Alright, I'm sure this has been asked before but I was going through my equipment today looking to unload a few items I no longer need. I came across two unused SanDisk Extreme III memory cards. Both are 4gb SDHC cards and both are marked as offering 30mb/sec 200x read and write speeds. The difference? One is labelled as Class 10 and the other as Class 6. More specifically, the class 10 card simply reads 30 mb/s and the class 6 card reads "30mb/s Edition."

I am aware of the SDHC performance classes and what they mean. But, in the particular case of these two SanDisk cards - both advertising 30mb/sec 200x read and write speeds - is there any functional difference assuming they are put into service in the same device? Or could these simply be two otherwise identical cards, just labelled differently because they were manufactured at different points in the evolution of standards?

Any SanDisk users out there who can speak from experience?

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Panasonic G5 and Panasonic GX1, Lumix 14-45mm f/3.5-5.6, Lumix 45-150mm f/4-5.6, Lumix 100-300mm f/4-5.6, Lumix 14mm f/2.5, Lumix 20mm f/1.7

Olympus E-PM2, Zuiko 9-18mm f/4.0-5.6, Zuiko 14-42mm f/3.5-5.6 II R, Zuiko 14-150mm f/4.0-5.6, Zuiko 40-150 f/4.0-5.6 R, Zuiko 15mm body-cap lens, Zuiko 17mm f/1.8, Zuiko 45mm f/1.8

I'm leaning towards this one. The way SD card classes work is that Class X means "at least X MB/s write speed". Specific classes are billed as sufficient for specific things -- I believe that Class 10 is advertised as sufficient to record typical 1080p HD video. I figure it's unlikely there's much difference between your cards, which are much faster than the Class suggests.

Right now I have Class 10 Sandisk sdhc cards in the GF-1 & G2. 1st set of class10 cards. Previously used class 6 sandisk sdhc cards. Don't really notice any speed difference for single shots. Shooting RAW still images.

The GF1 only need class 6, not sure if it can benefit from higher speed of class 10 for writing, but may be faster reading from a card reader to download images.

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Gf1 + 20mm f1.7 +LVF1 14mm F2.5, DMC-L1, ZD12-60, Summilux 25mm :D, ZD 70-300, OM 50 1.8, 24 2.8, 70-200 3.5, Metz 48

the class rating is a MINIMUM speed

the 30mb/sec 200x is a maximum speed

So, only the class 10 guarantees 10mb write speed.

i.e. in this case
the class 6 = 6mb to 30mb range
the class 10 = 10mb to 30mb range

You can find utilities to test cards. I have a Kingston 16 gb Class 4 that tested at 9.5 Mb/s

It's not that simple though. The speed of the card varies depending on the size of what is being written, that's why the manufacturer can't just say it's a 30mb/s card.

As an example if you save a 1mb jpeg v.s. an 8mb raw, the time is not always just 8 times longer. Similarly streaming a video will not have the same write speed in mb/s as a small jpeg nor the same speed as a larger raw file. I strongly suspect if you wrote 1 large 16gb file to that kingston card you would not get 9.5mb/s.

The only reliable number you can really count on is the class number minimum mb/s. It gurantees that no matter what you're writing, it will write it at at least that speed. The rest is just a temporary or situational bonus if you can get it.