Value isn't what you thought you got on Black Friday. For a painter it is the spectrum from white to black. Many students I encounter do not express the full range of values in the motif before them. Many times I see a set of mid values. Usually, this is an inability to see the values and/or translate them onto the canvas (colour really confuses this issue). There is often a good deal of confusion acquired elsewhere. In the concept of objective painting where one is trying to paint what you see, this value scale is very important. This type of painting is a good foundation for other concepts. This is because value supplies a framework that holds the painting together. Here you recognize and use the full range of values present.
The values available to be seen change. For example, in bright sunlight one often witnesses the full range. Shadows are crisp and dark, the sky is the light and bright source of light and the motif might show both hi lites and accents.
Bright Sun
In an overcast situation the values tend to be close together, shadows non existent, and no hi lites and accents exist. The value range may be quite limited.
Semi Bright Overcast
These two lighting conditions lead to two separate concepts of painting often confusing the painter or forcing the comment "no sun so I can't paint". These are light and shadow, and local tone painting. Rembrandt was a light and shadow painter. Van Gogh was a local tone painter in his final years. The Impressionists on the other hand developed the concept of compressing the value scale used (to go hi key ignoring the total value scale witnessed) and to emphasize colour. However, in order to do these things well one must be able to detect and paint what you see (level 1 painting).
Value and Shape Thumbnail
My thumbnail for what I saw on an overcast day. You will note that the recognized values are noted for each shape. The next step is transferring these to the canvas. That is the starting point for painting the concept.