The density value of Zone V, as with the other middle values, is dictated more by developer choice, agitation sequence, and dilution. I don't consider it a point on the curve that I can precisely control, its density is a function of the combination in general. How the middle values form on the curve is a main reason for testing, IMO; it's simple enough to obtain the speed point and upper limit of negative density that defines "N" development (i.e., a Zone VIII density of 1.3 in my case).
Frankly, if I had to test every film I use, for every possible combination of developer, camera, lens, meter, agitation variations, temperatures, moon cycles, etc, I'd abandon photography altogether.
I'm certainly not knocking anyone who wishes to conduct extensive and technical testing, but, at the end of the day, I really wonder if that translates to more successful images.
Zones are assigned a position on the log exposure scale. Essentially, a zone merely describes a level of log exposure. The short answer, IMO, is yes, zones are defined by a specific amount of log exposure. Log exposure alone, however, can completely define the "x" axis of the H&D curve. With the creation of the ZS, AA & Archer assigned each 0.3 log exposure interval (i.e. a one stop interval) a descriptive term, called a zone to be always designated by Roman Numerals.
Well, now I'm willing to revise my statements. What I'm really after in the end is optimal printing. And optimal prints means knowing your process front and back, particularly how to rate each film and getting to know what that film is going to give you, especially at its limits. I did the bracketing thing once and I could hardly tell the differences frame to frame. I'll probably end up comparing bracketed exposures again, but after I've set the "boundaries" of my film(s), so to speak. I plan on filling a lab notebook very quickly...
My analogy* is that I want to tune my instrument, and that's best done one string at a time. If I try to judge tuning while playing songs - i.e., making pictures - it's a LOT harder to know what you want to fix.
(* note: analogy doesn't work for non-stringed instruments)
Optimal printing starts with great negatives. Good fresh chemicals and a eye for contrast adjustments is good for printing too.
For film, I only use two types of film (tmy2 and fomapan100). Any more than that, and it gets too complicated to be really good at. You too can be average at film-of-the-week. To know what to expect, how a film will work, and the printed image will end up, in different lighting, you need to keep minimalist with regard to film choices.
I also limit my developer choices to max of two. Most people probably only use one developer. I use xtol or PMK right now. Either makes a nice general purpose developer, but PMK is shines for snow scenes or golden hour scenes where the brightness range is extreme. xtol is nice for general purpose, overcast, and artificial lighting where the scene contrast is more normal.
I'm also of the non-laboratory-testing persuasion of just shooting at some different exposures and figuring out what you like best. I'd shoot a scene, not a wall. Something with shadows and highlights, etc... When you get the photo with the best shadows, if it ends up having too much contrast, shorten or cool the development slightly next time.
Well, now I'm willing to revise my statements. What I'm really after in the end is optimal printing. And optimal prints means knowing your process front and back, particularly how to rate each film and getting to know what that film is going to give you, especially at its limits. I did the bracketing thing once and I could hardly tell the differences frame to frame. I'll probably end up comparing bracketed exposures again, but after I've set the "boundaries" of my film(s), so to speak. I plan on filling a lab notebook very quickly...
My analogy* is that I want to tune my instrument, and that's best done one string at a time. If I try to judge tuning while playing songs - i.e., making pictures - it's a LOT harder to know what you want to fix.
(* note: analogy doesn't work for non-stringed instruments)
Here's my slant on the music analogy. When I play stringed instruments with people who are dependent on sheet music, it's stale and lifeless. They get every note just right and it's exactly the same every time around. Yawn. When I play with people who play from the heart and memory, the music is alive, pulsing and vibrant, even though they have less technical mastery.
I dont know if this has anything to do with photography, but it does illustrate my photographic philosophy too. For me the point isn't accuracy, its music, and technical mastery is not necessary. Some minimal technical knowledge is, and the rest (arguably the best) comes from a different part of the human being. But this is your thread not mine so I'll shut up now.