NB: My updated lecture notes on tone (i.e. lightness) are here: Tone_s.pdf (right click download)
Lightness (sometimes called tone) is the master value of an image: more important to human perception than hue. It governs our ability to see shapes and forms.
Three value lightness structure
It helps to understand the lightness structure of an image if we simplify the image and separate the lightness values into three bands: light, dark and middle.
Using this understanding, this painting by Rembrandt can be broken down into these three regions:
It is interesting that these three regions maintain three completely different geometries. In this way, lightness can be understood as constituting the structural skeleton of a painting.
Five value lightness structure
In reality, a three lightness separation undersells the real complexity of a well constructed image. To a painter a more usable simplification can be had from a five value separation, with the black and white extremes as thin regions of separate values.
These serve the highlights and occluded shadows as separate values.
Most paintings have a tonal range that extends from black to white. However, some don't. Observe that in this painting by Gwen John (below) the range of light values extends from dark gray to light gray. However, this is the exception, not the rule, and most paintings contain at least one point of black and one point of white. It must also be remembered that three tone structure does not necessarily mean that the middle tone be mid grey. Light, middle and dark are entirely relative within the image. The middle value of a painting can as easily be light grey as dark grey.
Also... the light, middle and dark does not necessarily equate to white, mid grey and light. These three values are perceptually defined and correspond to the perceived extremes and average of the painting. In the two paintings below by Manet, the lightness values, though extending across a similar range, are distributed in different ways (as evident in their histograms.
Edge softness values
The edges between regions of different lightness values should also be considered. By blending the five tones we are given the intermediary tones. A consideration of these values is vital if we are to get shadows to work, especially when we need to paint Shadows, particularly in differentiating between cast shadows and form shadows.
Infinite value range with glazes
Consider the ball on the left below. It was made using a simple five value lightness structure (three base plus two extremes). In order to push the far ball into the distance a further layer of lightness values is needed. This can be achieved by apply the image in the middle as a multiply blend mode to achive the nuanced result on the right. Glazing offers an easy (ish) way to nest multiple levels of lightness values inside of each other.
The old masters used a technique identical in principle called glazing. Here, on a painting by Titian (who worked at a time when glazing was not widely used) I have painted a set of glazing values to achieve a more modeled look. This includes fluffy, cast shadow (of the front figure onto the back figures) which is very difficult to get any other way.