What I am trying to do is test the accuracy of the ND filters. I started with no filter and then shot the same still with each of the ND8, ND16 and ND32, manually moving the stops to counter act the filter to see if the exposure would remain constant. All 4 photos were shot within a 10 minute window at dusk.
With no filter, the EV was -1.3 ISO100
ND8 EV -.7 ISO 800
ND16 EV -1 ISO 1600
ND32 EV -1.3 ISO 3200
Does this mean that the ND8 and ND16 filters are not true or could there be other factors? Cheers.
ND filters are actually very simple once you learn the terminology:
ND2 = 1 stop reduction
ND4 = 2 stop reduction
ND8 = 3 stop reduction
ND16 = 4 stop reduction and so on
All else equal (ISO and Aperture), every step up in ND filter will half your shutter speed.
For example if the exposure of the scene is 1/1000, F2.8, ISO 100 with a naked lens, leaving aperture and ISO alone:
ND4 will drop you to 1/250 (4 times slower)
ND8 will drop you to 1/125 (8 times slower)
ND16 will drop you to 1/60 (16 times slower)
And so on and so on.
You need to set up a controlled scenario if you want to test the accuracy of the filters - pick only one variable in the exposure triangle to manipulate.
On your Mavic pro, you also have aperture to play with, which changes things - you can get away with far fewer ND filters because you have another variable at your disposal - just don't go beyond about F4 as diffraction will begin to degrade the image at really small (high F number) apertures.
The other way you can test it is leave the exposure alone and just change the filters in manual mode - you should see the EV drop by the exact same ratio as above (naked lens at 0.0EV should show -2.0EV with a ND4, and so on - you will probably hit the limits of the meter before you run out of ND filters though).