The depth of field created by a lens can roughly be defined as the space between the nearest sharp subject and the furthest sharp subject. In other words, you could call it the field of acceptable sharpness within an image.
The image above shows great depth of field.
To control depth of field using aperture is probably the most commonly applied creative aspect of photography, and the use of depth of field easily helps define the style of a photographer.
When the depth of field is shallow, or a shorter range, it creates a usually welcome separation between the subject and the background (and also sometimes the foreground). When we look at a near object with our own eyes, the background goes out of focus, and depth of field works exactly the same way with a lens. So, it is a much more natural image when the background is somewhat out of focus. You can control the depth of field using 3 variables:
The larger the aperture (or the smaller the f number), the shallower is the depth of field. In other words, something like f2.8 is good when you want to isolate the subject by throwing the background and any possible foreground out of focus. This works well for portraiture where you want the viewer’s attention to go to the person in the image alone.
The longer the focal length, the shallower is the depth of field. Conversely, the shorter the focal length the more is the depth of field at the same given f number. So, where you want to maximize the focal length and get everything in focus, you would want to use a wide angle in preference to a normal or tele lens.
This works well for photographers shooting landscapes…on the other hand, sports images almost always have very out of focus backgrounds; because sports photographers, just like wild life photographers use long lenses of 300mm focal length and even higher.
Subject distance is probably the least used depth of field control, and that is sometimes surprising because it is the only tool that comes pretty much ‘free’. Aperture and focal length on the other hand come with hefty price tags for every change of equipment.
Anyway, all you need to do is move in closer to the subject to make the depth of field shallow! This is why close up images of insects have such an amazingly out of focus background…unless special techniques are employed to actually have the reverse effect and increase the depth of field.
Image below of the Port Fairy lighthouse is a great image, but shows no depth of field.