First off, let me say that I'm not a disciple of the zone system. I'll go along with the underlying philosophy of pre-visualisation, but IMHO there are more int uitive ways of getting there. I favour incident light measurement, coupled with compensation for the specific subject to be photographed, or effect to be acheiv ed; so my question is really aimed at devoted users of the zone system.
The reference point in the zone system (and for the calibration of exposure mete rs for that matter) is stated as being "mid grey", zone 5, or 18% reflectance. N ow 18% reflectance is only two-and-a-half stops down from a theoretical 100% ref lecting diffuse surface, which one would expect to be rendered very near flat wh ite in a print. A whitewashed wall comes close, for instance, and this isn't eve n Zone 8, so where do the higher zones fall? It seems to me that zones higher th an 8 are only of relevance if a source of illumination or specular reflection is included in the picture, and we are willing to let objects which should normall y appear near-white in the print slip down the scale to grey.
These days, you can't even expect the film to compress the highlights by flatten ing off toward maximum density. If you look at a film like T-Max, or even Plus-X developed in T-Max developer, the density curve just keeps going up and up in a straight line, putting the densest highlights well beyond the range of the toe of the paper curve. Remembering that the zone system was thought up when a typic al film (or plate!) had an "S" shaped curve.
The conventional zone system answer to this is to add even more zones beyond 10 to accomodate the wider density range, but how are these extra zones to be accom modated in the print? Reduced development and contrast will simply make our prin ts look flat. The only answer that I can see in reality is to burn the highlight s and/or dodge the shadows. What do you do with a contact print?
How many users of the zone system regularly make use of zones more than plus or minus one stop from the "norm"? I'll confess that most of the time I end up exp osing within a stop of what the incident light meter recommends. How about you zone system users? I'd be interested to know how much the "expert" exposure devi ates from the idiot-guide on the leaflet included with the film (i.e. June to Se ptember, cloudy-bright - 1/125th at f/8).