Jeff Cables reports you shouldn’t put SD in Canon EOS 5D Mark III if you want speed in CF UPDATED

by planetmitch5 Comments

Jeff Cables has reported that putting an SD card in the Canon EOS 5D Mark III – even if you aren't using it – slows down the speed of writing to the CF card. tipped us to this report

UPDATE – It should be noted that Jeff has clarified his post (simply having the card there doesn't impact write speed):

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. I am not sure why you would want that (except for overflow), but this is the case.

I haven't tried this but thought it was significant enough that it should be reported!

Jeff Cables reports:

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 …

Read Jeff's entire report! Jeff Cables Blog: Why you should not put an SD card in your Canon 5D Mark III if you care about speed.

What do you think?

Have you noticed anything like this?

Image from Jeff Cable's Blog

  Note: it is our policy to give credit as well as deserved traffic to our news sources – so we don't repost the entire article – sorry, I know you want the juicy bits, but I feel it is only fair that their site get the traffic and besides, you just might make a new friend and find an advertiser that has something you've never seen before  
[source: CanonRumors post]


  1. Does this have any play while shooting video I wonder?
    I have shot video onto an SD card so far without a problem but have in the past had problems with 133x in my 60d. If I could get those same problems (video recording stops suddenly) on my mkIII than I will probably stay away from them. I don’t do too much burst photography so that doesn’t concern me.

  2. I have also found this to be the case. I’m a concert photographer, and during certain critical moments of a performance, I have to rapid fire to get the shot I need. This weekend, I noticed that my 5D Mark II reached it’s buffer more slowly than my Mark III, which had dual cards in it. Didn’t think all that much of it, as it didn’t cause me to miss any shots, but I’m happy that Jeff was able to find the root of the issue.

  3. It’s worth reading through (or at least skipping to the end of) the comments to the linked article. Several people point out (and Jeff also confirms here that the issue only arises if you’re attempting to write to both cards at once (which is documented in the 5D3 manual).

    If you only have another card in there as overflow, etc. then there is no issue. The “even if you aren’t using it” bit, as reported here and elsewhere, is therefore not the case.

  4. so the cf card only defaults to the 133x when you’re writing to both cards at the same time, is that correct? if the sd card is only being used when the cf card is full, the problem above does not exists, am i right?

  5. I can tell you one reason you’d want it in there… so you can install Magic Lantern to the SD card but shoot to the CF card. Glad to hear that’s not a problem. I am ordering a couple 1000x Lexar CF cards and an 8GB 133x for Magic Lantern.

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