You can enhance the rhythmic effect of many subjects by compressing perspective with a telephoto lens. You can do the same job with a wide angle, depending upon your subject and your choice of perspective. Usually, our concern with contrast has to do with maintaining detail in the highlights or shadows of our images. These contrasts, when incorporated into our photographs, can create visual tension and a stimulating effect. The shape of the poppies also provided a contrast with the shapes of the background blossoms.
Contrast also works on the grand scale.
Hillsides of colorful trees with one or two standing out in a different color can be a study in contrast. Position your objects and elements in the frame where you intuitively feel they're right. Sometimes, placing objects beyond those intersecting points, away from the center, can create more visual tension. Watch the amount of negative space that results in your composition. Too much emphasis given to empty space will draw the viewer away from the main focal points of the image.