By varying the size of the aperture it is possible to vary the Depth of Field . Depth of field is the term for the distance that is in focus in front of and behind the point of focus of the lens. For example, if the lens is focused on a tree, things at a certain distance in front of and behind the tree will also be in focus. The distance changes with the aperture used. Bald Eagle with injured wing Seneca Park Zoo, Rochester, NY Larger apertures give a shallow depth of field.
Smaller apertures give a longer depth of field.
This can be put to creative use. When taking a portrait, for example, it is customary to use a large aperture. The result is that whatever is behind the subject is thrown out of focus, making the subject appear to stand out of the photograph. To render the background sharp in this instance would be distracting. This photograph was taken with a 300mm f4 lens.