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Registered: ‎09-07-2015
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Memory Card Recommendation


I am about to get a 5D Mark III and am looking for a recommendation regarding memory cards. I remember reading that the Mark III writes at the slower speed of the 2 cards.  Is that correct?


I'll be using the camera for general, not professional, photos.


Steve F

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Posts: 980
Registered: ‎11-14-2012

Re: Memory Card Recommendation

Cf 100 MB /s. SD 21 MB/s.
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Posts: 3,847
Registered: ‎06-11-2013

Re: Memory Card Recommendation

The CF card slot is a UDMA7 slot (it's extremely fast).  But the extremely fast technology for SD cards is called UHS1 and the SD card slot on a 5D III is not a UHS1 slot.


The UHS1 standard was extremely new just about the time that the 5D III was released, so depending on when all the engineering on that body was finalized (long before it went into mass production) if the UHS1 standard wasn't far enough along, it's understandable why the 5D III doesn't have it.  Every Canon EOS camera that came out since (that uses SD cards) uses the UHS1 technology.


But that's not the wierd bit.  The wierd bit is, several photographers claim to have tested this (I have not) and the claim is that even if you configure the 5D III to write exclusively to the CF card slot and ignore the SD slot, merely having an SD card inserted in that slot causes the write speed to be slowed (it slows down to a speed "as if" it were writing to the SD even though it's ignoring the SD card.)  I can only imagine that they are both connected to the same IO "bus" and Canon slows the bus to the least common denominator if the SD card is present.


These photographers claim that if you're shooting fast action photography, eject the SD card and only use a UDMA7 CF card -- and your writes will complete faster so your camera clears the internal buffer fast and is thus ready to shoot more frames sooner.


I have not verified this myself and have not read any articles that show specific data (how many frames of continuous burst for RAW can I shoot with just a CF card, vs. a CF and SD card both in camera before the buffer fills and the framerate slows.?)  


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