You can enable or disable depth of field for each viewport panel.
This behavior is different from that of Maya 2015 (and previous versions). Previously, if you enabled Depth of Field in your camera Attribute Editor, depth of field was always enabled in the viewport. The Depth of Field attribute in the camera Attribute Editor is now decoupled from the viewport depth of field control.
In addition, you can include the depth of field effect in your render.
To see depth of field in your viewport
To include depth of field in your render