OBJECT ISOLATION

An object can be seperated from its environment by various methods. E.g. you can use a very wide lens to sort the scene into distinctive layers. However, while you seperate the object the environment is still visible which may be disturbing because -say- the background is very ugly. Sometimes there’s a workaround for this problem: we choose a very small depth-of-field so only the main subject is in focus while everything in front or behind the focus plane gets blurry and therefore virtually unimportant. Have a look at the 1st sample below. The blue marble to right right sucks the view from the first look. This is a natural reaction because the brain scans for the most contrasty subject first. The isolation of the object due to its “outstanding” sharpness is very significant here.

by Michael Wagner
Wanna see a perverse example … Imagine to be in the Himalayas at a nice sunset and all you shoot is a beautiful rose … The result is not all too bad I think!

A small DOF is also a common technique for portrait photography. Usually it is quite difficult to find the right balance between people, that are chosen to be the main subject, and their environment. A sharp background is often distracting here so a large aperture should be used to focus the attention on the point of interest.
Basically like this one.

Source: Photozone