Thank you both to fill some holes in my knowledge swiss cheese
My question was not oriented this way but this is great Information. Is there a way to know what nd filter tout use to get to shutter speed to 1/60 when frame rate would be 30 fps? Or would it be better to play with fps to get to the correct ratio?
Ps I'd did not had the time yet to try again the event settings.
If you use ND8 (3-stop filter) then more likely 90% of the time you'll be just fine. My frame rate is set to 30 fps and it seems GoPro is always trying to take 1/60 exposure, it will automatically adjust the ISO (within your MAX allowance set in Protune) to achieve 1/60 exposure (when my frame rate is 30fps). Further below I'll try to explain why I think this is the case.
I'm in sunny Australia and ND8 occasionally is not enough.
(I'm actually using SRP Blufix air ND4/CP filter but because it's in combination with CP, it is effectively more or less ND8 filter).
If it's overcast or you shoot just before sunrise or just after sunset you may need to use ND4 instead of ND8.
GoPro has a fixed aperture of f/2.8 so it only has the ISO range to auto-adjust itself in order to achieve correct shutter speed (relevant to your fps setting). If you set in Protune your max ISO allowance of 400 then GoPro has a 2-stop range to play with and to match correctly with 1/60 shutter speed. As long as the lighting conditions are such that GoPro can fit within this 2-stop ISO range while keeping the shutter speed constant at 1/60, then you are save.
Because the aperture is fixed at f/2.8, you absolutely have to use ND filter if you want to keep shutter speed down at 1/60, without the ND filter in normal daylight your shutter speed would be say 1/250 or higher.
In normal photography with standard camera you can adjust your aperture to achieve desired shutter speed but with GoPro you cannot do that so instead we have to use ND filters. For example with standard camera when the light conditions require you to shoot at f/8 1/60 ISO100 with no ND filter, in terms of the amount of light hitting the sensor on GoPro this is equivalent to f/2.8 1/60 ISO100 with ND8 filter.
I've been collecting data on this since I got my Solo, but that's only 3 weeks. Before that I had a cheap drone that could not fly higher than 30 feet and had a very bad/cheap built in camera, Solo is my first real drone that allows me to fly with GoPro.
While I have not found any documentation on this, from my tests it appears that GoPro will always try to use shutter speed 1/60 (when shooting at 30fps), now of course I don't know it for certain because we have no EXIF/Metadata data to check when shooting video on GoPro but we can get EXIF data when taking photos - and what I found is that nearly all of my photos (when shooting with ND filter) were taken at constant shutter speed of 1/60, only ISO is not constant, ISO is different for every photo. I believe that somehow GoPro will try to use same shutter speed for photo that is relevant to whatever is my current video fps setting. The only time any of my photos were taken at shutter speed other than 1/60 was when I could even tell the light conditions were such that it fell outside the auto-adjust ISO range (either way too bright or way too dark).
I think it's a good idea to take few photos between your videos, you can then learn a lot from the EXIF data what works best for you in different scenery and different lighting conditions.
Just to give you an example, here are 3 photos (raw, not post-processed), we have no control of shutter speed settings but in each case GoPro took them at 1/60, but ISO was auto-adjusted by GoPro to allow for the constant shutter speed. From left to right, ISO from EXIF shows : 184, 248 and 139. On that last shot it was already near the limit, if the light conditions were only 2/3 of a stop brighter it would've already run out of the 100 limit and then GoPro would've already adjusted the shutter speed to faster than 1/60.
Also interesting to note that the 2nd and 3rd photo are from the same flight, as you can see GoPro is auto-adjusting for different values of ISO in order to keep shutter speed at 1/60 as you are moving around or tilting the camera which then results in different scenes.
I took another photo of the sunset (similar to the 3rd picture) but taken on the previous flight 30 minutes earlier when the sun was higher, hence light conditions shooting directly towards the sun were much brighter and there ND8 filter was not enough, my ISO was already down to 100 and GoPro "decided" to use shutter speed 1/120. In that scenario it would've been better to shoot at 60fps or use a different filter. In fact for such scenario (shooting bright sunset) I have a different filter SRP Blufix air graduated ND8-ND16 filter (top half of the filter is ND16 and bottom half of the filter is ND8).
