Although a lens can only bring objects at a single distance from the camera into critically sharp focus, other parts of the scene in front of and behind the most sharply focused plane appear acceptably sharp. The area in which everything looks sharp is called depth of field. Objects within the depth of field become less and less sharp the farther they are from the plane of critical focus. As the distance increases, things eventually become so out of focus that they no longer appear sharp at all. The near and far edges of your depth of field are usually not visible as exactly defined boundaries. Instead, sharp areas imperceptibly merge into unsharp ones. At normal shooting distances, about one-third of the depth of field is in front of the plane of critical focus (toward the camera), and two-thirds is behind it (away from the camera). When the camera is focused very close to an object, the depth of field becomes more evenly divided.
To see the depth of field on an SLR-type camera, you press a depth of field preview button that stops down the aperture to the one that will be used to take the photo. On point and shoot digital cameras, the best way to check depth of field is to take a trial picture and then play it back on the monitor where most cameras will let you zoom it and then scroll around it to examine details.
Often it doesn't matter so much exactly what you are focused on. What does matter is whether or not all of the objects you want to be sharp are within the available depth of field so they appear sharp. There are reasons you may want parts sharp or not. You may want only a part of the setup in sharp focus to give it emphasis against a softer foreground or background. On the other hand you may want everything sharp to show details. To control how deep or shallow depth of field is, you have three factors to work with.
- Aperture size. The smaller the size of the lens aperture (the larger the
f-number), the greater the depth of field. The larger the aperture, the shallower the depth of field.
- Camera-to-subject distance. As you move father from the subject you are
focused on, you increase depth of field. As you move closer, you decrease it.
- Lens focal length. Zooming out to a wider angle of view increases depth of field. Zooming in decreases it.
Each of these three factors affects depth of field by itself, but even more so in combination. You can get the shallowest depth of field with a lens zoomed in on a nearby subject using a large aperture. You get the deepest depth of field when you are farther from a subject, with the lens zoomed to a wide angle, and using a small aperture.
When you get the camera really close, don't expect much depth of field. It's best to arrange the objects so the most important part falls on the same plane. That way, if part of it is in focus, all of it will be. Another thing to try with a zoom lens, is to use a wider angle of view. This will give you more depth of field if you don't also have to move the camera closer to the subject (doing so will offset the advantage of the wide-angle lens).
Shallow depth of field has its own benefits, so you don't necessarily have to think of it as a problem. An out-of-focus foreground or background can help isolate a subject, making it stand out sharply.
A large aperture gives a shallow depth of field (top). A small aperture gives greater depth of field (bottom).
Capturing Maximum Depth of the Field
When you want as much depth of field as possible, there are things you can do to obtain it.
- Don't focus on the part of the setup closest to the camera. Since one-third to one half of the available depth of field falls in front of the point on which you are focused, focusing on the front wastes one-half to two-thirds of your depth of field. This may cause some other parts of the object to be unsharp because they are not included in the one-third remaining depth of field. If you focus one-third to one-half of the way back, you will shift the available depth of field back to include more of the setup while still keeping the front part acceptably sharp.
- Increase the light on the setup so you can use a smaller aperture.
- Zoom the lens out to a wider angle of view.
- Move farther away from the subject.
- Switch to aperture-priority mode and select a small aperture such as f/11.
Using Selective Focus
One way to make something stand out is to use selective focus. When everything in a picture is equally sharp, the viewer tends to give equal attention to all parts of the scene. But if some parts are sharp and others are not, the eye tends to be drawn to the sharpest part of the image. You can selectively focus the camera and your viewer's attention on the most important part of the scene
if you restrict the depth of field so that the significant elements are sharp while the foreground and background are less so.
Digital cameras have great depth of field so you have to really push the limits to see the effects of selective focus. Move close, zoom in, and select a wide aperture. When using selective focus, here are some things you can do to reduce depth-of-field:
- Reduce the light on the setup so you can use a wider aperture.
- Zoom the lens in to enlarge the subject.
- Move closer to the subject.
- Switch to aperture-priority mode and select a large aperture such as f/4.
- Focus in front of or behind the subject to waste some depth-of-field.
In this photo, the point of interest is the lens cap. Using a large f/stop and selective focus, the camera on which the cap is mounted has been thrown out of focus. This dramatically emphasizes the lens cap.