Depth of FieldTo follow is an explanation of what Depth of Field is and how to get more or less of it in practical terms only, but to understand DoF properly you have to come to terms with something called the “Circle of Confusion”. I am not going to explain what the Circle of Confusion is because I am not able to do better in explaining what it is than the countless websites that already have perfectly good explanations as to what it is. I do recommend that if you have an enquiring mind that you go ahead and do a Google search to find out more detailed information about the subject.
Technically speaking, the term Depth of Field is defined as follows: Depth of field in a photograph is the point in front of and behind the front focal plane, that is considered acceptably sharp by the human eye when viewed at normal viewing distance (+ 30cm). Technically speaking, when you look through the camera and focus the lens, only that which is on the front focal plane (point where lens in focused) is exactly sharp, but in practical terms there is a zone in front of and behind the front focal plane in which objects appear to be in focus. It is this “zone” that is referred to as the “Depth of Field”. The size of the DoF is determined by three things;
The rule is that the closer the object you’re focusing on is, the less DoF you will have, and the farther away the object is that you are focusing on the more DoF you will have. Try it for yourself by taking two photos using the same Aperture, one of something very close to the lens, and another of the tree across the road, and compare how much of the detail in each photograph is “in focus”. Secondly, the larger the Aperture you are using (the smaller f/ number) the less DoF you will have, and the smaller the Aperture you are using the more DoF you will have. Try it for yourself; take two pictures of the same object at the same distance away from the camera, one with the largest Aperture you have on your lens, and another with the smallest Aperture, and compare the results. The most important but least obvious aspect of optics that affects how much DoF you have, is the size of the photograph. And the general rule of thumb is that the larger the photograph is going to be displayed, the less DoF you will have, and the smaller the photograph is displayed the more DoF. 

