Often I have seen students work very diligently on a “tonal” subject—only to have an arbitrarily added background or surround adversely affects the vast majority of value relationships throughout. Well—before you throw any ol’ value around your subject you should consider two related perceptual effects that could significantly affect everything that you have done.
In the 1960s, Hiroshi Takasaki from the University of Shizuoka, Japan and the Carl C. Semmelroth from the National Institute of Standards and Technology discovered and studied a lightness contrast effect that they dubbed “crispening.” This effect was described as an enhanced perceptual difference between samples as their lightness approached that of their background. In other words, perceived contrast between two similar values may appear greater if they share a background that is also similar. Let’s look at the graphic to better understand this: If you look at the numbers placed over the 3 strips of value—you will notice that the numbers that near the missing number (the number that matches its background) seem to have a greater disparity (perceived higher contrast) than those that are more distant. For example, the values of the numbers 7, 8, and 9 against the black surround here seem to differ more in value than the 7, 8, and 9 against the middle gray or white.
Later (around 1967), C. J. Bartleson and E. J. Breneman of the Eastman Kodak Company Research Laboratories also documented an effect involving perceived contrast variation observed among areas of different value within an image when the image is viewed against a light or dark valued background. As you might suspect, they dubbed this phenomenon, the Bartleson-Breneman Effect (B-B).
Generally speaking, as a subject’s background becomes darker, the values of that subject appear lighter, darker values are “compressed,” and the grays appear more similar (less contrast). When a background skews lighter, values may appear darker, darker values still tend to compress, and grays may appear to be less similar (more contrast). If you look at the grid of value patches at the center of the graphic, you will notice how the values and their relationships appear different (as described) as the background changes.
So to recap: Crispening refers to our tendency to perceive greater contrasts between similar values that share a surround (background) that is also similar while B-B refers to how a dark background may make values appear lighter with grey steps smaller while a light background may make values appear darker with greys appearing further apart. Keep these things in mind when choosing those values (or colors) that will surround those subjects that you have been working so hard on.