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?
|Leaving the office, Central London. by Edward48|
from Your City - Night Shift
|Running free by LassiM|
from My Best Photo of the Week
|Everyone's happy at Disney World! by Pixney|
from Disney World