I have a .9 (3 stops) ND filter, but I'm finding them not dark enough when shooting long-shutter waterfall shots. To get 1" shutter speeds using my .9 ND filter, I'm having to use ridiculously narrow apertures like f/22 and up, which gets the shutter speed I need, but results in serious softness. The lighting conditions are bright diffused late afternoon sunshine and I don't have the advantage of nice shaded waterfalls here.
I've noticed that there are darker ND filters: 1.0/1.2/1.5/1.8, which is a good number with out getting too dark in late afternoon bright sunshine? Forgive the question, but would stacking two .9 ND filters be at all efficacious?