rings as bokeh effect of a mirror optics lens

Bokeh (Japanese "boke" = blur or haze) is how the lens draws unsharp image areas. The effect becomes notable when the unsharp back- and/or foreground has significant contrasts, especially when small light areas contrast to darker surroundings. A typical bokeh effect is given by some lenses with six-blade aperture. Light points of the unsharp fore- or background are projected by these lenses as hexagonal lighter areas onto the image. Portraiture needs lenses with a pleasant or a soft undisturbing bokeh. Almost round apertures given by multi-blade diaphragms are always a good precondition for a nice Bokeh, but don't necessarily deliver one. It depends on where in the lens the diaphragm is placed. In some extra-short lens constructions and zoom lenses necessary compromises might reduce nice bokeh results despite of nearly round aperture.