I wrote about that topic about 11 months ago for the first time, and it's time for an update. :) Today, I do not wait until after sunset. The time frame to make long exposures then is just too short. Instead, I'm using a 6-stop and a 10-stop neutral density filter, often in combination with a polarizer (another 2.5 stops).
The best conditions for long exposures are actually overcast days (cloud movement looks really cool too!). There's enough light to make it easy to set up the camera, and in the 30-60 minutes before sunset and after sunrise, the light is not as strong and you can get some rrrreally long exposure times with ND filters. This is a 90 second exposure at ISO100, f/16, with a 6-stop filter, made 15 minutes before sunset:
There are a couple of challenges with long exposures, and I want to describe the methods that I have developed to comfortably work around them in this post. Quite naturally, there are a number of different approaches to tackle these challenges. My methods may not be the most efficient, or easiest, or most straightforward for anyone else but me. I just like to share them for the sake of sharing. :)
The two most obvious obstacles are composition and metering. Obviously, I compose without the filter on - with a 10-stop ND, you hardly see anything in the viewfinder. Live view might work even with the filter on, depending on the type of camera you use (for example, I've heard that Canon has better gain than Nikon).
When I began working with long exposures, I also metered without the filter and then either did the math, or used the "PhoForPho" Android app to calculate the correct exposure. I don't do that anymore though, because the precise calculations often do not match the "filter reality" (a 10-stop filter might not be exactly 10 stops, but maybe 9.5 or 10.5 stops - and at the exposure times I'm aiming at, 1/2 stop can be quite a difference - if you're at 2 minutes and need 1/2 stop more, that's 1 more minute!).
Instead, I put the filter in place once I have my composition, and set the camera to a really high ISO (depending on the situation, that could be ISO6400, for example - 6 stops faster than ISO100) and do a test exposure - hey, it's digital, lets utilize it's advantages, shall we? :) Important at that point: cover the viewfinder, or the meter readings will be way off. WAY off.
I aim at relatively short exposure times for the test exposures because I only look at my histograms. It might be necessary to open up the aperture as well (which means I might be using f/4 for my test exposure instead of f/11). I often do a couple of short test exposures that way.
Once I found the best exposure setting, I first set the target aperture and adjust the exposure time accordingly (3 clicks on the aperture dial equals 3 clicks on the exposure dial), then I slowly work my way down with the ISO. I skip three stops - 9 clicks - down at once, then adjust the exposure time. In between, I sometimes do another test exposure, like this one, at ISO1600 (1 second):
Eventually, I reach the 30 second limit of the camera and have to switch to "Bulb" mode - but from that point on the math is fairly easy. If I'm at 20 seconds and ISO1600, then the next stop (ISO800) is 40 seconds, ISO400 is 80 seconds, and ISO200 (the base ISO of the D700) is 160 seconds, or 2 minutes and 40 seconds. Easy enough even for a math dork like me. :)
Then all that's left is to dial in this exposure time in my timer-remote, and press the "Start" button. :) I'm using a 3rd party remote (I believe it's from a company called JJC) since I just didn't see myself programming a timer/alarm on my phone every time and keep watching the camera. No no no. Things should remain simple, and programming a timer release that controls the camera is simply the easiest. And that's it!
The advantage of this method for me is that I can NEVER forget to lower the ISO after metering. Been there, done that. 12 minute exposure at ISO6400 instead of ISO200. Quite a lovely result, really. ;-)
Oh and, one of the problems that I haven't found a working solution for is using the polarizer, or rather, seeing when it is actually reducing glare. I'm using a thin mount polarizer to minimize vignetting on the wide end of both the 16-35/4 and the 24-120/4 so it has to be the last filter when stacking, and in combination with a 10-stop ND, it's quite hard to see when it's rotated to the right position.
