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.