ND filters to slow down exposure - help

[Bob Salomon]

If you stop down to f22 (from sunny 16), the speed changes to 1/30, not 1/15, so the thing ends in 4s with ISO 50 and a Yellow

And you can also add exposure compensation on bellows extension... Anyway if stacking 3 ND filters + Yellow better if they are of top quality, Hoya HD3 or the like, or using a ND32 or ND64, IMHO 4 stacked filters are a lot of filters.

And, at small apertures you are into diffraction. That plus the questionable quality of many ND filters does not get you the best results.