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Quick Tip: Optimize Canon 5D Mark III Write Speeds – Avoid Using SD Cards

Gear Talk Tips & Tricks

Buffer Sluggish on 5D Mark III? Ditch the SD Card!

Photographer Jeff Cable discovered something interesting about the 5D Mark III when he was doing some high-speed shooting–it’s slow, but only under certain circumstances. His advice? Pass on using the SD slot if you happen to be writing the same image to both cards and care about clearing your buffer quickly:

“…YOU DO NOT want to put a card in the SD slot. Why? Because, for some reason unbeknownst to me, Canon decided to build the 5D Mark III with one very fast CF slot which supports the newer UDMA7 protocol and a standard SD card slot which does NOT support the high speed standard (called UHS – for Ultra High Speed). This is really strange because many other cameras have come out with UHS1 compatible slots over the last year. Without UHS support, the top speed that can be achieved by the SD card is 133x. This is true even if you purchase a 600x SD card and insert it in the camera. The best you will get is 133x…”

Why is this happening? The 5D Mark III defaults to the slowest card that is in the camera at the time. If you want to take full advantage of your professional CF card, leave the SD slot alone–save it for times when speed isn’t important but having backups or more storage is.

On any camera with dual card slots (this includes pro bodies, such as the Canon 1D x and Nikon D4, as well as the Nikon D800 and Sony A99), it will default to the lowest card speed that is currently in the camera. What is different about the 5D Mark III, as observed by Jeff Cable, is that the SD card slot won’t write any faster than 133x no matter what kind of SD card is inserted. We are hoping that this will be addressed with firmware updates. [/learn_more]

Below you can read Cable’s findings in full, reprinted here with permission:

Why you should not put an SD card in your Canon 5D Mark III (if you shoot to both CF and SD and care about speed)

I am going to start this by saying that I really like my Canon 5D Mark III cameras and use them for shooting everything including Bar and Bat Mitzvahs, landscapes, portraits, sports and more. But just after receiving my two cameras I started to play with the memory slots and experimented with the best way to use both memory card slots. My first thought was…it would be great to shoot RAW images to both cards as a backup measure, and my second thought was…I could shoot RAW to the CompactFlash card and JPG to the SD card.

Well…after some testing I have determined that, if you care at all about high speed shooting or clearing you buffer quickly, YOU DO NOT want to put a card in the SD slot. Why? Because, for some reason unbeknownst to me, Canon decided to build the 5D Mark III with one very fast CF slot which supports the newer UDMA7 protocol and a standard SD card slot which does NOT support the high speed standard (called UHS – for Ultra High Speed). This is really strange because many other cameras have come out with UHS1 compatible slots over the last year. Without UHS support, the top speed that can be achieved by the SD card is 133x. This is true even if you purchase a 600x SD card and insert it in the camera. The best you will get is 133x

So…the only reason to use a really fast SD card is for faster downloading after the shoot.

At this point, you might be thinking, “why would the SD card slow down all of the data transfer of the camera including the CF card?” It turns out that the camera will default to the slowest card inserted. So, if you have a 1000x CF card in slot one and any SD card in the second slot, the very best buffer clear that will achieve is 133x. When shooting sports or any type of images with burst mode (6 frames per second), this is crippling. I want to shoot a bunch of images, have the camera clear the buffer as quickly as possible, and then keeping shooting more. Why would I want to clear data at 20MB per second when I could be transferring at 90MB per second or better? For this reason, I almost never use the SD slot in the camera. I want to take full advantage of my Lexar 1000x Professional CompactFlash card.

If you are a photographer who shoots in a studio or does not shoot in burst mode, this may not be a big deal. But if you care about clearing your buffer, you need to be aware of this.

One more thing. Most of the time, this is a hardware limitation and can not be solved with a firmware upgrade. Even more of a disappointment!

To read more from Jeff Cable, visit his blog here.

Thank you, Jeff, for bringing this to our attention and allowing us to share your findings!

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Alexandria Huff is a portrait photographer based in San Francisco. Her tutorials can be found here on the BorrowLenses Blog, 500px, Shutterfly, Snapknot, and SmugMug. She specializes in studio lighting. Follow her work on 500px.


  • Peter says:

    This explains alot, thank you for the information. I recently did a wedding and the camera was reading very slow on my CF card because I was shooting Raw to the CF and jpg to the SD card for back up.

  • I just got my MkIII yesterday and was excited about adding an SD card to the mix. I figured most of the time I’d just leave the card in the camera but not use it. But now I’d have an emergency option if I either fill a card and don’t have a backup or forget to bring a CF card at all (I’m sad to say that happened once).

