Key is the range of values in a drawing, and manipulating those values is a little like playing music. If you play the same song in a higher or lower octave, it’s still the same song even though it sounds different. With a drawing, changing the key lightens or darkens its values, but underneath it’s still structurally the same scene.
Here is an example of altered key. The photo on the left is unaltered, and I darkened the lower end key of the one on the right. The altered photo has more depth, and the bird stands out more.
Altering the key lets you play with mood, focus, and composition. It also allows you to draw more sculpturally and differentiate color in gray scale.
Mood
Key expresses lighting, and lighting gives us cues about the scene in many subtle ways. Light tells us what time of day it is, or what time of year. Time and season are evocative.
Alter the key slightly to express a different time sense, and the mood changes.
Change the over all key to inject emotion into a piece by indicating “lightness” or “darkness” of mood.
Focus
Shift the focus by making one area lighter or darker than the rest of the drawing.
Composition
Modify the “weight” of light and dark values to help balance the composition.
Pushing and Pulling
Extend both extremes of the value scale, and you’ll see a more three dimensional effect in the drawing. At the drawing club, we call this “pushing the shadows” and “pulling the highlights.”
Defining Color in Grey Scale
Alter one part of the drawing to differentiate between two colors that are nearly the same value. Red and green are notorious for looking different in color but the same in gray scale. (See this post.)
Cameras See Differently Than We Do
Cameras can’t record all the values our eyes can see, especially at both ends of the scale. They often can’t pick up the detail in the shadow areas and make them too dark, and they often obliterate details in highlights by making them too bright.
Lighten the key of shadows and darken the key of highlights to correct for your camera’s poor eyesight.
Don’t think about it too much.
Listen to your inner voice as you work. Be sensitive of the need to lighten or darken, shift the focus, or add more detail. That inner sense always knows how to take the next step needed.