Gray cards : the 12% - 18% controversy

Started Apr 13, 2013 | Questions thread
Roland Karlsson Forum Pro • Posts: 27,206
Re: Gray cards : the 12% - 18% controversy


It was not really meant as a trap. I was really curious what you meant with "the middle". Because, in my opinion there is no middle.

E.g. 12 bit RAW data goes from 0 to 4095, and its linear. As the eye is logarithmic, the best definition of "the middle" might be the geometric mean of 0 and 4095. Unfortunately, the geometric mean of 0 and 4095 is sqrt(0*4095) = 0. Oops! This is of course so as log(0) = -infinity.

Now, there is noise of course. Then, the middle might be the geometric mean of the noise and 4095. But, then the middle is depending on the noise level. Not really well defined.

Moreover, its not really relevant, even if it exists. Cameras have different dynamic range and so have real scenes.

The idea to put the grey of a grey card at 127 (or whatever) in a gamma 1.8 8 bit image has its merits. But only if the camera generates gamma coded 8 bit data, and if the scene has a well controlled lighting of suitable dynamic range. If you have 12 or 14 or whatever bits of linear data, or even better 16 or 20 bit linear data (or 24 bit?) then the whole idea with putting grey cards at certain values is not so interesting any more. The same for low and high DR scenes.

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