    Are you saying that just having an SD card in the camera will slow things down or do I have to be actively writing to the SD card to encounter this problem.


    • I’ve had cheap (slow) SD cards in my 5D Mark III’s since day one in case of an emergency. I don’t actively write to them but they are sitting in the camera. With that said, I have notice no difference in write speeds to the CF, whether the SD card is in camera and not active or out of camera completely.

      I hope that helps

  • Zach says:

    I am wondering does this affect shooting video? Any thoughts?

  • Sean says:

    Another reason I like the 1D-X: 2 CF slots.

  • Ana says:

    O mesmo se aplica a Canon T3i? Eu já tinha ouvido algo sobre mais não imagineo ser assim tão diferente…..

  • Pilou says:

    Nô slow Down using SD until you save RAW on it

  • bob lionel says:

    I just tested this, and it’s not true at least on my 5D Mark III. Using a Lexar 1000x 16gb card on fast burst mode, i get 17 shots rapid fire before it slows down and still shoots about 1 shot every second or so(shooting RAW) with a sandisk SD card also in the camera. I pull the SD card out, fire in burst and I get 17 shots before it buffers and then about 1 shot a second. In both cases, the light goes off and buffereing stops after about 5 seconds or so. If I switch to the SD card I get about 12 shots before it slows down and then I get about 1 shot every 3-4 seconds. After I quit, the red light stays on indicating that the photos are buffering to the card for probably 45 seconds. These results indicate that the SD card has no effect on my 5d Mark III writing to the CF card.

    • Ben says:

      Bob, your test just proved what Jeff is talking about. The problem only occurs when both cards are being written to at the SAME time. Your test (apparently) does not include that scenario. If both cards were being written to in your tests, after 12 frames the camera would slow to 1 frame every 3-4 seconds. This might improve if jpegs only are written to the sd card but I am not sure.
      The bottom line is – I don’t know what Canon has against the UHS-1 standard even though the cards and the standard has been out for a while and other manufactures have been implementing it. I think only the new 6D is using it. This problem would be almost a non issue if the SD slot supported UHS-1.

      • T says:

        I just went up to re-read. And he clearly states that simply having a CF card in the slot, even when it is NOT being written to, causes the slow down. He advises to leave the slot empty unless you are actually using it.

        • slvrscoobie says:

          ‘it would be great to shoot RAW images to both cards as a backup measure, and my second thought was…I could shoot RAW to the CompactFlash card and JPG to the SD card’ – Right, if your writing the same or similar files to both cards at the same time, it will be slow. But if you use them one at a time the SD will still be slower than the CF, but the SD card being in the slot will not affect the CF write speeds.

          • I also want to add to Scoobie’s reply that writing RAW files to both cards will be quicker than writing a RAW file to the CF and a JPEG to the SD. It’s a matter of pushing the same file to two cards rather than creating two different files for two cards.

            You can see the buffer capacity change in the viewfinder when you make changes to the file types.

  • […] Faudrait-il éviter d’utiliser des cartes SD dans le dernier 5D Mark III si vous voulez optimiser la vitesse d’écriture? – Borrow Lenses […]

  • I’ve also tested this. I’ve tested with a Lexar 1000x 32GB CF and a Sandisk 30 Mb/sec sandisk SD as well as other combinations. I’ve tried one at a time and with both cards in. I’ve tried writing to just one and writing to both. In a 20 sec. burst of time (which means buffer more than filled) I get 93 full size raw images if I only write to the CF with both cards insered. I get 93 raw if only the cf card is inserted. If I only have the SD inserted, I get 30 images. If both cards are inserted and I write to BOTH, I get 51 images, so could be true when writing to both … or maybe it just takes longer to write to two cards, but CLEARLY FALSE when writing to just CF card when both are inserted.

  • […] From quick research I found this article pertaining to the camera he tested (Canon 5D Mark III). Blog @ BorrowLenses Basically the article claims the speeds between the SD and CF slots are different. And more […]

  • […] gebruiken. Best sandisk extreme (pro) of lexar 600x (1000x) Het SDkaart slot op een 5DIII is kreupel Design is destiny in my hands, fate in your eyes. BD2222BE / Fototoestel, wat glas, iets om […]

  • Rick Spaulding says:

    I don’t understand why this article is still posted on your site. It is quite obviously incorrect. It’s yet another example of Alexander Pope’s observation, “A little learning is a dangerous thing.”

