Bokeh - Wikipedia, the free encyclopedia: "The other characteristics of a lens that cause it to produce pleasing bokeh are complex. Some lens designs are known to produce a certain kind of bokeh, but most photographers do not fully understand how the lens design influences bokeh; they merely note whether a lens produces results that please them. For example, mirror lenses produce a 'doughnut' bokeh that is generally considered unpleasant."
bokeh: "Photographers know that one of the characteristics that separates photographic imaging from drawing or painting is the matter of focus."
Boke "depth of field" - Google Search