I like doing value studies. When I'm out in the field, I always do at least one thumbnail sketch in my sketchbook using a ball point pen. It's a great way to visually organize the value structure and think about what's important, and what should be edited out.
If I'm in the studio, I still do these pen-and-sketchbook thumbnails, but sometimes I like to do them with paint. I have a lot of scrap pieces of canvas which are perfect for these little studies.
The idea is not to copy the photograph, but to figure out how to simplify the value structure and tweak if necessary to come up with a design which, with very small amount of information, communicates the sense of time and place in the original photo.
The main challenge in doing these is to reduce the number of values to three or four. Dark, medium dark, medium light, and light. Sometimes a simple composition only needs three values, sometimes as many as five. But no more than five. If I need more than five, I probably won't develop it into a full color painting because it would be too fussy and will lack a clear statement or impact.
You may see more than four or five values in my value studies, but when I start out, I only have three values. I may end up with a few more due to softening of edges or transitioning one shape into the next, and these "in-between" values are ...not exactly incidental... but just minor enhancements to make a design make visual sense. If you were to squint down, they simplify back (or they should, anyway) to four values.
In the above example, you can see that the shadows on the ground are much lighter in my sketch than in the photo. This is a decision I made consciously, to make good use of the four values I had at my disposal; since I had to use my darkest value for the tree silhouettes and the windows in the doorways, I wanted to distinguish the ground shadows (and the same shadows creeping up the side of the building) by using a lighter value. A couple of benefits in making this decision - the shadow areas are much more luminous, which is fitting because we are outside where a lot of ambient and bounced light make everything lighter. (the contrast in the photo was looking too much) Secondly, it allowed me to separate the paved walkway from the grass (?) area flanking it.
So the point, again, is not to copy the photo, but simplifying and organizing the value structure - how best to convey a given light/shadow situation with just four values?
While doing these studies, I often have ideas about editing that I hadn't considered before starting the study. And this is an excellent place to try these ideas out, because well, these are just quick small studies–most are 5 x 7 or 6 x 8 ish and only take 10 to 20 minutes. If I try out an edit and it doesn't work out? Big deal. I'll do another study or keep working on it. I don't feel like I've wasted hours or days of my life trying things out (which may be the case if I were to be making these decisions on a big, full color painting).
There's no need for detail, or subtle modeling. It's really easy to see what's going on in the study, even though it's very simple and crude. What does that tell us? We really don't need much to make a scene "believable". So why do we feel the need to put more stuff in a painting? Hmmm. something to think about, eh?
Painting, especially painting en plein air, is sometimes intimidating and frustrating. Often we may talk ourselves out of trying to paint a certain scene because we feel like it's too difficult. Or we have a hard time seeing anything worth painting, even though we are surrounded by a ton of potential paintings.
I find that these quick value studies are incredibly helpful in getting over that fear and tackle a task that seemed at first to be too difficult. The reason is that these are first and foremost, exercises in simplifying the problem. If we can simplify the problem, we can simplify the answer, no?
If you can get into a habit of doing these quick value studies in limited values, you have a great advantage over someone who's trying to paint every little thing in view. Simplifying forces you to consider what's important and what can be ignored. It forces you to make a clear statement, and it forces you to think about hierarchy of impact in your composition. Without the awareness of which, you really don't have a statement. And without a statement, what is the point of painting this scene?
This last pair shows a value study, and a color one, both done from observation. Thanks to the simplified value study, I was able to maintain a simple structure in the color one, and not get caught up in the little details in the background - and there were a lot of visual activity there which I didn't bother to include. The value study told me it was OK not to paint them.
I encourage you to do these kinds of studies when you don't have a whole lot of time to devote to bigger painting projects. They only take 10 to 20 minutes, and you only need black and white paints, and scraps of canvas or some other surface. Don't think of them as little masterpieces. Don't even think of them as something to keep. Crappy composition? So what. Do another one and make improvements. A lot can be learned by doing these studies using snapshots as references - photos that were never particularly well composed and were never meant to be made into frame-worthy paintings. They're just studies and practices, much like a musician practices his scales.
My musician friends tell me it's boring but necessary. Here we artists are better off; these are not boring to do! Far from it. I really enjoy doing these, and you will too. Don't believe me? Just try it.