While there are many photo situations, such as a landscape, where you goal is to make the entire frame sharp, there are other times when you might want to limit focus to a very shallow area of the frame. This technique is called selective focus and it requires restricting what is called the depth of field, or the near-to-far area in a frame that is in acceptably-sharp focus. Selective focus is particularly useful in outdoor portraits (human or animal) because it enables you to subdue distracting backgrounds and focus attention on just your subject. In this shot of a wild horse that I photographed on the King Ranch in Kingsville, Texas, I used the technique to "pull" the horse out of somewhat distracting surroundings.
There are three primary things that affect how much will be sharp: aperture, focal length and your distance to the subject. All other things being equal, using a wider aperture (f/5.6 instead of f/11, for example) will create a much more shallow depth of field. Longer focal length lenses also limit sharp focus. In order to separate this horse from the dense underbrush, for example, I combined using a long (300mm) lens and a wide (f/5.6) lens aperture to limit sharp focus to just the horse. Also, because I was relatively close to the horse, the depth of field was even more shallow.
Depth of field is not as complex or scary as most books make it sound. Just remember that to limit depth to a narrow zone, use a wider aperture, a lens with a relatively long focal length (longer than normal, at least) and get as close to your subject as you can. For a very thorough look at depth of field, check out my book Exposure Photo Workshop.
9 hours ago