• Andreas, what you’re wanting to do is compare two different lengths of measurement that start at the same point and want to end at the same point. If we lay both lengths down with one end of each flush at the same point, one of the lengths will be 1.20 – 1.00 = 0.20 Δ log-H longer than the other. If one end of each measurement begins at the metered exposure point on the film’s characteristic curve, an exposure of 8 / film speed, the end of the shorter, 1.0 Δ log-H, length measurement will fall at 0.8 / film speed where the film density is 0.10 over film base plus fog. The end of the other length of measurement, 1.20 Δ log-H, will be 0.20 Δ log-H further down.

Under these conditions, the 1.0 length measurement will fall at a relative Zone I 2/3, but the problem is Zone I needs to fall at 0.10 over Fb+f. So exposure needs to be increased by 2/3 of a stop. The 1.20 length is moved to the right so that the ends of both measurement lengths are at the same point, but now the starting ends fall at different points. A variable has changed with the 1.20 Δ log-H length of measurement, but not with the 1.0 Δ log-H length of measurement. It’s not possible to make a direct comparison under these conditions.

I don't know if this is going to help or not. It's a breakdown of exposure for an average scene luminance range for a film speed of 125 without flare. The Zone indications are on the left. The metered exposure is on step 10 at 0.064 lxs. The speed point is Δ1.0 log-H further down on step 20 at 0.0064 lxs. Zone I falls at step 22 or 2/3rd of a stop below speed point.

Exposure for 125 speed film.jpg