most sources I´ve read divide light and shadow into 3 separate areas, those being - light, halftone and shadow. In my opinion this isn't clear enough and could lead to confusion. It is better in my mind to divide it into 3 groups consisting of 5 values altogether. They are as follows:
Group 1. highlights
Group 2. Areas in light
Group 3. Areas in shadow
above are some examples of the tones, the third one is white and the forth one is black, notice how the shadow tone of the white is only a 5.5 value
Group one is the spectacular highlights and light sources ONLY, this never includes light tones such as the light areas on a dull object such as cloth. group 1 is the only group when any of values of any object can approach white, this applies no matter what the local value of the object is. Spectacular highlights and light sources have to be at least 1 value lighter than the areas in the light and will almost never drop below a 4 value in brightness.
Group 2 consists of 2 closely bunched tones the lightest one being the highlight or full light areas of an object and the slightly darker halftone value of the object. It will never be lighter in value than a 2 value on the scale (allowing room for spectular highlights) but surprisingly it can go as dark as a 7.5 on the value scale for very dark objects. so a black object in full light could be a 7.5 value on the scale whereas a white object in that same light would be a 2 value.
In group 3 we have shadow areas and dark accents. Shadow areas are any area no receiving light from the main light source (contrary to what many people say they are certainly not areas that receive no light) this includes refleced light. A note on reflected light here, there is rarely a change in value in the reflected light( a percievable one at any rate) reflected light should be shown using only color temperature. Dark accets are the areas that recieve very little of the main light source or the reflected light. Such areas include the dark area where a object touches another, cracks and other small areas. accents ae small, reflected light tends to get everywhere, making the accents to big or using to many will destroy the picture.