When the subject of a picture is on one side of the middle, it must be close to a pivot point. If it departs from the center, it must be balanced by a small weight element on the other side to create a visual balance. If you were to look at an actual scale, this is what the principle would look like visually (see below).
Painting below, “Mr. and Mrs. Andrews” by Thomas Gainsborough, demonstrating the steelyard principle from the book Pictorial Composition: An Introduction by Henry Rankin Poore. Notice how the figures on the left-hand side of the central vertical are balanced by the trees and the haystack on the right-hand side.
In the painting above by Caravaggio, called “Beheading of Saint John the Baptist,” notice how he is using the steelyard principle in his design. If you cover the circled area on the right-hand side with your thumb, you will immediately discover that the composition feels unbalanced.
Painting above, by Henri Fantin-Latour, demonstrating the use of the steelyard principle through the application of value. In the bottom image, notice that when you remove the subtle highlight on the glass, the balance of the composition changes drastically.
I came across the two paintings below in a book on composition in art. The author claims that the paintings by James Whistler are imbalanced because of edge flicker and improper placement of the figure. I strongly disagree with this assessment. When you crop the images in this manner, it destroys the delicate balance of the paintings and ruins the compositions.
Pictorial Composition and the Critical Judgment of Pictures By Henry Rankin Poore
Artists and photographers that are interested in learning more about balance in composition can download a free copy of the book Pictorial Composition and the Critical Judgment of Pictures by clicking here. The material presented in this book, even though it was first published in 1903, is the same information available in the book Pictorial Composition: An Introduction by Henry Rankin Poore.
Photograph above from the book Pictorial Composition and the Critical Judgment of Pictures demonstrating the steelyard principle in perspective