[Brightcolours]

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.

Of course you do not need to stack them back to back. You do when you have a CIRCULAR pol filter in front, as that one is "directional". But if you have one linear pol. filter, it goes in front of the circular one, and you can just stack them. You can also of course stack two linear ones. Or do silly stuff and use two linear ones back wards, or back to back, or front to front  :ph34r:

 

You can test if a pol. filter is linear or not my holding it in front of a LCD screen backwards and rotate it. If you can change the amount of light coming through (and even black it out), it is a linear one. If it does not allow that, it is a circular one.

 

My guess is that your linear one actually is not linear.

 

What happens with a linear pol. filter is that light waves with a certain polarization (direction of the wave) don't fit through the slits of the pol. filter film. If you turn the slits, that light can get through. If you put one pol. filter with slits in a horizontal direction, and one behind or in front of it in vertical direction, all light will be blocked. If you adjust the filter in front, you can vary the amount of light coming through. If you put the slits of both filters in the same orientation, you get the most light through.

 

This is exactly what the variable ND filters are, 2 pol filters. 

 

In the SLR AF age, they added a layer behind the pol. film because when filtering  a certain direction of pol. light under certain conditions the AF sensor might not see anything anymore. So, the added layer changes the let through polarized light's wave orientation again, it turns the direction of the waves. 

 

One can use linear pol. filters just fine on (D)SLRs, by the way, just be aware of that in certain rare situations your AF might not focus lock. Good linear pol. filters give better image results than good circular pol. filters.

 

As for ND filters, I have a 9 stop Hoya HMC ND400 which has no colour cast, and a Marumi ND8 (3 stops) which has no colour cast either. I actually chose the Hoya ND400 after reading some tests which found it was neutral (no colour casts to speak of), and it is very decently priced (and does not introduce unsharpness issues). The Marumi is very good too, and very inexpensive.