hi, and thank you for your answers.
paul is right, its not the aperture blades. they appear clearly behind those blades when hitting the stop down button. well, at least i learned that they seem to be the shutter blades.
and as i said, this happens sometimes with the shutter release button AND sometimes with the stop down button.
the lens is brand new. i guess, i'll take it into service and let you know what it was.
jorma.