Basic Technique Digital Photography "Depth Of Field"

Literally Depth of Field (DOF) means depth of field. In the world of photography, DOF is technically means the range or variation of the distance between the camera with the subject of the picture to produce variations in sharpness (focus) images that can still be accepted (no blur). In other words, DOF is used to indicate a particular room in the photos that get special attention by the eye because of differences in sharpness (focus)
In general, Depth Of Field is affected by 3 things:The main focus of the camera distance
Width sharp space proportional to the square of the distance of the object. If we change the distance between the camera with the object of 3x (even further - by sliding the camera back from its original position) then the width of the space will be sharply 9x original width.Aperture diaphragm
Width sharp space proportional to the diaphragm. Example: if the diaphragm is raised 2 stops from f / 8 to f/16, then the width of a sharp space will be 2x the original width.Focal length lens used
The width of the space is inversely sharp focal length of the square. In other words, the width of a sharp space will be 4x the original width if we change the lens from 100mm to 50mm (the lens focal length and a half from the original).The more wide-angle lens, the more extensive region of space sharpness. This means that when the camera is zoom out, objects that we shoot will be increasingly free to move forward or backward within a certain distance from the camera and still look sharp / focus. Sharp narrow space in telephoto shots, also called narrow DoF, while sharp wide space in the wide shots called DoF also extensive.
The more open the aperture, the more narrow region of space sharpness. This means, set the focus in lighting situations that less is more problematic because the diaphragm must be opened wide and the object will not be free to move closer or away from the camera because it will be out of focus (out of focus).
The combination of telephoto (zoom in all the way) and the diaphragm is opened wide, will lead to sharply narrow space. One example, when shooting telephoto (tight shot) a singer who did a concert at night with minimal lighting, we must be careful in adjusting the focus, as there are few singer moves closer or away from the camera, then he will be easy to out of focus.
source: http://www.arielz.net/fotografi/teknik-dasar-fotografi-depth-of-field