So what is the rule of thumb on when and what ones to use
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Was thinking the same yesterday.
More a case of do you need them
Lots of arguments either way, itâs a good start to know what they do, then you can answer.
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Basically they reduce the amount of light entering the sensor.
In stills photography this allows you to either use a wider aperture, narrowing the depth of field (how much of the background is in focus) or use a slower shutter speed to induce motion blur (think milky moving water or blurred moving subjects).
Not sure about the advantages in video so now I am listening!
UV filters donât have a huge effect on digital but I always use one on my DSLR lenses. Why? They are a cheap (ish) way to protect the glass of the lens.
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In a nutshell to get the shutter speed as close to double the frame rate as possible.
The influencers and film makers would tell you that the 25fps/50th second is the closet to cinema feel you are going to get.
It does work, as for the advantages on still photography I donât buy into it
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In what way?
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As Chris says the best thing you can do is watch some YouTube videos for an indepth explanation and then decide if they are something you need or want to use. Essentially they are sunglasses for your camera lens allowing you to decrease your shutterspeed to create motion blurr. Depending on if you are making video or still photos you use them differently. For still photos you can use them in higher light levels for long exposure photography to capture motion in running water for example.
I havenât made any videos yet but from what I gather you use them to match your shutter speed to your frame rate which gives your video a more natural motion blurr. There are rules to follow. The shutter speed should be equal to double the frame rate (25fps = 1/50 second shutter speed or 30fps = 1/60 second shutter speed). Below is an example of the effect. Its called 180 degree rule if you want to Google it.
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All the things you mention I can pretty much do with a half decent camera.
In bright sunlight you canât get a 1/2 s exposure to blur motion.
Even putting the ISO as low as it will go on my 60D I sometimes need ND in the summer
Decrease the shutter speed.
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Sorry I meant increase the time the shutter is open, my bad lol edited to decrease shutterspeed
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Never had the need to.
My background is in sports photography where generally Iâll want as much light in as possible, I can motion blur in other ways without sticking an inferior piece of glass in front of a very expensive one.
I see it for video, just (especially in our climate) for stills I donât.
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Ditto.
And then you see people saying in one post they always shoot 60fps so that they can get quality slow-mo, and in the next post that they always use ND filters for the âcinema effectâ. This is a load of tosh, because if you are using 60fps and the 180 rule, youâll have a shutter speed of 1/120 sec ⌠and you wonât be getting much cinema effect at all. 
Also, in most drone videos, one should be looking for gentle pans/tilts and one is too far from anything moving sufficiently fast across the image to really benefit for the 180 rule, even with correct settings.
The biggest issue I have with ND filters is that I often shoot stills and video in the same flight ⌠so one of them will be compromised without multiple flights and Fiddly-A-F filter removal/replacement. I tend to prioritise the stills.
So - unless there is something really special that deserves correct use of the 180 rule (ideally 24 or 25fps, max 30fps) I never bother with an ND filter.
Also remember, you donât have variable aperture on the MA, so the ability to use ND filters to modify that isnât something relevant to your drone.
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That is fair enough Chris, but that is not to say that others shouldnât use them. I have used them for landscape photography e.g. waterfalls, to make the water silky smooth or to blur the clouds moving across the sky. Also, when you see e.g. looking down a pontoon into a lake with an etherial feel to the water. That is not a shot that could be captured without an ND.
I agree with you that ND filters do not give you a âcinematic effectâ and presume that what people mean by that phrase is injecting a little motion blur into the video. What will give you the cinematic effect is a combination of shutter speed and aperture - sometimes this can only be achieved by cutting the amount of light entering the camera which might need the use of NDâs.
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And there is the difference.
I like slow shots of water.
I quite often want shallow dof with a slow shutter.
There are times when I want the shot but canât wait until its getting dark. Or the shot isnât available at lower natural light levels.
Horses for courses 
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I donât think anyone would suggest they are useful for drone stills? I guess only if you were doing a hyperlapse which is video anyway.
I was talking photography in general. But you can still take shots of moving water with a drone.
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I like the milky flowing water look and so, with the M2P gimbal able to hold still for up to 2 seconds, I am able to get into a position with a drone above a waterfall, not otherwise achievable without one, and get that look with the use of an ND filter.
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Fixable in post ;o)
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Chris, I meant getting into position over a waterfall 
.
I realise that many things are fixable in post, but the more you get right in camera (whether a drone, phone or Nikon D750), the more natural the image looks and less work is required in post.
Personally I am not a fan of coloured filters or special fx filters, but I know some people enjoy using them. I do use Polarising filters, ND grads and NDâs when they are required to achieve the look I want in my Nikon photography, so donât see any reason not to use in my drone photography when time (and battery levels) allow.
I suppose that itâs all âhorses for coursesâ and down to whether people want to be bothered to go with the expense and extra âworkâ required.
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A useful app to use the correct filter!
https://play.google.com/store/apps/details?id=au.com.nisifilters.ndhelper