Our eyes are a magic organ. They adapt to the color of light
and make adjustments so fast that we don't even know they are. In many respects they have all the limitations of a camera lens, but we never know it..
A final photograph can be very different from how it looked to us as we took the picture. When you decided to photograph a scene everything looked equally sharp. When you saw the print something changed.
This zone of sharpness is called the depth-of-field, and it extends in front of and behind the point that you focused on. The area of focus is determined by three factors
- The f-stop
- The focal length of the lens
- The distance you are from the subject.
Changing anyone of these elements allows you almost complete control over the depth-of-field in a picture.
When most of the picture is sharp you can say that the depth-of-field is large. When only part is sharp, we say depth-of-field is narrow. The one you want is based upon the desired effect of the photo. |
More about the three factors which affect depth of field.
1 The aperture.
Simply put - the smaller the aperture, the greater the depth-of-field.
To keep as much area as possible sharp, you should set as small an aperture as possible. Keep in mind that the larger the f-stop number smaller the aperture and the greater the area in focus.
Depending on lighting conditions and film or ISO setting, you may need to use a tripod or support because the smaller aperture means a slower shutter speed.
If you are shooting a flower or a portrait you may want less area in focus. In this case a larger aperture will work.
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2 The focal length of the lens.
The wider the angles of the lens the greater the area in focus. On the flip side of the equation the longer the lens the more narrow the area of focus. For portraits a general rule of thumb is to chose a telephoto that is twice the length of the normal lens.
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3 The Camera-to-Subject Distance
The closer you get to the subject the more limited the depth becomes. In fact, when shooting close-up subjects the depth of field can be measured in it can extend to just a few millimeters in front of and behind the subject.
So that's the theory, but what does it all mean in practical picture-taking terms?
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 This picture of the old nail is another example of depth of field. The first shot was taken with a wide open f-stop, or portrait mode for the point and shoot people.
Notice that the nail is in focus as is the wood near it. As soon as you look past the nail you see that the rest of the barn. The effect is to isolate the nail and draw attention to it by removing the background.
Compare the first image to the second image below. With the greater depth of field the barn siding distracts from the nail.
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Putting Dept of Field into Practice
1 If your goal is to have everything in the photo sharp:
Subjects such as landscapes, groups, and interiors are usually desirable sharper. Using a wide-angle lens set to a small aperture, (higher number) will give you more depth-of-field.
There is a technique worth mentioning called hyperfocal focusing. This will allow you to give the impression of increased the depth-of-field.
As a rule-of-thumb, there's twice the depth-of-field behind the subject than in front of it. So if you photograph a distant subject such as a landscape and focus on infinity you waste lots of depth-of-field. By focusing a little closer, you'll extend the depth-of-field at the front so it comes nearer to the camera, while still making sure that infinity falls within the depth-of-field behind the actual point of focus.
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| You can guess the hyperfocal focusing distance, but life is much easier if your lens is marked with a depth-of-field scale. This used to be regarded as an essential feature, but with the development of wide-ranging zooms many manufacturers now omit one. If you do have such a scale, simply line up the infinity mark against the mark for the aperture you've set and, although the image in the viewfinder will look out-of-focus, the finished image will be sharp from front to back. |
2 Main subject sharp with background completely out-of-focus.
There are some subjects where you want the main subject to stand out strongly from an out-of-focus background. Portraiture, where the emphasis is on the person, rather than the location, is probably the most popular area. What you need here is a use a telephoto lens at its widest aperture. It's also worth moving the subject as far away as possible from the background - in cramped studio situations this is often impossible but outside against a wall or foliage it's usually easier. Take care, though, that you focus accurately, as the limited depth-of-field will require accurate focusing. Old masters would have the eyes in focus and the ears soft or out of focus.
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3 Main subject sharp, with background out-of-focus but still recognizable.
Sometimes throwing the background completely out-of-focus is going too far. You want to show the subject in its natural environment, but with the background toned down to it doesn't compete for attention. A person on the beach, for instance, an animal in the zoo, or a flower in a garden. Here a standard to short telephoto lens, somewhere from 50mm to 135mm, is ideal - especially if it's coupled with a middle range aperture of around f/8.
This example of a dogs nose is an excellent example of these factors in play.
 Notice how the dogs nose and ears are soft. This increases the impact of the photo by bringing the viewers eyes to the dogs eyes.
A simple way to do this is to use a telephoto. A wide angle lens would tend to distort the dogs nose and make it appear larger.
Like with all photography techniques you need to experiment and learn your equipment.
Notice the image below. See how the dept of field is narrow. This draws your attention to the model as opposed to letting it wander around the field of flowers. This is a common portrait technique.
Submitted by Art @ Artsphotogaphy.com a Kansas based photographer providing family portraits and event photography services.
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Summary
Overall, the practical use of depth-of-field can be summed up as follows:
To maximize depth-of-field and have as much sharp as possible use a wide-angle lens, set a small aperture and stand back from your subject
To minimize depth-of-field with only a small zone of the scene sharp use a telephoto lens, set a large aperture, and get closer to your subject.
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