I was reading yet another article on how to calibrate for the Zone system, how to ensure that a half-step of exposure on paper would correspond to a full Zone change, yadda yadda.... When suddenly it dawned on me: What's the unit for the tonal scale? It's all fine and dandy to want to standardize, but perhaps we should start by having a way to mean the same things when we say grey, light grey, dark grey, etc? Between what our eyes accept as full black and full white, how do we split the interval so that tones appear more or less equidistant? Let's say we take sound for comparison. Between 440Hz and 880Hz I have a full octave. I could play whatever frequency if I want (that's what synthesizers are good for), but I limit myself to a particular set of frequencies. That's a scale. On a scale, each note is separated from each other by a given criteria: ratio of pitch, multiple from a given pitch, etc. As far as I understand musical theory, that's called temperament. It's what ensures aural distinguishability between notes. Now, if we translate that to the visual arts: on a sheet of white paper, I can dilute India ink with water to obtain a continuum of greys between black and white. Perhaps it would be useful to me, as a painter, not to have an infinity of steps between black and white, but rather a more handy set of grey tones. Let's say I want ten, labeled G1, G2, ... G10. How will I ensure that they are sufficiently distinct from each other? What measuring tool can I use to measure grey (the equivalent of frequency in Hz for sound), and what intervals between these measures should I respect so that these greys are perceptually "equidistant" from each others (the equivalent of temperament) ? I know that's what Munsell and others have done on color system, but I'm not interested in the whole chromatic wheel: I'm just interested in the ten steps it employs to represent value. How can I measure them practically? The logic behind all that abstract thinking is simply this: once a good grey scale is established, one works backwards to figure out the technical means to achieve it (paper development, film development, film exposure, in that order) and to move and down (what technical adjustments are needed to go from G1 to G2). But the fundamtental question is: How to establish a good grey scale in the first place?