Quote Originally Posted by ic-racer View Post
Simplest reason it that 18% reflectance just is not the middle value of a glossy silver print.

More detailed explaination is that each type of paper has a different maximum black (remember all the threads on Dmax ). The D-max, then determines what the D-middle (D-Middle = 1/2 D-Max) is going to be. It is not one universal value and is highly dependent on the paper.

The "Zone" followers are always posting transmission log D values of their negatiaves and assigning them zones, but they almost never divide up the paper reflection densities into the appropriate zones. I don't know why they leave this step out. But if they did they would find that the middle is about 36% for paper with a D-max of 2.0.

The 18% card would match the middle value of a paper with a D-max of 1.48.

Again, the 18% card is an exposure tool for times when you need an approximate incident reading and have only a reflected meter.
I don't think that D-max/min actually matters in my argument.

I see using the gray card in a reference shot as much more than an approximation. In fact it calibrates the print to the scene. It factors out exposure errors and even differences in film or development choices.

Let's say I've done my paper testing and I can program my EM-10 or color analyzer to reproduce middle gray (the Kodak card tone) reliably on whatever given paper is in the enlarger, and I set the enlarger properly. I should get really, really close to a "real world" match every time.

My argument does assume real rather than relative placement and that shadows and highlights are simply allowed to fall off the paper where they may in a straight print.