I took this photo with a Takumar 1:1.4/50 at f-2.0. The bokeh circles, which I recall were caused by very bright reflections off some swampy grass, are not round. They are clearly the exact shape of the 8-element diaphram in the lens and you can even see the corners where they hook a bit. Is this normal? How can you get a rounder image or prevent the clearly octagonal shape?