If you follow this blog or especially my YouTube channel you know that I often preach about the merits of using filters - and in particular ND filters on smartphones.
ND filters help control the amount of light that hits the sensor and thus affects the shutter speed - and that of course affects motion blur. And lack of motion blur is the biggest giveaway that you're shooting mobile video.
Since (most) smartphones have a fixed aperture you control exposure with ISO and shutter. And you always want to use the lowest ISO possible, and so to obtain proper exposure without using ND (neutral density) filters the shutter has to really speed up. And while fast shutter speeds can look great in photography (to capture a clean frame of a "moment" in time), they make video look "staccato" and like you're watching a zombie movie.
With traditional cameras ND filters are also routinely used to help get shallow depth-of-field. Since those camera do have a variable aperture by adding an ND you can shoot with a wider aperture, which in turn that will allow more light in and create a shallower DOF look. But again, in smartphones the aperture is fixed so it doesn't work like that with them.
As smartphone video has gotten better and better over the years different lens & filter manufacturers have started paying more attention to this market - and now today there are lots of great options for adding filters.
Recently there have been three great announcements and product releases for smartphone filters from three different companies
3. Moment 67mm Universal Filter Mount (this is a link to their main site, then look under Filters)
I've done videos about the first two products and plan to do one on the new Moment release, too.
All these filter systems are primarily geared at the new iPhone 11 Pro three-camera system, although the Moondog and Moment will work with dual and single camera devices, too. The PolarPro will not, however, I believe they do have another model in the works for the regular iPhone 11.
So if you enjoy shooting video on your smartphone but sometimes wonder why your footage doesn't look better or more "professional", then definitely try adding an ND filter to your kit. It's seriously the number one accessory I recommend.
P.S. If you want to learn more about cinematography and smartphone video then consider my course, "Smartphone Cinematography 101" and SAVE 10% using this link.