Depth of Field
Depth of field is (loosely quoting from an old professor of mine) “that area both in front of and behind the plane of sharp focus where objects appear in reasonable focus”. Whew. But really, that statement pretty well sums it up.
A camera focuses on a plane–the plane of sharp focus. And that is a plane in geometric terms: a flat area of infinite height and width, but infinitely small DEPTH. This plane is parallel to the lens plane, and with 35mm SLR cameras (and most others) also parallel to the film plane:
Now comes the voodoo part. Because lenses are not (all) flat, there is an area in front of and behind the plane of sharp focus that has reasonable focus– our depth of field. There are several factors that effect depth of field, but let’s start with a general description. The amount of focus can be described as a curve on a graph, starting between the camera and the POSF (plane of sharp focus), and ending somewhere behind the POSF. The curve rises a bit sharper at first, and tails off slower, giving you more distance in focus behind than in front of the POSF:
Here the grey area under the curve indicates the amount of focus.
The factors affecting depth of field are aperture setting, distance from the subject, and focal length of the lens. The first is the easiest to control, and simplest to remember: the smaller the aperture (bigger ƒ number), the more depth of field at a given distance. So, if you are six feet from your subject, ƒ11 will give you more depth of field than ƒ2. The second factor, distance from subject, is pretty straightforward as well: the farther from the subject, the more depth of field at a given ƒ-stop. These two are the most easily controlled. The images below illustrate the combinations.
Shallow depth of field: a combination of wide aperture and short distance to subject.
Long depth of field: small aperture, longer distance to subject (note treeline in distance).
The third factor, the focal length of the lens, is of concern, but unless you have more than one lens, may be beyond your control. The shorter the focal length of the lens, the more depth of field, at a given ƒ-stop and distance to subject. Thus, a 35mm lens, set at ƒ8, six feet from the subject, will yield a longer depth of field than an 80mm lens at ƒ8 six feet from the subject. (There will also be other differences in the images, explained in lenses).
To sum up: more depth of field -> smaller aperture, farther distance, shorter focal length of lens. Less depth of field -> wider aperture, closer distance, longer focal length.
Standard Aperture Sizeseach is ½ or twice preceding or following size(one stop) | ||
 | ƒ/1.4 | Largest opening, shortest depth of field |
 | ƒ/2 | |
 | ƒ/2.8 | |
 | ƒ/4 | |
 | ƒ/5.6 | |
 | ƒ/8 | |
 | ƒ/11 | |
 | ƒ/16 | |
 | ƒ/22 | |
 | ƒ/32 | Smallest opening, longest depth of field |
