I'm not sure why, but I'm getting, at times, images like the one below, while doing long exposure with the 15 big stopper.
I had this problem today for the first time, and not with all pictures. What could it be?
Note: I'm using a full-frame mirrorless camera.
Light is reflecting off the back of the filter. You can plainly see the reflection of the front of the lens bouncing off the back of the filter.
Some type of makeshift hood may solve the issue if the light is entering in front of and passing through the filter. But I suspect the offending light is not coming through the filter.
With 100mm square filter holders, sometimes light can leak in at the sides of the holder through the small gaps between the filter and the lens in the areas where the "springs" that hold the filters in place are cut out. If the hood alone does not solve the problem, try covering the side holes with some tape that leaves no residue such as gaffer tape. If the top and bottom of your particular holder leaves gaps between the filter and the 'back wall' of the filter holder, cover those as well.