18% gray (or 12.5%) are NOT related to "middle" gray of a silver print. (The 18% gray card is an exposure tool, not a printing tool).
We know the log D range of reflected values in a silver print is roughly 2.0 log D (glossy paper approximation).
The half-way point would be 1.0 log D.
The conversion from log D to percent reflectance is:
log D = Log10 (1/percent reflectance)
For an 18% gray card, the log D would be 0.74, which is darker than the middle gray in a silver print on glossy paper.
Last edited by ic-racer; 04-07-2011 at 12:15 PM. Click to view previous post history.