Ivory And Ebony
The name that is on everyone's lips is Big Stopper - it is the catchy tag for the Lee company's 100mm x 100mm resin filter. It fits into their standard holders and drops 10 stops of light. I wish I had invented the name Big Stopper - it is the sort of thing that you can bandy about at a camera club meeting and sound really cool.
Oh, would that you could go into a shop and buy one. Like into our shop, for instance. Because of their great popularity and the drought that has been affecting the English filter fields...we never seem to have enough of them delivered to satisfy the clubmen. Please do not think that I am criticising English manufacturing practice - I am sure that if you wanted a Quad amplifier or a Manton shotgun you could find them at any corner store...
But for whatever reason, the Lee filters are hard to get. We have found another good answer for the landscape photographer. B+W make 1000x filters in screw-in sizes that will do the same job as the Big Stopper. Kenko make an ND 400 that gives you 9 stops of darkening. And Promaster make a wonderful variable neutral density filter that looks as though it would do 1 to 10 stops. To prove it to myself I put one over a light box at the two extremities of adjustment - have a look at how dark it gets.
The video people can also use this sort of a variable filter to do fade-outs at the end of video shots.
I should purchase one in the largest size lens that I use, then adapt it to smaller lenses with simple step-dow rings. The whole deal would be cheaper than Lee, but don't let that influence you.