The wrong aperture has ruined many “singular theme” compositions, including the portrait shown here. Do you notice the difference between these two “identical” portraits? One has a clean background while the other has a cluttered, and thus distracting, background. The smaller the aperture, the greater the detail beyond the subject. When it comes to singular-theme compositions, you will want to use a larger aperture to limit detail in the background and focus attention on the main subject.
Top: Nikon D3X with Nikkor 70–300mm lens at 240mm, f/16 for 1/60 sec., ISO 200; Bottom: Nikon D3X with Nikkor 70–300mm lens at 240mm, f/4 for 1/1000 sec., ISO 200