Why two grey cards?

[delboy]

Please excuse my ignorance, but surely 18% grey is 18% grey? The two cards represent two different grey values don't they?

MT>>>>18% Gray was designed for exposure setting not WB. See more below.

[MichaelT]

The light gray is best suited for most RAW converters, which work in the linear space (C1, ACR, etc), and the darker card is better for a JPEG workflow where you are setting a midpoint using levels or curves.

[delboy]

I'd like to know some more about this. Can you point me to a thread where this is discussed in greater detail please?
I use C1 Pro with Canon RAW files at present, on the whole most of the time the conversions are good but I still get some problems, particularly from strong tungsten lighting.

[MichaelT]

Even with strong tungsten lighting you should get good results using WhiBal with C1. While the C1 slider only goes to 2K, if you do a click WB it will go lower and should yield good results. Of course camera limits come into play as well in this case.