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?
|I see you by Phocal|
from Animal eye reflection
|Apocalyptic Sunset by Impact Photo|
from A wheel good photo!
|AU4_6418_BB-35 by DaveInHouston|