[Studor13]

You can get the same effect by mounting a high quality linear pol. filer on a linear (or circular) pol. filter.

 

I don't know how true this is but I am absolutely certain that you can't just stack them onto each other. Apparently, you need to stack them back to back. So your first problem is to get a female to female or male to male filter ring. Good luck trying to find one.

 

I have a linear polarizer and have tried to achieve this variable ND filter effect by screwing in a CPL onto the lens and holding Linear one backwards with my hand. It didn't work for me. Maybe I needed to have held them in a different combination. Don't know really but I'm not going there again.

 

As far as brands go for standard NDs, I have a 3 stop and 5 stop Hoya, and a 10 stop Heliopan. Heliopan is good. Hoya leaves a blue cast. And with a CPL which is around 2 stops, I am basically covered. For example, if I drop my ISO to -1, I have 12 stops with the Heliopan 10 stop filter. Or if I bump the ISO to 400 the 10 stop filter becomes an 8 stop filter. And so on.

 

Filters are not big to carry. You can easily pack 6 filters in one small case.

 

Sometimes when you want an exact exposure at an exact time of day you need a certain ND filter. In the shot below I needed around 5 minutes but it had to be at time when the full moon was in a certain position above the mountains. For this type of shot you need a 3 stop ND.