In many discussions about zone system that I have seen (including this thread), it seems that the starting point is to meter the important shadow and reduce exposure by 2 stops. So this implicitly indicates that one is using a reflective meter on camera. A handheld meter only tells the exposure of 18% gray. If you put a handheld meter in front of an important shadow area and take the reading, you cannot simply reduce the exposure by 2 stops. Imagine that we have a scene that includes a building under sunlight. The shadow of the building include some light, grey, and dark objects. If one takes a reading of the shadow with a handheld meter, and reduces the exposure by 2 stops, it means that the grey objects in shadow are put in zone III, and any objects darker than grey are lost. But if one uses a reflective meter and takes a reading of the dark objects in the shadow, that becomes a different story. I guess my questions is how much shadow details does one want to preserve then.
Unless I misunderstand you, it sounds to me like your hand-held meter is an incident light meter. Not all hand-held meters are incident light meters - they also can be reflected light meters which is what we're talking about here. Ideally a spot meter is best for the Zone system as you can't always walk up to your shadowed object to meter it - for example a landscape scene.
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