If you have two polarizers of the same size you can experiment crossing their axes close to, but not quite, 90 degrees. That gives you a great deal of neutral density cheaply. I did this to photograph a nearly total eclipse of the sun with a 300mm lens on a Nikon loaded with Kodachrome 25 in the 1990s. It worked well.
You’ll have to experiment with a meter, such as that in an SLR, to determine precisely how many stops of light are held back by the pair of crossed polarizers.
When you find the setting you want, you might want to draw a diagram of the relationship between the polarization axes indicator marks of the two polarizers so that they can be positioned in the same way relative to each other on the lens you use.
Even if you don't cross the axes, the combination of the two polarizers will hold back about 10/3 = 3.3 stops lengthening the exposure by at least 8X.
Last edited by Ian C; 01-01-2012 at 09:07 AM. Click to view previous post history.