The towel is simply a proxy for judging when textured highlights you can\'t see in the camera feed, like eyes and teeth, are correctly exposed. Its unlikely a towel will be perfectly neutral. The important thing is that its similar the reflectance of the white details where detail is needed. By coincidence the red channel in skin, which also can\'t be seen in the feedback, also clips the same time as the towel making it a good quide to avoid blowing skin highlights. The towel is like the canary in the coal mine used to warn the air is getting bad. if I see the towel or any other white textured highlight clip in camera warning when \"chimping\" it tells me I\'m at the cusp of losing highlight detail in the RAW file. With experience comparing the camera warning to the RAW files its easy to corrolate the two.
A gray card is a process control guide for White Balance. There are many ways to set WB, but the only practical why to measure it in editing is from the RGB values on an object known to be neutral. For example you can use and ExpoDisk to set WB, but there is no way to verify it is actually neutral unless a known neutral gray card is put in a test shot and measured. Also with a gray card in a test shot its possible to \"click to neutral\" on one image with the card then paste the WB settings into other RAW files taken in the same light. Setting Custom WB off the card makes the color neutral on screen when opened but doesn\'t alter the RAW capture, saving the copy / paste WB settings step.
The reason WB better off a gray card rather than a white one are:
1) Gray cards are manufactured to reflect equal RGB in all light. Even the newer Kodak ones (contrary to what some say).
2) With a white card there is a chance overexposing it might clip one or more channels which would skew the WB. Gray falls in the middle of the sensor range so that\'s less likely to happen.
What the process does is ensure all neutral tones are reproduced neutral (i.e. gray balance) and had it called \"gray balance\" instead people wouldn\'t be as confused about it
Chuck
Oct 20, 2009 at 12:17 PM
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