I'm not sure how the incident meter can be used to meter both the high and low values since the diffusing dome is designed to "average" all the light falling onto it when positioned on the side of the subject that is receiving the light. If one intends to meter both the high and low values, then it best be done with a spot meter so that an average of the two can be obtained.
Originally Posted by Paul Howell
Hellen said earlier that too many people have different views of specular light. To those who take AA's zone system seriously (and I am one), then his definition should be accepted and goes like this: "Polished mirrorlike surfaces yield specular reflections where most of the source light is refelcted in a beam...........Since all scintillations are direct reflections of the light source, they are far brighter than a diffuse area, and they can lend a sense of briiliance to a photograph. Unless the specular areas are large, the duffuse and specular reflections combine to produce an average diffuse luminance value read by the meter. It is usually best in practical terms to try to direct the meter at areas that do not contain strong scintillations" (i.e., specular reflections).
So, what I get from that is this: without a doubt, the spot meter makes the best utilization of the ZS (i have no idea about BTZS), and do not concern yourself too much with specular reflections as long as they can be kekpt to a minimum on the negative.