Heya, [IMAGE'S LINK: https://flic.kr/p/qDZHS3]IMG_3382
Eh, I don't think it's necessary at all if your camera has live view. You can see the histogram real time. Even with the filters on. I use my Big Stopper on my setup and just throw it in live view and see my exposure histogram and go from there. Once I hit 30 seconds, I can do the simple math in my head if I need to go longer. But I'm often trying to stay at 30 seconds to keep it simple so I have less gear.
When I'm doing as long as 8 minute exposures, I use an intravalometer of course, but I don't need a light meter. I can do basic math in my head and count stop on my fingers like any kid can (not trying to be snarky, but it is that simple). So I go a head and meter my scene with the live view histogram and get it where I know I want it based on that histogram (I tend to gently over-expose a touch, using RAW, but avoid clipping highlights, to make shadow recovery cleaner). Whatever I meter my settings to, I just consider what the shutter will be once I throw on my 10 stop filter, and 3 stop grad filter (sky). 10 is an easy number to do in terms of shutter. Or you can just literally count it out. Double the shutter for each stop and just count out 10 stops. I often go for an easy to calc shutter so I can quickly gauge out 10 stops.
I tried using an app to calc exposure for long exposure with ND, but at the end of the day, I'm faster with counting stops in my head. A stop is a stop no matter how you achieve it. 10 stops is super easy, it's not even something you have to count for shutter, but if you're getting different with your aperture, well, that's one stop each time too, so again not a big deal. You can count it out. So I do everything without a calc and without a light meter. I use the live view histogram and count stops in my head on my fingers. Seems to be working fine for me.
When using a 10 stop filter, it's easy. If you are at 1/30s before the 10 stop filter, you're at 30 seconds after the filter. You don't even need to do math. That's the beauty of using 10 stops.
Here's my filter-less image (I like to do before / after to see the difference). I went to ISO 100 for obvious reasons. F11 for DOF purposes, no other reason. So the only real setting I'm using is shutter speed. And I moved that around until I saw my histogram roughly rounded in the middle and then pushed it a touch to the right. This gave me a shutter of 1/60s.
, on Flickr
Then I wanted to throw on my 10 stop filter. So I knew my shutter would go to 15 seconds. But also I can count the stops (1/60s -> 1/30s -> 1/15s -> 1/8s -> 1/4s -> 1/2s -> 1s -> 2s -> 4s -> 8s -> 15s on my fingers). I like to gently expose to the right especially when using the 10 stop filter, because the exposure will not be exactly 10 stops because light will change over that time a little bit often. So I tend to go a bit longer by a stop. So I went to 30 seconds, which is just 1 stop more than the metered 15 seconds.
And here's what I got with the 10 stop. I also had a 3 stop grad ND for the sky (soft edge) which is what I always use. For early morning skies, I find 3 stops is sufficient to even it out. For bright days or sun involvement, I find 6 stops is the starting point. But I don't even bother calculating the sky. I know it's going to be 3 stops or 6 stops for me, from the foreground exposure, or at least close enough to be ok with it. [IMAGE'S LINK: https://flic.kr/p/ryGNCo]IMG_3381
, on Flickr
And same process here from the same day. I already knew my settings, so I didn't need to keep redoing the stops. I opened my aperture a little to get more light because I noticed it got a wee darker between some clouds moving. And just kept exposing. You only have about 30 minutes in the morning and evening I find to get that light. [IMAGE'S LINK: https://flic.kr/p/qDZKwL]IMG_3377
, on Flickr