    Why didn’t you think to test this before regurgitating Mr. Cable’s false assumptions?

  • martin says:

    I have a problem with the CF disk, once I delete all the images on my laptop and then place the card back in the Camera the notification appears saying card Full. I cannot find any info on how to resolve this, so if any of you guys can shed any light on this it would be much appreciated.

    • When you pull the images off your card onto your computer, you are merely sending exact copies from the card, to your computer. When you put it back into your camera, you need to go into the blue menu items in your camera and select to erase the images….or you can go to the yellow menu section and have it format the card.

  • […] – Optimize Canon 5D Mark III Write Speeds: Avoid Using SD Cards by Jeff […]

  • Not only is this article not correct, but it is horribly irresponsible advice to anyone who has a 2-slot camera. Don’t take advantage of the redundant backup feature? Really?

    • Jeff Cable says:

      Joshua – I assure you that this is correct information. I agree with you that redundant backup is ideal and I use this all the time in my 1DX. And…people can choose to use the SD card slot as long as they know that it will severely limit their burst mode shooting.

      • Rick Spaulding says:

        Again, I will state, this is not correct! Simply putting an SD card in the camera does not slow down the write speed. Period. But don’t take my word for it. Try it!!!!

        • Tom A. says:

          This is a silly argument. “This is not correct!” The this you are referring to is one part of a decent article. Do you disagree that the write speed to an SD card of any speed, is relatively slow compared to a average CF card? Do you disagree that using both card slots at the same time slows down the otherwise fast write speed of a good CF card? There really is no point of solely writing to the CF card and having an SD card in the slot. You missed the value of the article and proved a meaningless point. Congrats!!!!

          • Rick Spaulding says:

            I agree that writing to an SD card is slower.

            I disagree that simply putting an SD card in the camera slows down the write speed to the CF card. (if you need that explained you shouldn’t have replied to this post)

            I don’t use an SD card for backup of the CF card. I use it for overflow if I’m in a situation where I can’t change the CF card fast enough. It allows me to keep shooting until I have a second to change cards. As a live event photographer that happens on pretty much every job. Yes, then it is slower but it’s better than nothing.

            As for your comment about making a meaningless point you are obviously speaking (or typing) before you think. You might try using a more civil tone in your posts next time. It only makes you look like a loud mouthed idiot but, hey, if the shoe fits!

  • I’m tired of this. I’ve shot thousand s of images and tested my 5D MkIII with all of the above options. What do I do? I use a very fast CF card and write to it, but I do have an SD card in that slot for emergency overflow. I don’t want to not be able to shoot when that fish eagle flies over dragging a baby. As soon as my ’emergency shot’ clears, I swap out the CF card, confirm that the camera is writing to it and keep shooting. The camera is definitely slower writing to the SD, but is not slower writing to the CF when an SD is in also.

  • Jeff Cable says:

    Let me see if I can clarify all of this in one post:

    It is absolutely true (and can be easily reproduced) that when WRITING TO BOTH THE CF AND SD CARDS AT THE SAME TIME in the 5D Mark III, the performance is greatly reduced. I wrote the original post to let people know that this is the case. Honestly, I should have chosen a different title, since there are times when people WILL want to use both cards. Overflow is a perfectly good reason for having an SD card in the slot. And…for people that shoot in single shot mode and are not bursting out frames, they do not care about about buffer clear, and slowly writing to both cards may still be an option for them.

    I think it is important for people to know all these facts. How you use the information is up to you, as we all have our own ways of shooting and our own preferences.

    With all of this said, I am glad that people are talking about this. It should be a useful discussion amongst us passionate photographers. There is no reason for anyone to be confrontational about this. In my opinion, the greatest thing about photography is the sharing of information and the way that we all strive to learn more and shoot better.

    Hope this helps,


    • Jeff, well stated. I agree completely.

    • joshuacmitchell says:

      My big issue with this article is that you at no point go over the benefits of using the second slot. Instead you have “DO NOT USE” in capitals, circles with red lines drawn through them, and basically imply that the camera is slow to a point of uselessness if using the second slot. I find it irresponsible because some photographers are going to take your word for gospel, cards are going to fail, and there will be no images backed up to save the day. The fact of the matter is as an event photographer I shoot writing full size raw backups to the second slot, on silent mode (further slowing things down), and I have learned to anticipate the buffer. Would Canon have made a better camera if they had addressed the issues with the second slot? Of course. However as a photographer with paying clients it’s more important to me to protect their images than it is to machine gun through a shoot.

