Got a super-fast Canon 5D MkIII? Love that you can just pop out the SD card and slide it straight into your Retina iPad via the camera connection kit? Not so fast – literally. Photographer Jeff Cable has done the math and found that the camera’s SD slot is slow, slow slow compared the the CF slot, and then it actually gets worse.
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.
This, says Jeff, means that the maximum in-camera write speed for even a 600x SD card is 133x. Worse, the camera defaults to the maximum speed of the slowest card you put in there. Thus, you can put in a 1000x CF card and you’ll be fine, but if you put any SD card into the other slot, the whole lot is slowed down to 133x.
This reminds me of wireless networks, which are slowed to accommodate the worst device on the network. Modern routers like Apple’s AirPort Extreme and Time Capsule get around this by using two radios working at different frequencies. It’s hard to see why Canon didn’t come up with a similar solution, although I have a feeling it might have something to do with the DIGIC processors having some trouble with the mixed bandwidths (although this is just speculation). Either way, it looks like using the second, slower card slot in any camera is a bad idea.
(And finally, a terrible pun: If Jeff shot portraits and asked the models to sign a model release form, would it be called a Jeff Cable Release?)