Create a simple scene.
Select the Persp View.
Select View->Camera Attribute Editor... from the Persp Window's Menu.
Under Depth of Field:
Check Depth of Field
Select Display->Show->Cameras to display the Camera icons.
Dolly out the orthographic views so you can see the camera icon in relation to the item you would like focused.
Select Create->Measuring Tools->Distance Tool.
Click near the camera, and then click near the item you want to be in focus.
Adjust the locators to get a more accurate distance.
Use this Focus Distance in the Depth of Field menu in the Camera Attribute editor.
Lower the Fstop to make the image less focused.