Centering as a Composition ToolCentering your subject means that your subject is in the general center of your frame. It does not mean that the subject is always exactly centered. In fact, some "centered" subjects can still be broken down to the rule of thirds by applying the rule of thirds to pieces of the subject rather than the whole subject. For example, the eyes of a portrait may work via the rule of thirds although the portrait subject is centered. Centering can be used to draw attention to a subject, to create a sense of space, to create a sense of size, and to overcome location difficulties.
Drawing Attention to Your Subject
Centering is an effective way to draw attention to your subject when there are very few other items in the composition. When taking photographs of stand alone objects where background and foreground are not shown and the subject will nearly fill the frame, centering works to maintain focus on the subject itself. Portraits (animal and people) and illustrations are good examples of this. Also, centering can be used to good effect on the other end of the compositional spectrum when the composition is extremely busy. When there a lot of objects in a frame that compete for attention, centering a strong and different type object can draw attention to it. This works much like when you are working a jigsaw puzzle and are faced with many similar pieces. If you place a puzzle piece of different size, color, or shape in the middle of the group your eye will be drawn to that difference.
Centering to Create a Sense of Space or Size
Centering is also a very effective tool for creating a sense of space and size. By centering a subject that is surrounded by smaller (or larger) objects, the size of the subject can be emphasized. For example, a house photographed in the middle of a large pasture area can create a sense of smallness. You can also use centering to create a sense of belonging to a space or a sense of loss. By photographing a child completely surrounded by toys, you create a feeling of the child belonging to the space around the child. Conversely, by photographing a child surrounded by toys but with a small empty space around the child before the toys begin will create a sense of separation or loss. In both images child is literally the center of attention. In the second image, because the blank space would be centered with the child - it becomes both part of the center of attention and a barrier to the toys.
Centering to Overcome Location Difficulties
Not all subjects or backgrounds/foregrounds will allow a photographer to compose according to the rule of thirds or any other compositional rule. In some cases, the only shot you can get is a centered shot. Sometimes there are distracting foreground or background objects, or there is an element to the side of subject that would intrude on the image were the subject not centered. When events like this occur you must make the most of the situation and center the subject. A centered subject will certainly command more positive attention than a subject with a lamppost directly behind his/her head.
Centering can also be used in conjunction with creative lighting techniques (spot lighting, halo effects), spot focus, spot/selective color, and many other techniques to enhance the effect of the centered subject.