The possible brightness values in the scenes you photograph range from dark shadows to bright highlights. When you photograph a scene, pay attention to the direction from which the light is coming.
When an object interrupts the light, a shadow is created. Light and shadows are powerful compositional elements. The human eye is naturally attracted to the brightest areas of a scene. You can use this to your advantage to create a compelling image.
When you photograph a sunset, the sun is the brightest thing in the image. It’s also your focal point. Using the Rule of Thirds, position the sun on a power point. (A power point is where the borders of two sections intersect, after dividing your image into thirds.) This will draw your viewer’s eye to the most important part of your photograph.
If you don’t bring some order to Mother Nature’s chaos, you won’t have interesting photographs. You bring order to the chaos when you compose your images. Knowing how to use the element of light brings order to the chaos and creates a visually compelling image. As you become more proficient at photography, you’ll use rules of composition instinctively.
Brightness can also work against you when you take a photograph. Bright areas at the edge of the image give your viewer an escape route from your photograph. If you compose your photograph so that dark objects or areas are at the edges of the image, the darkness acts as a frame to keep your viewer's attention in the image.