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?
|Glass by Hobbyfotograaf|
from High speed photography
|Northern water thrush by APenza|
from A Big Year - birds
|4th of July Outfit by Jack Sofield|
from - Old Age - (Black and White only + A Border)