Photography Calculators

Exposure & light

ND Filter Exposure Calculator

An ND is just stops of time. Ten stops turns 1/250 s into about four seconds; 16 stops turns it into minutes. Work the unfiltered shutter first (spot-meter or sunny 16), then stack the marked density. Always take a test shot — cheap NDs are not the stop they claim.

Inputs

Units
s

Metered time without the filter. 1/250 is 0.004.

stops

ND1000 is 10 stops, ND64 is 6, ND8 is 3. Stacks add.

Results

Filtered shutter
4.096s
Filtered shutter
0.068min
Filtered shutter
0.0011h

Recommended gear

Calculation method

Each ND stop doubles exposure time. New shutter is the unfiltered time multiplied by two raised to the ND stop count. Ten stops is ×1024, so 1/250 s (0.004 s) becomes 4.096 s. Minutes and hours are that same time divided by 60 and 3600 so you can set a bulb timer without doing the division on a wet rock.

Exact formula

t_new = t_base × 2^(ND stops). Minutes = seconds/60, hours = seconds/3600. Limits: Marked vs measured density, colour cast, vignetting and film reciprocity failure omitted.

Video 180° shutter

Daylight at 24 fps wants ~1/48 s. That is often 6–10 stops of ND, not a long exposure. Use this to size the ND, then set shutter angle on the camera.

Bulb and intervalometers

Past 30 s you need bulb. An intervalometer or camera app that times bulb is safer than holding the button for four minutes.

Frequently asked questions

Do stacked NDs add or multiply?

Stops add. 6 + 3 = 9 stops, which is a 512× time factor. Two filters also mean two extra glass surfaces and more flare.

What about variable NDs?

Treat the ring marking as a guess. Cross-polarisation at the long end eats stops unevenly. Meter through the filter or chimp a test shot.

ISO while the ND is on?

This tool only scales shutter. If you also change ISO or aperture, use exposure-shift after you have the filtered time, or fold those stops into the ND field.

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