As it says there "The term responsivity is also used to summarize input–output relationship in non-electrical systems. For example, a neuroscientist may measure how neurons in the visual pathway respond to light. In this case, responsivity summarizes the change in the neural response per unit signal strength. The responsivity in these applications can have a variety of units. The signal strength typically is controlled by varying either intensity (intensity-response function) or contrast (contrast-response function)."
In the case of film, the output unit would be HD density.
It got more complicated for me when I started developing and printing for myself. I found out that when you used some developers the ASA you needed to set on the meter changed. Then I found out that negative density differences could be changed in enlargement - but you got some tonal changes, some of which could be addressed by using different contrast paper (or, with multi-contrast paper, adjusted quite finely) - and sometimes the effect that you wanted required a different exposure/development/printing combination. And, how you developed the film changed the grain, strange that.
Bob, my friend, person I admire…
A Marshall more famous than I once wrote, “The medium is the message.”
I don’t fully understand that, but I do understand that the presentation and delivery of the message matters. It’s not enough to be right. Look at the recent pandemic: Public health officials had to desperately make sure they were clearly conveying the message. Being right was not enough. (It’s an analogy; please, let’s not discuss Covid).
The Northrups present very well. Sometimes they even present useful and accurate information. Presentation matters. Being clear and understandable matters. It’s why YouTubers buy expensive lighting and cameras and do makeup.
I think we have developed very strong consensus around the art of exposure, although we are still struggling to provide an approachable and concise summary of it all. We will, and then we should move on to other topics such as lighting, composition and so forth.
Your mind is different from mine; when I take photos, I think about DoF and my focus point but I don’t do the math; I approximate. I simply can’t keep the math in my mind. Maybe you do, which is remarkable, but I think 99% of people cannot.
All I remember was 400 ASA wouldn't get a fast enough shutter speed to do indoor basketball for the college paper in 1970.
We would set 1600 ASA and then leave it in the tank much longer.
That wasn't post processing, it was development. And it changed the ASA.
We get that, but, again, BEGINNERS… they just bought the camera last week and want to take shots of their kids playing softball and they need to get started somewhere.
I have already argued for becoming a skilled photographer with your phone and then, maybe, buying an interchangeable lens camera -- and I'd recommend a MILC. And I've said that, once you do that, you should start out in M mode. Ignore the exposure meter; you've got the preview in the EVF.
However, using Northrups' example, it is better to have a less good presentation and be right than to have a good representation and be wrong. Albeit, most people want entertainment, not learning.
We all often use the experience to set the camera for desired DOF and focus point. However, it is important to periodically reevaluate what we have been doing for a long time and maybe change it if it helps our photography.
And start with practice, not an actual match: that is take an hour or so changing one thing at a time, don't be afraid to get lousy shots, but try to understand what causes them.
There is no "the math" involved in actually deciding on the settings, and you should include Time her as well as Aperture.
There is some math, or rather physics, in explaining what a setting does and why it works or not. Somehow this generates a lot of yesbuts and whatabouts.