Originally Posted by Kirk Keyes
A couple of things, first a difference in reflected measurements of 3 or 4 stops is very different from measurements of Illuminance. If you have a shadow, however light and a bright "side" you can use the incident system as explained by Davis. OTOH like any metering system it does requires some experience and the situation you describe is a perfect example. There are times when an SBR of 5 just simply wont do, you have to use an SBR of 4 or even 3 and this can only be determined by experience.
All the exposures in the BTZS are based on the shadow measurement, so in the case of a SBR of 3 or 4 you would still use the meter setting given to you by the shadow reading. Supposedly you would have already made your film tests and know how you should adjust the EI for extreme expansion.