People talk a lot about values as if they there is a correct value for light or dark in a subject. I don’t really think that there is though. Very little is said about Key or Value Scale which is I think of equal importance.
Key for those that haven’t met the idea is the range from light to dark in a painting. So a painting that has most of it’s values in the lighter range is called high keyed. Setting the key of a picture is a choice made by the artist rather like setting the exposure on a camera.
If you decide the lightest value in your picture and the darkest value then it is the relative positions of the values in your picture that matter. You can choose a narrow key in which case the lightest is pale grey and the darkest mid grey, or a wide key in which the lightest is white and the darkest black. The same picture can be painted in either key, neither is more correct or incorrect than the other. It is the relationship between tones that matters and gives coherence and believability to your picture. I have briefly I think dealt with this before but this I hope is clearer. I’m going to use a plein air of Rotherhithe which lends itself to the subject.
First The picture as I painted it.
At the bottom are two value scales. The bottom one the full range of possible values the top the ones actually used. Below is a diagram to show the spread of values.
As you see the lightest possible lights and the darkest possible darks are not used in the painting. The space between the lines is the Value Range or Key. The top strip is just the area between the lines stretched to fit.
Here is the same image in grey scale. So how would the painting look if I had used the full range? Below I have done a photoshop adjustment that gives some idea.
Here it is. As you can see the picture is more contrasty and the two range strips more or less match. It still looks fine because all of the values have been stretched to fit and the relative positions are unchanged. This is rather using an elastic ruler. You are stretching the whole thing but it still has twelve divisions equally spaced. Here it is in grey scale.
As you see this looks fine and probably about how I would choose to draw the scene in black and white. See that the two strips now more or less match. So what happens if we go the other way? This time we will narrow the value range rather than expanding it.
It takes a moment to adjust your perception so look at the image above in isolation for a few moments. The mood has changed but the picture still works we have no difficulty in believing such a day and mood of lighting is possible. Below is the same thing in grey scale.
As you can see it has rather the feel of a foggy day, but is still perfectly plausible. In fact faced with this subject you could have painted in any of the value ranges above and it would have been perfectly valid.
So remember, when you start a painting first make a decision as to the Key. Put dabs on your canvas of the lightest and the darkest you intend to use and then stick to that range of values and place your other tones between those extremes. It is your decision as the painter not necessarily set by the subject, though some subjects obviously lend themselves to a particular range. On advantage of this approach is that if, when your painting is almost done, you need to punch an area up you will have the means rather than being stuck in the position of needing a white that is whiter than white!