The problem with ultra-wide lenses is that a bulbous front element makes it impossible to attach a filter directly to the lens. A tried and tested method to create more space between lens and filter through the use of a filter holder and a lens adapter ring. Systems such as the Wonderpana by Fotodiox and the SW-150 by Lee make the use of all sorts of filters available for photographers with deep pockets. Like the enormous filters themselves, these systems are often expensive and only attach to one lens. Enter STC with a 6-stop ND filter that you could use with the widest of lenses, and for just 80 bucks.
It’s essentially the same system as the STC Astro-Multispectra filter, reviewed here. The idea is that you lock up the mirror, put in the filter and prevent the mirror from coming down, while never touching the delicate sensor at the heart of the camera. This nifty approach enables the use of filters on lenses without filter threads.
A 6-stop ND filter is ideal for use under broad daylight to slow down waterfalls enough to get them nice and streaky, without blurring them completely.
Another great time to use a 6-stop is right before sunset, when the sun is about to dip below the horizon and you wished the water was flat as a mirror.
STC claims that the ND64 filter is free of the dreaded color cast, due to its water and grease resistant coating and “Simulative Linear Calibration.” The filter is just 1mm thick and features the high quality Schott B270 optical glass from Germany.
The 80 dollar neutral density filter weighs just 9 grams and is available for Canon and Nikon full-frame bodies.
Source: FStoppers (https://fstoppers.com/gear/now-available-6-stop-nd-filter-ultra-wide-lenses-without-filter-holder-171238)