Using a tiffen big stopper
This may be way off, but I wonder if light could be reflecting off the aperture blades and then back off the filter. Were your tests that show the bright disk done shooting into a light source, whereas the in-store tests done with lighting from above?
|Steamin' Mad by ahrensjt|
from Angered Subjects (Street Photography)
|Smile by Olymguy|
from Ultra Asian Indian Female Faces
|Space Shuttle Cockpit- by vbuhay|
from Aircraft Control Stick