ND Filter Exposure Time
Long-exposure time behind an ND filter from the metered shutter and the filter in stops, factor (ND64) or density, with stacked filters and film reciprocity.
The shutter time with the filter, the filter strength in stops, factor and optical density, the total for stacked filters, and — for film — the time after a reciprocity correction from the datasheet.
Example: Metered at 1/125 s, a 10-stop filter needs 8.2 s; an ND64 plus a 3-stop filter turns 1/60 s into 8.5 s.
Two to the
stops.
How ND strength is labelled three ways, how the time multiplies, what stacking does, and why film needs more than the arithmetic.
Three labels, one strength
Filters are sold by stops (10-stop), by filter factor (ND64 passes 1/64 of the light) or by optical density (ND1.8). They are one number: stops = log₂(factor) = density ÷ 0.301. ND64 is 6 stops, density 1.8, factor 64.
The time
Meter without the filter — through a 10-stop filter the camera often cannot — then multiply the shutter time by 2^stops: each stop doubles it. Stacked filters add their stops. Past 30 s most cameras need Bulb mode and a remote.
Reciprocity
Film loses sensitivity in long exposures, so the arithmetic time underexposes. The Schwarzschild correction t^p, with the exponent p from the film’s datasheet, is applied above one second; digital sensors need none, so leave p at 0.
What the number does not settle
Strong filters add colour casts and let light leak through the viewfinder; the time is right, the white balance and the eyepiece cover are yours. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.