Types of Light and Value Planes by Don Finkeldei: There are  4 value planes and 5 types of light that you need to deal with as a minimum in a landscape painting .  These are broad generalities - not an absolute rule.  Understanding the types of planes, different kinds of light sources, direction of the light source and how each type of plane is influenced by the light and how they relate to each other will help you create a more believable painting.

THE 4 TYPES OF VALUE PLANES
As a general guideline, all landscape paintings should follow these rules.  Of course, there are instances where you can, and should, break them…. But make sure there is a logical physical reason and support for breaking them.

Note:  I must give credit to John Carlson since I think he first explained the relationships of  value planes and  types of light.  Also I have to give credit to Clyde Aspevig, Scott Christensen, and the many other artists who gleaned  the teachings of Carlson and promoted them.

1. SKY: The lightest value plane in a painting.  Overall, the average value of the sky will be lighter than any other portion of your painting.
   
   
2. GROUND PLANES: This is the second lightest plane.   It’s on average lighter than the upright planes (for instance, trees and buildings.  The reason the horizontal ground plane is the second lightest plane is because it receives more light from the overhead sky per unit area than the other types of planes.  It makes perfect  sense if you think about it.
   
   
3. SLANTED PLANES: Since a slanted plane (slope of a hill or side of a mountain) doesn’t generally receive as much light per unit area from the sky it's generally lower in value than the horizontal ground plane.
   
   
4. UPRIGHTS: Uprights can be anything from trees to buildings.  Anything with vertical surfaces.  These are the darkest planes (lowest in value) because uprights recieve less light than any other type of plane from the overhead sky, and, in the case of trees, there’s a lot of little pockets of darkness (shadow under things).

THE 5 TYPES OF LIGHT
To create a believable landscape painting you should keep in mind the type or types of light affecting the object or area that you are painting.  Not only should you keep the types of light in mind, but also the value planes discussed above and the relative lightness/darkness scales you have to work with on each value plane.  All objects need at least three types of light modeling to make them look real (light, shadow and midtone).

1.  LIGHT: This is the sunny side of an object, or the side that gets the most light.  A tree may have a light side (the side or area where the sun is shining on it).
   
   
2. SHADOW: Obviously, this is where the light doesn’t shine -- the shadow side of an object.  This area is darker in value than the light, as you would expect.
   
   
3. MIDTONE: A lot of people forget about the mid tones, half way in value between the light side and the shadow side.  There are always mid tones in curved rounded objects.  Even a tree must have mid tones.  It’s the transition area between the light side and a shadow side.
   
   
4. ACCENT: An Accent is the darkest or lightest area in an object (which contains light, mid tone and shadow areas).  Never overdo accents.  Take a tree for example:  in addition to the light, mid tone and shadow, there may be a few accents, the very dark tree trunk, or the light side of a white aspen trunk or tree limb.  Just a few, don’t overdo them.  I generally wait until the object is modeled before adding the lightest accents.  I always put in the darkest accent before modeling the light/midtone/shadow.
   
   
5. REFLECTED LIGHT: Reflected light is most commonly overlooked and misunderstood.  An object’s intrinsic color and value is affected by light reflecting from other objects near to it.  You should always keep in mind the objects near what you are painting and consider whether the near object(s) might cast reflected light upon the object you’re painting.  For instance, let’s take an example of a red barn and a green shed next to each other.  The sun is to the right.  The green shed is to the left of the red barn.  The red barn’s shadow side is facing the green shed’s sunny side.  The green shed will reflect into the barn’s shadow enough to change the color of the red barn’s shadow to a greenish tint.    Another instance where you should consider reflected light is at the base of buildings (where the ground plane may be reflecting on the building near the ground).  If the ground plane is grass green, bring some of that green into the color of the side of the building near the ground --- and even some up higher.  I know you’ve seen paintings where the building seems to be floating… and not realistically founded on the ground.  Reflected light is the reason (and softer edges)… there’s generally always a smoother transition of color and value between the ground plane and the building. 

Keeping in mind both the type of value plane and the type(s) of light affecting an object will improve your painting.  For instance, the light side of a tree (an upright) should be darker than the light area of the ground plane.  The Shadow side of a tree should be darker than the ground plane in shadow.

As stated earlier, there are instances when you can and should break these rules.  Here’s a situation where you can and should:  If the sun is on the horizon, an upright with a side facing the sun would be higher in value than the ground plane.  The upright that is facing the sun  is reflecting more light than the ground plane would (because the angle of light on the ground plane is very low).