This is kind of interesting. I've put together three exposure models comparing the subject reflection densities and Reflectances for Zone V = 18%, Zone V = 12%, and the standard model of exposure. As anyone familiar with the Zone System knows, Zone VIII falls three stops above the metered exposure point (Zone V). Zone I falls four stops under the metered exposure point for a total subject luminance range of 7 stops (2.10 logs). The standard model of exposure is slightly different as it doesn't base exposure from the metered exposure point but from 100% Reflectance with a 7 1/3 stop (2.20 logs) subject luminance range. If Zone V equals 18% Reflectance, what would Zone VIII equal? How about when Zone V equals 12%? And there are other implications depending on the model used. A little fun and a little food for thought.