I bought a Vario ND 2-400 filter today and tested it out. I think it goes up to +8 stops, and I was using it darkest to let less light in.
I used high f-stop, ISO 200, and up to 10 seconds. However, anything longer than ~5 seconds became mostly white, and I couldn't see anything.
Question: how do people do exposure times of something like 1 minute without everything become overexposed?