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I've been using this on android for a while, which seems to work well enough and lets you use any ND stop values (for those of us who stack filters to odd ND values)
The way I've done it is to use an exposure calculator like this.
No idea if this is a stupid method or not, but what I'd do is put the ND filter on (I had a variable one so I couldn't use actual stop calculations) and then just mess about with the camera until the photo was exposed. Usually really wide aperture, stupidly high ISO and for a few seconds. Not looking for it to look good, just looking for a known "this is exposed" to enter into the app.
I'd input all that in the calculator then start tweaking the values to what I wanted to actually use. Usually I'd start with the aperture set that to whatever I wanted, then change the ISO downwards and see the effect on the new calculated exposure time at the bottom. Once I hit the right ISO or decide the exposure time is getting too long I just use Bulb mode and time it out.
You can totally do the math in your head or just learn the common values, I just found this easier. Some apps will have the option for a 9 stop filter so all you'd have to do was expose without and then the app will give you the time with, I just had to go the long route as it was a pretty crap variable one.
ISO 400 and F2 can get you some good stuff handheld if you're steady and there's reasonable ambient light. The Exposure Calculator app can help you figure out your shutter speed (google play store link but also available on apple https://play.google.com/store/apps/details?id=com.quicosoft.exposurecalculator.app&hl=en_US)
Am I understanding how to use Exposure Calculator app to use ND filters correctly:
+1. A longer shutter speed would a) brighten up the image and b) make the firefly trails longer which would fill up more of the frame. I imagine they are very dim so might have to brighten them in post. I like the concept. In the future I would do a test exposure at high ISO then use something like this to caluclate the correct low iso exposure. You will definitely need a tripod.
Some may not be photography specific, but I end up using them in photography situations:
Exposure Calculator: [link]
ND Filter: [link]
Photo Tools: [link]
The Photographer's Ephemeris: [link]
TimeLapse Helper: [link]
Clear Outside: [link]
Exsate Golden Hour: [link]
OSMTracker: [link] (records gps signal and creates a GPX file which can be imported into LR and will automatically tag all your photos with the GPS coordinates. Handy for those of us that don't have a GPS unit and are too cheap to buy a dedicated one. Can also be viewed in Google Earth / Maps).
Smart Voice Recorder: [link]
Of course I don't mind ��