Henry outlines an in-camera test very similar to Adams. A few things puzzle me, but I will limit this to the x-axis of the curve plot. He retains the use of relative log H. Ok. But I don't get how you can start the scale at 0.0 where the two axes cross in a test like this.

Henry begins with a metered exposure of the grey card and places it on Zone V (arbitrary since it is not a Zone System test per se). He then recommends stopping down (or reducing shutter speed) 6 or 7 stops to start the series of exposures so that you ensure you will start below a net D of 0.1, and of course also include a frame shot with the lens cap. This is all fairly straight forward. But if you call the lens cap exposure 0.0 on the x-axis, which exposure is 0.3? 0.3 relative to what? The first non-zero exposure could also be any other log number relative to the lens cap exposure. I don't get how you can do this unless you are somehow able to determine one of your first non-zero exposures happens to be precisely the threshold exposure for the film/processing combination.

To me, in an in-camera test, once you base the series of exposures on a meter reading, the only reference point (ie 0.0 on the relative log H scale) is the middle grey exposure. You'd then label the x-axis with negative relative log H values below the metered exposure, and positive relative log H values above the metered exposure.

I think I'm missing something here.

Thoughts?