Rebel T3i SD Card Types and Maximum Speeds, SDHC, SDXC, UHS-1 Discussion

Started Feb 6, 2012 | Discussions thread
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Burner100
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Rebel T3i SD Card Types and Maximum Speeds, SDHC, SDXC, UHS-1 Discussion
Feb 6, 2012

Just ordered up a shiny new T3i and started to research memory cards. Turns out that what should have been a simple exercise turned out to be quite a bit more complicated.

The first question to answer is, "why care?". The answer is quite simple, two reasons really, 1) the camera requires at least a Class 6 card running at 6MB/s to record HD video but some folks have reported issues with some cards and not others, a faster card is less likely to hiccup on recording, 2) the speed with which the camera can dump photos from the internal buffer to the memory card is critical when snapping many shots in quick succession - when the buffer fills no more shots can be taken until the buffer empties enough for additional photos to be taken - the faster the memory card, the faster the buffer writes out the photos and the sooner additional photos can be taken.

The confusion occurs because there are new very high speed SD Cards available, such as the Sandisk Extreme Pro line, which can support speeds at 45MB/s and even 95MB/s depending on which card is selected. Importantly, the 45MB/s and the 95MB/s cards leverage a new communication protocol called UHS-1 (ultra high speed 1) but are also backward compatible with the old Class 1-10 rating system. The T3i supports SDXC cards, and is advertised as such, and the SDXC cards have additional capacity exceeding 32GB and also use the exFAT file system to handle the larger size. However, it is not automatically the case that all SDXC cards support UHS-1 (as far as I can tell) as an example, some could just be a Class 10 card. Now, this gets even more complicated because it is apparently also possible to have an SDHC card (32GB or less) which supports UHS-1 but is not considered SDXC. Furthermore, there are two versions of the UHS-1 protocol, one running at a bus speed of 100mhz and running at a bus speed of 208mhz. Each respectively correlating to a maximum write speed of 50MB/s and 104MB/s. Keep in mind that a Class 10 card is rated for minimum throughput of 10MB/s write speeds. Theoretically, a new UHS-1 card could be 5-10x faster than a Class 10 card depending on the actual performance of the Class 10 card and the supported bus speed of the host device interface and the speed of the UHS-1 card itself.

I was unable to find reliable information regarding what protocols and bus speeds the T3i supports so I called Canon. I was told that the camera supports the UHS-1 protocol at the 100mhz bus speed and 50MB/s write throughput. This is a very big deal because even the fastest Class 10 cards only deliver actual throughput of 20-25MB/s from what I was able to determine. Minimally, these new UHS-1 cards should deliver about double the throughput of the prior fastest Class 10 cards, and 5x the throughput of the slowest Class 10 cards. In practice, the new UHS-1 100mhz protocol will not deliver a full 50MB/s write throughput due to overhead in the protocol itself. This is why Sandisk rates their card at 45MB/s write speeds instead.

When I asked about the faster 208mhz bus speeds, I was told that the T3i does not support that speed and so spending the money for the super fast 95MB/s memory cards would likely not offer any meaningful performance gains. Despite having been told this by Canon, I'm still tempted to buy a small one and do some benchmarking to see if there are't any performance gains to be had.

Bottom line - if you want to snap lots of photos fast, like for any kind of action shots, then you will want to buy at least a UHS-1 compatible 45MB/s write speed card.

I hope this helps some of you.

Andrew

Canon EOS 600D (EOS Rebel T3i / EOS Kiss X5)
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