Previous post: Dominance or Emphasis.
Gradation is a gradual change, an increase or decrease in the strength of visual elements such as value (brightness or darkness), color, saturation, clarity, texture or sharpness. Gradation can add a feeling of depth to an image and direct the eye to the subject.
The hills in the foreground are relatively clear. but the clarity for the hills that are further back gradually decreases as the distance increases, due to the misty conditions at the time.
A gradation of changes can create a hierarchy, with a dominant element drawing the eye and appearing most important, with other elements attracting to a lesser and descending amount.
Hierarchy creates a scanning pattern., with the eyes following the dominant feature, to the secondary and then further elements. Gradation can add a feeling of depth to an image and sometimes help direct the eye to the main subject.