http://photo.stackexchange.com/quest...m-bokeh-effect
Have a look at where the graphic called optical vignetting and then think of a hood in place and how this could impact it. As mentioned, not sure if it's correct. Only reason I knew about this was when I got my 70-200 I was confused about the cats eye bokeh on the edges when wide open and some of the responses seemed to indicate the hood could have an effect. If you think about the shaped bokeh lens hoods you can buy that make heart shapes, diamonds etc then there could be some truth because even when the hood is out of view, it still impacts the bokeh
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