The 6-bladed diaphragm makes itself visible on out of focus areas in obvious and subtle ways.
This example is rather a worst-case, yet with many images the results are quite pleasing.
As shown below, the diaphragm shape with its six blades is readily visible in out of focus areas, along with a bright edge, drawing even more attention to the unnatural shape.
Even on the sky area, the image rendition of details and sky has unpleasant harshness to it.
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