Depth of Field.
Depth of field refers to the range of distance that appears acceptably sharp. It varies depending on camera type, aperture and focusing distance.
On DSLR cameras and some more advanced point and shoot camera you can have full control over the aperture. While the shutter speed controls the duration of light hitting the sensor the aperture controls the amount of light hitting the sensor. The aperture is the part of a lens that dictates how much light is let through to the sensor – if it’s wide open, lots of light gets through. If it’s closed down, not much light gets through. In essence, it performs the same as the pupil of an eye. If you are in a dark room, the pupil is open; sunlight, the pupil is small.
Depth of Field always extends 1/3 in front of and 2/3s behind the point of focus. No matter whether the DOF is deep or shallow, it always follows this formula. This fact becomes more valuable when you do macro photography.
Depth of Field decreases as the distance between the subject and film plane decreases. You have VERY little DOF to work with when doing macro photography and are focused just a couple of centimeters away, but you have extreme DOF when focused at a point near infinity.
Different lenses can have different apertures – for example, a cheaper lens may only open to f4.0, not letting in as much light as a more expensive lens that will open to f1.6 or more.
The depth of field does not abruptly change from sharp to unsharp, but instead occurs as a gradual transition.
All lenses have a Hyperfocal Distance (hyperfocal distance is a distance beyond which all objects can be brought into an “acceptable” focus) for a given f/stop. If, for example, the Hyperfocal Distance happens to be 16 feet for a particular lens/aperture combination, everything from one-half that distance (8 feet) to infinity appears to be in focus. If your lens has a DOF scale, line up the infinity symbol with the f/stop you are using and you have just set your lens to its Hyperfocal Distance for that f/stop.
Depth of Field sounds like a good thing and usually it is—but not always. If you want to produce dramatic portraits you’ll want to limit.
Bokeh. In Japanese is means “fuzzy” and in photography it’s used to describe the parts of a photograph that are not in focus. Anyway, some lenses are optimized to produce attractive bokeh.
There are also various ways of calculating DOF online. http://www.dofmaster.com/
Here you will find a variety of charts, downloads and online resources for DOF or hyperfocal distance.