This effect is a blur whose radius differs for each pixel and depends on pixel depth.
      
                          The radius for a given pixel is known as the 
                    
                      Circle-of-confusion
                     for this pixel and can be configured
      thanks to the 
Coc Function parameter.
      
                          By default this function is linked to a 
                    
                      Lens DoF
                     model, but it can be changed to 
                    
                      Simple DoF
                     to use settings
      or to any other 
                    Function
                    Function that maps a number (percentage, time, etc.) to a value
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                    .
      
                          One key parameter inside 
                    
                      Lens DoF
                     and 
                    
                      Simple DoF
                     is the 
                    
                      Focus Distance
                     that determines the distance at which objects
      are perfectly sharp. By default, this distance is linked to the focus distance of the camera, which itself by default is linked to the camera
      distance to target.
      
                          If preferred, the focus distance can be overridden directly from within the depth of field modifier. This is made by enabling the 
                    
                      Focus Distance
                     option
      inside the DoF function and entering values manually there.