GB CF Card. However, for some unknown reason, Canon does not support the faster UHS-1 (ultra high speed) standard for SD cards, which means that no matter what SD card you use, the 5D mkIII will always revert its speed back to 20MB/sec instead of the faster 45MB/sec UHS-1 speed.

What’s worse, when there is both an SD and a CF card in the 5D mkIII, the DSLR will automatically default BOTH write speed to the slowest card. This means that expensive 90MB/sec professional CF card that you are using will perform like a cheap 20MB/sec SD card. So if you are a photographer who needs to shoot a lot of photos quickly, the best advice that Jeff suggest is to only have your CF card in your 5D mkIII.

So have you experienced this issue when taking a lot of photos at once? Post Production Pye has verified that this phenomenon is something he has noticed.

To learn more in-depth of this issue, you can visit the original article on Jeff Cable’s blog. Thanks to for the find!

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Joe is a fashion and commercial photographer based in Los Angeles, CA. He blends creativity and edge with a strong style of lighting and emotion in his photographs.

Q&A Discussions

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  1. Basit Zargar

    Never new it until i read it !

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  2. Roman

    What is I am using SD only to screen my CF card and protect shots, does it affect the speed?

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  3. Tim

    Then there really is no reason to buy the more expensive cards with fast speeds other than unloading your card to a computer after a photo session.

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  4. Ron

    I use the SanDisk Extreme Pro 16 and 32GB Compact Flash cards as well as the SanDisk Extreme Pro 32GB HC1 95MB/s SD card and I don’t experience any slowdown in the writing process. I updated the firmware recently but I can’t say that update had anything to do with the writing speed. I use the SD card specifically as a backup to my CF Card. I set the camera to write to both cards so that I feel comfortable that I’ve got the files for certain.
    Since I shoot mostly corporate and media events and no live sports, My Mark iii isn’t pushed to the limit because of multiple frames per second. But I have used the HDR setup and I’ve found that the shooting speed and processing time is extremely quick when the camera is set for the 3 exposures needed to get a quality HDR image.

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  5. Sean

    I have used Sandisk SD and Compact Flash cards in my camera and and both are 90MB/s cards that i have and i don’t see any difference between them and i down load the files using a Sandisk usb 3.0 multi card reader & writer when downloading to the SSD in the macbook pro retina and both cards download very quickly and when i’m ready to get my next 5D eather the mark 3 or the next model after that i would use SD and Compact Flash. It looks like he was using a card that was to slow and i did have the same problem with a Compact Flash card that i did use. I won’t buy cards that are slower then 60 MB/s in SD or Compact Flash. 

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  6. Souvik De

    Does this hold true for Nikon D800/E

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  8. Krisztián Storm Kocsis

    Hmmm… lately I hear a lot of Canon Fail stories… Come on Canon, you should take much more care, you start to damage your reputation.

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    • Matt Corkum

      Except that there have been tests that show the 5DMKIII’s buffer and write rates still best the D800 because Nikon’s internals don’t match up with the crazy megapixels of the camera and it slows the entire process down.

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  9. Adrian Staicu

    If this is not a hardware problem, the writing speed is probably still 90 MB/s on CF, but the camera buffer is emptied at the slowest speed making it look like the CF is slow. Depending on how the software was written, and how data is send to storage, this may be avoided writing data to CF first and making a backup of that data when idle, and could be fixed in an firmware update.

    If it’s a hardware limitation… someone will probably get fired.

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  10. HawaiiGeek

    curious, is there a test out there on the D800 with current FW?

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  11. JesseDavis

    I use the SD card slot to load Magic Lantern and video only. For photos, I record only to the CF, no speed issues here.

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  12. Enrico Pretto

    Is possible that when the camera use both the slot the slow speed of the SD make the buffer full after a short time and the camera have non place in the buffer for save the photo also if the speed of the CF is the same?

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  13. Scott Painter

    Well I guess if you are a spray and pray photographer and the thought of
    you being limited to only 14 frame raw bursts in 3 seconds like the 5D
    mark ii predecessor is worry some, you have a point. 

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  14. Scott Painter

    Interesting article

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  15. guest

    The article written by Jeff was a while ago and you should really do more research or at least read the long thread of comments. 

    As its been pointed out and stated by Jeff too…”As many of you have pointed out, the degradation in speed will occur when you are attempting to use both the CF and the SD to write files. If you have an SD card in the camera and are not writing to it, you will be fine.”

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    • spoiledbrt

       lol…so it doesnt matter whether the card is inside the camera or not since you are not writing it to the SD card

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