Design Elements need contrast
Visual compositions are, by definition, arrangements of one or more elements or forms. This definition, therefore, presupposes that those elements or objects are somehow distinct from one another: e.g. a series of lines, a combination of text and image, a diagram of the inner ear, or a movie poster with the title of the film and the different characters in that film, etc.
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But Not too much contrast...
It's true that contrast is critical, but a designer should try to obtain just the right amount of contrast. In his excellent book Visual Explanations (1997), statistician and data designer Edward Tufte discusses the contrast between design elements as more of a design strategy than a principle – what he refers to as the smallest effective difference – wherein one endeavor to, "make all visual distinctions as subtle as possible, but still clear and effective (73)."
He then presents a simple example – two detailed diagrams of the inner ear:
On the left is the original, with pointers as thick or thicker than the linework of the item being referenced, and uppercase letters at the end of these lines that refer to some lookup table. This requires the viewer to a.)tease out where the pointer lines end and the thing pointed to begins (quite difficult down there in the vestibule and semicircular canals...ouch!), and b.)then leave the illustration for some abstract list of definitions to see what she was just looking at, thereby losing focus.
On the right is Tufte's improved version: the pointer lines are perhaps half as thick, and labels have helpfully been placed within the illustration itself, obviating the need for a lookup table. These new lines are just pulled back in stroke thickness enough to create a more readable difference between the pointers and the things being pointed to. One other layer is information is added with contrast: by using all UPPERCASE letters for larger ear structures (e.g. EXTERNAL EAR, or MIDDLE EAR, etc.), and lowercase letters for the substructures within the larger structures (eustachian tube, cochlea, etc.). No color, no wild changes in scale, just thinning the strokes and using the contrast within a single typeface. Hence: the "smallest effective difference."
Another example from Visual Explanations:
The left-most two columns is an original analysis of various humanist typefaces. Some are filled in, and some are not – a confusing contrast choice seeing as it is implied that we are to compare all of these letterforms on equal footing. But note too that the grid, drafting lines, and notations are as thick as the letterform lines. Because of the lack of contrast between all these elements, it makes it visually confusing to separate letterform constructions from the artifacts of their construction.
On the right is Tufte's approach: he thinned down the grid and drafting lines just enough so that they are noticeable, but do not interfere with the letterforms. Note that he also unifies the fills for the A letterforms, so that they all appear to be equal in importance and intent. Lastly, the notations are made uniform and positioned consistently around each specimen.
The Focal point
Lastly, contrast is how one creates a focal point in a composition, as seen here in this very famous poster by French illustrator (and frequent barfly) Henri de Toulouse-Lautrec. Because of its dark value, large mass and scale, and highly contrasting yellow eyes, the titular 'black cat' is the first thing that grabs our attention.
