The now seem to be a range of extreme ND filters available (for those times you want exposures in seconds or even minutes to average out images)
The ones I know about are:
B&W110 (about 7 stops)
Hoya ND400 (about 8.5 stops)
Lee big stopper (10 stops)
All costing over £50 - far too much for the number of times I'm likely to use them!
An alternative I'm planing to try is to use welding glass, which is generally available in 'shade 8' (10 stops) to 'shade 13' (15.7 stops).
For around £2 as a 4¼" x 3¼" plate (which just about fits in a Cokin P holder.
My initial filter is a shade 10 which works out at ~13 stops (ND7,500) - possibly a bit much for the lower light levels in the winter, so I've just ordered a shade 8 as well.
I've seen photos done this way that look OK, though they do introduce a green cast (solved with custom WB or PP). Other potential problems are stray light (especially with the Cokin mount) & possibly flaws in the glass.
I've recently checked the spectral curves of my standard ND filters. None of these block near IR at all (& absorbance is NOT uniform through the visible spectrum - in particular a peak absorbance ~450nm is roughly twice that either side).
If the photo grade high ND filters are similar this would cause problems with my camera which is fairly sensitive to IR, welding glass should block IR (& UV).
Has anyone any experience with this sort of photography?
Or suggestions in reducing stray light? (Insulating tape will do for starters, but doesn't suit a long term solution.)