Our own Ken Lee has a couple articles about metering on his Tech page. They may be helpful. Scroll down to the heading - A Simpler Approach to Metering and just below that is The Myth of the 18% Gray Card.
Let me say my view, a grey card is a 18% fact, used as an standard. We know that the spot meter on the card will give the same exposure than an incident meter. If we set the exposure for the grey card then a white subject reflecting 90% will be some 2 1/3 stops over exposed, and black subjects reflecting 2% will be just outside the toe, as defined by ISO. From here we can decide what we do for each particular situation.
Also a grey card is spectrally flat, so it is a good reference for color corrections to restore neutral greys.
So IMHO a 18% grey card is simply a calibrated tool, an amazing one, but it's up to us how we use it.
We could talk about specific circumstances where that's true, but many people find a 1 stop difference between spot meter and incident meter.
I just check it both ways and make an exposure determination based on which type of meter I think was more likely to give me the correct exposure in that situation. (For example for a backlighted scene I would think the exposure recommended by my spot meter is more likely to be correct, in ordinary light, I would lean towards using the recommendation of the incident meter - unless I was making careful use of Zone System).
R-27 Kodak grey card includes precise instructions about how to use the grey card, it has to be angled depending light direction...
I find that the instructions in the R-27 are very well explained, this was really helpful to me.
For caucasian skin there is just one stop difference, but this is very easy to explain.
1) I take indicent metering of lingt illuminating a wall
2) If I take an spot metering... if the wall is white I'll real +2, it is black I'll read -2.
As we normally place white skin in Z-VI we have to overexposr +1 what we read with spot meter in the cheek. If it is black skin we well place the face in Z IV.
The spot metering depends on subjects reflectance, so we'll need to correct exposure depending on the zone we what that spot...
I agree...
IMHO it's just about understanding what we are doing. If we use the incident metering (with uniform light) we know that subjects reflecting 18% like a grey card will be in Z-V, and that white things will be in Z VII.
Instead, if we explore the scene with the spot meter we will know in what zone we will have each spot, for each exposure we consider.
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