Jan & Lee:
You have a point, and you exhibit a depth of technical thought. I have found that low intensity lighting creates a very contracted zone negative. This is due to the very effect you mentioned. In low illumination, one usually bases his exposure on the shadow, or rather the lowest needed zone (as the shadows are aped to be essentially black). This should cause the higher zones to receive correspondingly more exposure than they would receive under “normal” lighting. I do (did) much night and low light level photography. I even got the title of “unavailable light photographer” at the local art group. I do a lot of “-N” development, but I think my best efforts were produced with split D23 developer. It is possible to expand the gray scale three zones this way if one is careful and applies experimentation.