DOF is a range of object distance, which the image appears to sharp and focused.
Also a parameter describing the distance of imaging side (sensor side)is called depth of focus.
Tolerable level of blur is called permissible Circle of Confusion, or Permissible COC.
This represents the smallest diameter if a bundle of rays when being focused on an image plan.
The diameter of Permissible COC will be definedby each application, pixel size of camera and the person who actually measures.
DOF = 2(permissible COC * W.F#)/(Opt Mag)² = permissible COC / (NA*Opt Mag)