Turn off all lights. Your scene will render black.
Go to Materials
Enable Depth Cue
Set Depth Cue Color = WHITE.
Set Depth Cue distances to desired values.
(Turn down Render setting so it does not take long)
Turn off all lights does mean all light emitting materials as well. You can use SceneFixer to get these ambient sources.
Once created, you can use it for free in GIMP with Focus Blur, or in Photoshop with Lens Blur, or in paint.Net…somehow?