Depth-of-field. It rings terror in the hearts of beginning photographers.
At its worst, it's frightening concept developed by a mathematics-crazed genius bent on terrorizing the photo-taking world.
At its best, it's a simple method of selectively bringing objects outside the immediate subject area into (or out of) sharp focus.
Say, for instance, that you've focused on a row of flowers 10 feet away. IN one shot, the tree five feet beyond the flowers is also in sharp focus. In a second shot, the tree is blurred - out of focus. Even though you hadn't moved the focus from the flowers, the depth-of-field, or area of sharp focus, changed from one shot to the next.
The size of this area of sharpness increases or decreases depending on three factors: length of the lens being used, distance from the camera to the subject, and the size of the aperture, or lens opening. The shorter the lens, the greater the depth-of-field. The farther you are from the subject, the greater the depth-of-field. The smaller the aperture, the greater the depth-of-field.
What does this matter to you? Well, by knowing in advance of taking the photograph exactly what elements in the scene will and won't be in sharp focus, you can produce a more effective shot.
For example, if you're getting ready to photograph a friend standing in the park when you notice the trees behind him are too cluttered, too distracting, you can blur them out of the scene by decreasing depth-of-field so that only your friend is in focus. You may not be able to substitute a longer lens or to step closer to the subject, but you're probably able to adjust the aperture on the lens. By opening it up to one of its large settings (f/4, f/2.8, etc.), you'll be decreasing depth-of-field and eliminating the distracting background from the scene.
This is an example of using shallow depth-of-field to produce a more effective photograph.
Or the opposite may be true. Say that you're photographing your brother standing at the edge of the Grand Canyon and you want them both to be in focus. By setting the aperture on the lens to a narrow opening (f/11, f/16, etc.), you'll increase depth-of-field and accomplish your goal.
But how do you know how large the depth-of-field is at any given aperture? If you shoot one of the newer cameras, you may find a depth-of-field preview button. By pressing it, the lens will close down to the aperture you've selected and you'll have a preview look at what is and isn't in sharp focus.
If your camera hasn't such a device, it probably has a depth-of-field scale sandwiched between the focusing and aperture rings. Read the camera manual or talk to your favorite photo dealer for help in reading the scale. Once you learn how, it's simple to estimate depth-of-field.
Properly utilized, selective depth-of-field can enhance the beauty and effectiveness of your photographs.