I went out last night looking to create some images of Tacoma, WA in the dark. I haven't really explored much in Tacoma so this was a new treat. I started out in downtown and walked around with the camera looking for a shot.
I liked this hill but there wasn't much traffic to capture. I setup anyway, framed it, got my focus, then set my base exposure, the longest I could get was about 1 second without totally blowing out the highlights. I did the math, and my 3-stop ND filter would only get me to about 8 seconds, no good.
So I pulled out my shiny new 9-stop ND filter, spun it on, opened up the aperture a touch, did the math and wound up with about 6 minutes, thats in the ballpark. The first image was a bit too dark for my liking, so I let it run for 8 minutes the next time. It was still a touch dark, but the highlights were a bit more blown out than I like, now I just needed the luck of traffic on my side. I closed down the aperture by 1/3rd stop then set the shutter to 10 minutes and created the image above. The traffic worked out nicely and my highlights mostly survived intact.