Due to the nature of dense ND filters, any unwanted ambient light leaking through the camera or gaps in the filter holder may degrade image quality due to reflections off the filter itself. This is called ‘light leak’. For ND filter strengths of less than 6 stops, ‘light leak’ is rarely a problem. When using 6 stop filters or above, the following tips help reduce ‘light leak’:
When inserting filters, use the slots of the filter holder closest to the lens for ND filters so that its foam gasket creates a seal with the holder and will effectively prevent ambient light from leaking into the system.
If shooting with a DSLR, Cover up optical viewfinder of your camera. Some camera brands have a dedicated cover on the camera strap, otherwise, a strip of duct tape or blue-tac will suffice.