ND Filter & Long Exposure Calculator

The arithmetic assumes the filter is neutral, which no filter quite is — strong ND glass usually casts warm or magenta and wants correcting afterwards. On film there is reciprocity failure too, where a metered ten-second exposure needs considerably more than ten; digital sensors are effectively free of it. Nothing is uploaded.

An ND1000 is ten stops — and it is really 1024

Three naming systems describe one filter and none says "stops" out loud. The ND number is the divisor: an ND8 passes an eighth, which is three stops. The density is the base-ten log of that divisor, so the same filter is also an ND0.9. Converting between the last two is one constant — density × 3.3219 is stops — which no filter ring prints.

Up to ND256 the name is the divisor exactly. Past that they are marketing roundings, and all 4 of them are off by the identical -2.344% — because each one is the same substitution of 1000 for 1024. That is the very swap that puts 1/125 on the dial where 1/128 belongs, and in both places it is 0.0342 stops: nothing at all, and worth knowing about because it explains why two systems that ought to agree are always a hair apart.

Stacked filters multiply — an ND8 on an ND8 is an ND64

Stops add, so divisors multiply. Two ND8s make an ND64 and six stops, not the ND 16 and four that adding the numbers suggests — a shortfall of exactly two stops, and it widens from there. Two ten-stops stacked are twenty stops and a reduction of over a million to one.

What that does to a metered 1/125 is the whole point of the filter: ten stops turns it into eight seconds, and sixteen stops into nearly nine minutes. Each stop doubles the time, so the last few cost far more waiting than the first few — the step from thirteen stops to sixteen adds more than seven minutes on its own.