Hi, Fahim,
Doug, Bart...
Thank you for the explanation.
So, if I have you and me standing in open sunlit, take an incidence meter reading from either of our face,
plug it into the camera..I should do ok?
Well, one does not take an incident light reading "from" any subject. Recall that it measures the light arriving at a certain place - not the light reflected from some subject (which is what
reflected light metering does)..
Perhaps you mean, "take an incident light reading
at each of our faces".
In that case, if we assume your face has a lower reflectance than mine, yes, both faces would do fine. Mine would have a higher "relative luminance" in the image than yours, but that's probably what we want.
Or if you and me were standing in the same dark and did the same exercise, the exposure would be ok?
Yes (subject to all the words above, and assuming you don't mean "complete dark"). Ideally, my face would be end up with the same relative luminance in the image as before, and the same for yours (a different one from from my face).
With spot I generally point to mid-grey and compensate right or left based on the zone system.
First note that a spot reading taken on a target of a certain reflectance (suppose some known reflectance gray card) is equivalent to an incident light metering. Whether the result would be the same depends on the reflectance of the target. Most incident light meters are calibrated such that their "recommendation" for exposure is the same as we would get with a reflected light spot reading off a target with a reflectance of about 16%.
Now, as to making an adjustment with regard to the zone system, if you judge the gray things you metered on as worthy of being included in different zones, then that's fine.
Do I still do that, I the incidence reading was taken of my face?
Again, note that an incident light reading is not taken "of a face". It is taken of the light arriving at a certain place (perhaps where your face is, of where it will be for the actual shot).
But an exposure taken based on an incident light reading in effect already "plays" the zone system" for you. That is, it fulfills the zone system objective of having the recorded luminance of each object depend (only) on its reflectance (we rarely describe the Zone System that way, but that's what is really going on).
So, assuming you really want your face "darker" in the delivered image than mine, then using incident light metering you would not need to make any different "adjustments" to shoot your face or mine.
Best regards,
Doug