Still muddling through Dunn & Wakefield's Exposure Manual.
The following are my impressions and extrapolations. It appears to me that there are three basic ways to peg an exposure to a film curve and by extension to a print; shadow point, main light key tone (highlight), and middle tone pegging.
The shadow point method (normally measured via spot metering) appears to be a pure attempt to maximize the quality of the print by using a minimum but safe amount of exposure on film and quite regularly zone system ideas.
There is debate on how much buffer is important here to maintain a safe/workable exposure. This seems to me mostly a personal EI decision, a matter of calibrating the system to fit our own needs. This may be one reason we get such widely varying views on what EI is best for any given film.
This method in general protects shadow detail nicely, minimizes grain, keeps shutter speed faster, ..., and when combined with a contrast adjustment to film development or paper grade also protects highlight detail and with experience makes for easy printing.
On the downside, mid-tones though will typically "fall where they may" because both ends of the scale that surround them are fixed in separate decisions. Unless burning and dodging are also employed, the brightness of faces for example, may vary considerably from what we might prefer.
For landscape shots this method makes great sense, for portraits and other subjects, not so much, at least for me.
The highlight key tone method picks the main light as the basis for exposure; typically a white card, a gray card, a piece of paper, the palm of a hand, anything that has a known off-set from white is measured in full frontal light. Flat face incident meter readings taken when pointed at the main source light provide essentially the same setting for the camera.
This is a measurement of incident light without regard to the camera's point of view.
This method has the advantages of placing subjects very specifically on the curve and of protecting highlight detail for the final print.
What struck me in Dunn & Wakefield was where this highlight pegging was important, color and movie work.
And why, because the highlights are what catches our attention first and because of the importance of the human face and with movies the speed at which each frame is viewed, there is simply not enough time to explore the shadows and faces constantly changing brightness is disconcerting.
I personally made a leap of logic here in relation to a series or tryptic that is meant to be hung/seen together. Continuity becomes quite important.
Shadow point exposure settings will render faces/scenes at very different brightness levels from frame to frame as the camera moves from full front lighting through random points in the gamut of possibilities to full back lighting. These images may stand very nicely alone but may clash when hung together.
For this key tone/main light method Dunn & Wakefield essentially postulate that the typical center of interest normally resides at the higher end of the scale so when a key tone is fixed from frame to frame it is typically only the expendable (my word) shadow detail that is lost.
Since my favorite subjects are people, this method struck a cord as I was reading.
This key tone method also, for me, has the huge advantage of "set it and forget it" for a given situation. Subjects simply fall normally in relation to the main light, whatever that may be. Those in full sun look like they are in full sun, those in open shade look like they are in open shade, etcetera...
The very real risk with this method is in the underexposure of something important.
This underexposure risk is another driver that I can see for people making personal EI choices especially with high latitude negative films for shooting at 1/2 box speed.
This is also very much the situation toy & disposable camera shooters find themselves in. Essentially toy camera shooters choose the film/camera/process based on the main light we expect and we let everything else just fall where it may.
This method is easy for camera work but may require some extra effort at the enlarger.
The third method is based on mid-tone pegging. The middle way as Dunn & Wakefield call it.
It is a compensated key tone method that adds consideration for the view the camera has.
Readings for this method are normally made; using a flat face incident meter by duplexing, measuring pointed directly at the main source and measuring pointing toward the camera then finding the logarithmic average, or by using a single hemispherical incident meter reading.
This method gives precedence to the middle tones, "excess" low and high tones are simply left to fall where they may, even if that means they are lost.
This method of metering caught my attention because a) most of my important subjects are in the middle, and b) the decisions about where I want to place my main subject (compensation) are easy (normal, light, or dark), c) it is how I learned incident metering. I didn't really understand the difference between main light pegging/incident metering and compensated incident metering until studying this book.
This compensated method has the advantage of placing the main (key tone) subject very specifically and little risk of underexposure.
It also has, the very minor in my mind, disadvantage/necessity of having to set exposure for every shot and the slight risk of losing detail at both ends in a high contrast situation.
The extra effort at the camera is generally rewarded at the enlarger, prints from this compensated method should, if we use the technique consistently, require very little adjustment from the "normal" settings we might use. Theoretically we should be able to use standardized enlarger settings and get very predictable results.