Calculating depth of an object

hi all,

currently implementing DOF, nearly there just need to correct my focal point. basically the depth of the object is not being calculated properly. Usually, outside of unity, i would do:

 CVector4 EntityPosInCameraSpace = CVector4(m_EntityInFocus->Position(),1.0f) * camera->GetViewProjMatrix();
 m_FocalDepth = EntityPosInCameraSpace.z / EntityPosInCameraSpace.w;

and in unity i am currently doing:

     Vector3 focalPoint = Camera.mainCamera.ViewportToScreenPoint(m_FocalTransform.position);
     m_FocalPointDepth = focalPoint.z ;

as you can see in the code for unity i am not conducing a perspective divide as i do not have access to the w value focal point.

could anyone please tell me how to calculate the depth of on object.

Thank you (in advance)