Especially out of focus and near the corners, bokeh can often be unpleasant due to various optical aberrations: chromatic errors, a point spread function showing as coma-like or smeared or streaked halos. The lens diaphragm also has a role to play.
Hence a useful test is to observe an out of focus specular highlights as is done here.
Specular highlight bokeh
The lens was defocused here in order to study the behavior. The vehicle was struck by late-day sunlight on its tail-lamp creating a bright specular highlight.
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