This may be a long response, so apologies if it is.
tbh I cannot think of a scenario where I would ever be away from base ISO when using a stopper, not that it matters.
When using any ND filter you increase the exposure time by the number of stops. ISO is irrelevant here, by that I mean if yo are shooting at ISO 100 and you require 1 second to get the desired exposure without any filters and you then add a 2 stop ND, your exposure time doubles twice, so becomes 4 seconds. If for example you are shooting at an ISO of 8000 and you require 1 second to get the desired exposure without any filters and you then add a 2 stop ND, your exposure time doubles twice, so becomes 4 seconds. So ISO is irrelevant - well other than setting the initial exposure time - but irrelevant in calculating how much longer the exposure time is when adding the filter.
So when using any ND filter, get the exposure time required at whatever ISO you want and then double the exposure time for each stop of the ND filter. If I am using ND filters (and I do
) I am always at base ISO so I make sure that is set first.
For a 2 stop ND double the time twice. e.g. 1 sec becomes 4 seconds
For a 3 stop double the time three times. e.g. 1 sec becomes 8 seconds
For a 6 stop (called little stopper in Lee) double the time 6 times. e.g. 1 sec becomes 64 seconds (though on the Lee app it states a minute ...)
For a 10 stop (called big stopper in Lee) double the time 10 times. e.g. 1 sec becomes 1024 seconds (though the Lee app states 16 mins).
However, once you start to get to higher number of stops (e.g. 6, 10, 15 etc) the timings get less critical and may vary from filter to filter too. In the case of Lee the filters are hand made so there will always be slight variations with the denser, longer NDs. It is just a matter of starting with the maths and tweaking if required.
I don't know the nisi app but it is likely that the NISI app has two exposure times, one for without, one with. Set the exposure time for the one without, the filter density if that is setable (it is on the Lee app) and the extended exposure time should be displayed. The Lee app is really easy to use and will work for any 6, 10 or 15 stopper. Google it if you want it (app stores) I think it is still free.
When using a stopper, excluding ICM, you really need a stable tripod. The 'pod is more important than any form of cable release or other such device. On longer durations (anything over a minute really) any initial movement/vibration caused by mirror slap or pressing the shutter button won't be seen in the final image. However if the 'pod is unsteady throughout the shot then everything will be blurred including our intended subject. It won't matter if the intended subject is the sea for example but in most scenes there is usually something that is in sharp focus (rock formation, bridge, tree etc ...). However I will still use a remote release and mirror up and electronic front curtain to avoid any movement
You will not be able to compose the image with the stopper attached so you need to get that all done before attaching the stopper. SO, compose the scenes, note the exposure time, attach the stopper, calc the extended time, set the timer or use bulb and shoot. I usually take a shot without the stopper attached and check the blinkies, if it blows without a stopper it will most likely blow with one! So any adjustments should be made be that adding a grad or reducing the exposure time or living with whatever it is that blows. I will sometimes use aperture priority to get the inital exposure time and take the test shot, switching back to manual (though sometimes I don't too
).
It is important that you cover the viewfinder if you have one, if not you will most likely get a light leak there and it will ruin the shot. It should also go without saying that you will be shooting in manual mode too when using the stopper.
Hope that makes sense and helps some.