  • Sean Holde says:

    I think the article is excellent, I feel people make assumptions about the content of the article before reading the entirety. I am a wedding photographer and if I am using both a CF and SD card, writing to both at the SAME time. It is slower than just writing to the individual CF card. For example: CF card capturing RAW images with a SD card capturing JPG for backup. I can shoot faster if I just had the CF card in. Try it, the article makes perfect sense, I just think that people don’t read as much as they used to.

    • Actually, Sean….the problem is that you didn’t read the article properly. They claim that just having the SD card in the slot at all, will slow down the write speed to the CF card…even if you’re not writing to the SD card. The article says that ust the presence of the SD card slows the write speed to ALL cards in the camera….and that is not true. I use the SD card as a backup in case I fill up my CF card without realizing it, so I don’t run the risk of missing the perfect shot because I had just filled the card. I can keep shooting until I have a chance to put another CF card in, and I don’t miss an important shot. I never see ANY slowing of the write speed to my CF card, while the SD card is in the slot….and that *is* counter to what the article is saying.

  • Hachiko says:

    This is complete BULLSHIT. Jeff is full of crap and so is the girl here. Simply “putting a SD card into the slot” will NOT slow down the speeds. The only way the speeds come into play is if you WRITE TO BOTH CARDS AT THE SAME TIME. Again, the title and everything is just wrong. Simply putting a SD card in the slot does nothing. You have to write to both card simultaneously.

    • We seem to be in agreement. From the intro: “Pass on using the SD slot if you happen to be writing the same image to both cards and care about clearing your buffer quickly…” From Jeff’s section title: “Why you should not put an SD card in your Canon 5D Mark III (if you shoot to both CF and SD and care about speed)”. Having the card physically sitting in the slot isn’t problematic.

  • Scott Belton says:

    Do you all know how fast the CF slot can write? Lexar also sells a CF card that can write at 3400x. Will this allow me to take pics continuously in RAW?

    • Jeff Cable says:

      Scott – the 3400x card is not a CompactFlash card, it is a new format called CFAST. Not the same format.

      As for the write speed of the Mark 5D Mark III, the CF slot can take advantage of the 1066x CF cards, which is as fast as the CF spec allows. You won’t see anything faster in this format.

      I hope this helps.


  • Emanuil says:

    I am thinking of putting an Eye-fi card in the SD slot and use it to download/share images on the iPad. For the iPad i do not need full resolution JPEGs.

    Here is a question for those who did extensive testing with both cards BEING WRITTEN AT THE SAME TIME. I understand that the burst rate will be significantly reduced if you write RAW images to both CF and SD cards (or if you write RAW to CF and high res files on SD).

    Would the CF write speed be impacted if you write low resolution images (only for iPad viewing) to the SD card?

  • Jeff Cable says:

    Emanuil – yes, the CF write speed will slow down to 133x, even if you have a much faster card in the camera. And, it is not just the burst rate that is effected, it is the clearing of the buffer to shoot more.

  • Jeff, thank you for the quick answer. Are you saying that the CF speed is limited to 133x by some kind of software switch or is this purely determined by the 133x limit of the SD bus?
    If we write a full RAW 27MB file to both cards then, based on the bus speeds, the transfer speed will be 1066x to the CF and 133x to the SD. In this case the buffer will not be cleared until the RAW file is written to the SD and I can uderstand how the 133x is limiting both the write speed of the CF as well as the clearing of the buffer.
    However if we write a full RAW 27MB file to the CF only at bus speed of 1066x and we write the same image file but as a small S2 JPEG 1.4 MB to the SD at 133x then the RAW will be written in 27MB/160MB/s = 0.17s while the S2 JPEG will be written in 1.4MB/20MB/s = 0.07s. That means that the camera will be writing the JPEG 2 times faster than the RAW and so while the SD speed is 133x it will not adversly impact the CF. Unless the CF is limited by a software while the SD is in use.
    Appreciate your thoughts. Once I get the SD I will do the test but am really curious to see if someone has done this test or knows how the camera works.

  • Ceri Davies says:

    Nice article. I’ve recently purchased a SanDisk extreme 32gb 60mbs SD card to test out (£17!). I’ve been using CF cards up until now – are there any benefits SOLEY using one or the other. I don’t shoot sports or anything where i’m firing 15/20 shots off at one time. I shot 15 raw files last night on the 5D mk3 before i hit the buffer on the SD – without a CF card in. That’s more than enough for me. Just a curious post – thanks in advance for any feedback.

  • Laura r says:

    Has this been fixed by filmware yet ?

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