When we shoot using DSLR camera and the lens with the maximum aperture is wide, of course we want use it in maximum setting. Unfornately when we use maximum setting such as f/2.8 atau f/1.8 so we have small depth of field, viewfinder can’t give us the preview how background turn into blured (blokeh) , it will appear in the final result of photograph.

      Fortunately DSLR Camera maker give fasility named Depth Of  Field Preview. This button is usually located in the  front part of camera and the position in the next of lens. In Canon or Nikon for example in Canon EOS 5D and Nikon D200, this button is located in front of camera in the next of mounting lens.  On preview of the picture above, the green circle is location of Depth of Field prievew button in Canon EOS  5D.

     How to use it? In the wide aperture setting ( f/1.4, f/1.8, f/2, f/2.8 dll), press and hold that button while your eyes are peeking in viewfinder.  So the display in viewfinder will approach the final result of the photo where the depth of field is visible and the area in the outside of depth of field  turn into blurred. 

images sources : compareraja