I've discovered that this lens produces very different aperture shapes depending on what aperture is selected. The blades have a very unique shape, which results in the following variations in point light sources:
I have the pre-set Pentax 50/4 macro and also the Vivitar 55/2.8 (goes to 1:1) macro lens. Have not done a side by side comparison but my money is on the Pentax. My favorite feature: non-circular aperture opening at some apertures.
Mine looks the same. Hence, the slight star effect even wide open. I think it is normal for this lens. I've seen this in other lenses, too, although it is not common.
