Using an SD and CF Card Simultaneously in your 5D MKIII Might Be a Bad Idea

Using an SD and CF Card Simultaneously in your 5D MKIII Might Be a Bad Idea

Recently, Jeff Cable from Lexar published a blog post explaining a rather curious choice on Canon's part. It seems that the 5D Mark III was built 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). Without UHS support, the top speed that can be achieved by the SD card is 133x. Because the 5D MKIII defaults to the slowest memory card speed inserted in the camera, your CF speed could get short-changed by the SD card.

Jeff explains that he first discovered this problem when he decided to do something that makes perfect sense to a photographer: shoot RAW to one card and JPEG to another, or just shoot RAW to both to act as a backup. But because of the way they designed the SD card slot to work, 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.

What is most painful, but makes sense based on the decision by Canon to not give the SD slot UHS support, is that the 5D MKIII will shoot only as fast as the fasted 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... Why would I want to clear data at 20MB per second when I could be transferring at 90MB per second or better?"

An excellent question Jeff. What do you all think? Does this problem bother you? Would you have even noticed if Jeff didn't say anything? Let us know in the comments below.

[Via Jeff Cable's Blog]
Photo copyright Jeff Cable

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Ron Risman's picture

I noticed this immediately. I had a slow, class 6, EyeFi card in one slot and a fast CF card in the other and video recording couldn't keep up when writing to the CF card, despite it's fast 300x speed.   I no longer use an SD card in the camera unless I am shooting casual stills.

Jeff Lowman's picture

I have the Mark III and I shoot a class 10 32gb SD Card.  I was shooting a sports event running around and was wondering why such a fast card would have buffer issues.  This makes sense now, so I guess I will have to pick up a CF card if I want to run in RAW like I normally do.  It bothers me greatly as I am slowly making my transition to SD with the MiFi cards and iPad combo.

Satya Varghese Mac's picture

would a CF to SD adapter work if you just wanted to use one SD card at high speeds? It might be an option for Jeff. I don't know if using the adapter would slow down the card. SDs are so much cheaper.

Mr Blah's picture

the adapters would have to be built to sustain the faster speeds but it makes sense!

Danylo Bobyk's picture

I would highly NOT recommend using an adapter SD>CF , i have one and unless you want to risk some of your images to be corrupt.l.just dont do it. The are file errors one too many times and the card/adaptor locks up. Not something you would want if your time or images are important to you ; ) - probably are.

Michael Stephens's picture

Is there a noticeable slow down when shooting RAW on the CF and Jpeg on the SD card? Does the smaller file size of the Jpeg's allow the SD card to keep pace with the RAW's going to the CF card?

Delane Rouse's picture

I think this is the same for the Nikon D800...can anyone confirm? When I was shooting with an EyeFi card in the SD slot it felt slow as molasses so I removed the card. 

I'd love to know if it was just me. 

Delane Rouse
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Rex Wui's picture

The same rule applies for the D800 - read/writes at the slower of the two.  But the key difference is that the D800's SD slot supports UHS.

Kerry Loudenback's picture

Firmware fix?

Scott Thomas's picture

Is this anything Canon could fix with a firmware update?

Mike Burton's picture

upshot is...get the Nikon!

Wendy's picture

I don't like tinkertoys, thanks.

Paul Krol's picture