How does the meter reading of the highlights correlate to a time in the developer? I understand that you expose for the shadow and develope for the highlight but how do you calculate a "development time" for a "meter reading"????
This is essentially the "Zone System" see Ansel Adams "The Negative" or a similar book for the long version.
First, you would spot meter the darkest point where you want detail on the negative, let's say the reading is f5.6@100.
That spot reading is not your camera setting, this reading is normally defined as Zone II or III. (Personal preference and metering style is the difference)
The camera setting is actually supposed to be Zone V, which is the middle gray/mid-tone of the photo you are trying to shoot. (Side note, Zone V is what incident meters are designed to read directly).
To find the camera setting, you need to offset the spot meter reading (stop down) roughly 2 or 3-stops. (I say roughly because Zones are not always equal to f-stops, that's why Ansel and his buddies used roman numerals.)
In this example the spot meter reading of f5.6@100 would be adjusted to a camera setting of say f5.6@400 or 800.
The Brightness Range is what determines development.
To find the Brightness Range you now need to spot meter the brightest point where you want detail, this is Zone VIII.
In a normal scene, and only a normal scene, f-stops and Zones are equal.
If your shadow spot reading is F5.6@100 and your highlight spot reading is f5.6@6400, a 7 stop Brightness Range, you would use normal development.
If the high reading was f5.6 at 12500 you may want to shorten your development time. If your high reading was f5.6 at 3200 you may want to increase your development time.
This method works best for sheet film because each sheet can get it's own time based on the high and low readings in a scene.
With roll film, it is tougher. Adjusting development to make one shot better affects all the other shots on the roll and not always the right way.
