Controlling DoF by focal length
At any given aperture, depth of field gets shallower as focal length increases and vice versa.
This is especially useful to landscape photographers who often shoot with wideangle lenses to include a wide vista as the short focal length results in greater depth of field.
If you want to restrict DoF to make your subject stand out more, consider mounting a longer lens on your camera, or using the zoom to set a longer focal length.
PAGE 1: What is depth of field?
PAGE 2: Controlling depth of field via your aperture
PAGE 3: Controlling DoF by focal length
PAGE 4: Subject distance and depth of field
PAGE 5: Using all three factors to control DoF