Sometimes you’ll need to use several filters at once, and with screw-in filters you really only want to use a maximum of two because vignetting will become an issue at wider focal lengths. Also, the more of any type of filter you stack means there’s a greater risk of dust and fingerprints. Drop-in filters are better for filter stacking, but you must make sure the filters are clean. Lenspen and filter companies offer a variety of easy-touse filter cleaning products designed for both larger and smaller screw-in filters.
You need the right size adapter ring for drop-in filters
Stack several filters at once in the holder