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SD and CF Cards together in 5D Mark III


Have 32 GB SD card (45 MB -speed)

Depending on answer, may want to get a CF Card 32 GB (with faster speed)


Read that if both used at same time (not one or the other) then the speed becomes that of the lowest card.  True?

I realize there are many different reasons to have two cards as well as speed of cards.  I am not a profesional, but at least would be considered intermediate. Used to shoot print film years ago (60-70s) eyesight became poor, got cataract surgery now eyes good again, and the AF on the 5D MIII propeled me to re-enter photography. My lens is good too, 24-70 F2.8


Any help or direction to a valid and new explanation of my question will be appreciated.




You could use two cards to give you an in-camera backup by recoding the same images to both cards.  In that case the slowesr card will control how fast the buffer can be emptied.


Some people record RAW to the CF card and JPEG to the SD card. In that case the CF card will most likely control the buffer speed even though the CF card is faster. This is because the RAW files are larger.

John Hoffman
Conway, NH

1D X Mark III, M200, Many lenses, Pixma PRO-100, Pixma TR8620a, Lr Classic

was thinking same way if buffer speed as you suggest. also, thinking of video to CF


You may want to read this article by Canon:


The camera has an internal memory buffer.  When you take a single shot, that image reads off the sensor and is saved initially into a temporary memory buffer.  The camera then feeds that data to the memory card at whatever speed the memory card is able to accept the data.  Once the data is completely saved, the internal temporary memory buffer is cleared and the camera can re-use that memory space for more pictures.


It turns out, higher end cameras have larger internal memory buffers.  They can usually take quite a few images before the buffer runs out of capacity.  This allows you to shoot in continuous burst mode -- even if the memory card being used is quite slow.  Of course if the memory card is quite slow then the camera will slow to a crawl while it waits to finish sending each image to that slow card to make more space in the internal buffer to let you resume shooting.


If you have very fast memory cards, the first few images will have been completely saved to the card before the camera gets to the end of the memory buffer... and can immiately start reusing that space from the first few images to shoot even more images (the buffer is literally fillling and emptying at the same time... like a funnel.  Put a bigger hole at the bottom of the funnel and you can pour liquid into the funnel even faster without over-flowing the funnel.  In this case, the "bigger hole" at the bottom of the funnel is a faster memory card.)


But also keep in mind that JPEGs do not require nearly as much space as RAW images.  So while the 5D III's cap on RAW images ranges from 12 to 18 frames (depending on what type of cards you use), if you shoot JPEG and use very fast cards (e.g. UDMA 7 CF cards) the capacity is basically unlimited (the card will run out of space as you shoot thousands of images before the memory buffer actually fills up.)  For this reason, some sports photographers will shoot JPEG because they can capture many more frames before they have to pause and wait for the camera to catch up -- even though RAW images capture more detail and offer more adjustment latitude when it comes time to tweak your images on the computer.


If burst speed is important to you, you want to (a) buy the best memory cards you can get (e.g. SanDisk Extreme Pro SD and CF cards, or Lexar 600x SD carsd and Lexar Pro 1066x CF cards).


You do want to get them ONLY from vendors you can trust.  Coutnerfeit cards are easy to make (since they're all basically black plastic identical cards and the only thing distinguishing them is the label... it's pretty easy to print a fake label.)  


It's fairly common that someone will come to the Canon forum asking for help with a camera that stops recording after a few seconds when shooting video EVEN THOUGH the card they used in the camera is allegedly fast enough to handle the video. In most cases, when we can convince those people to buy a card from a trusted brand and sold through a trusted store, all their problems go away.


I've reached the conclusion that trying to save a few dollars on the cost of memory cards is likely to create more problems than it's worth.


Tim Campbell
5D III, 5D IV, 60Da

thanks for this information. I'll read the reference and return if need any more info.

"Read that if both used at same time (not one or the other) then the speed becomes that of the lowest card.  True?"


Yes, that is true but  in most cases it will be the SD card.  love my 5D Mk III but it does have one big flaw. The SD card slot can only handle about  133x or about 20MB/s.


If you are writing to both the CF slot and SD slot at the same time, then both slots will slow down.  To shoot at the maximum 6 frames per second, many people will only write to the CF slot so the buffer clears as quickly as possible.  

Mike Sowsun