ND filter · long exposure times
ND Filter Calculator
Meter the scene with no filter, pick your ND strength, and get the corrected exposure time. Or work backward: choose the look you want and see which filter gets you there.
Long exposure
Meter without the filter, then do the math here.
Set exposure with no filter on the lens and note the shutter speed. Each ND stop doubles the time. The calculator gives the corrected exposure, and the reverse section tells you which filter reaches a target look.
Corrected exposure
17 s
1/60 + ND1024, sold as ND1000 (10 stops)
Timer readout
00:17
Reverse: which ND gets you there
Seas and lakes flatten toward mist from about 30 seconds.
You need about 10.8 stops
ND2048
1/60 through 11 stops lands at 34 s.
Field notes: recheck the meter right before fitting the filter (light drifts fast at the edges of the day), cover the viewfinder eyepiece on long exposures, and expect roughly a stop of extra exposure error from cheap filters that run darker than labeled.
How to use it
Step 1
Meter without the filter
Compose, set aperture and ISO, and note the shutter speed the camera meters with no filter on the lens.
Step 2
Pick the ND strength
Choose the filter you are fitting, from ND2 (1 stop) to ND65536 (16 stops), including common stacked pairs.
Step 3
Read the corrected time
The result is the new exposure time. Above 30 seconds, switch to bulb mode and time it against the countdown readout. The reverse section shows which ND reaches a target look instead.
Frequently asked questions
How does an ND filter change shutter speed?
Each stop of neutral density halves the light, so the shutter has to stay open twice as long for the same exposure. The math is base time multiplied by 2 to the power of the filter stops: 1/60 s behind a 10 stop ND1000 becomes about 17 seconds.
Are ND2, ND8, and ND1000 numbers the same as stops?
No. The ND number is the light reduction factor, and stops are its base-2 logarithm. ND8 passes 1/8 of the light, which is 3 stops. ND1000 is marketing shorthand for ND1024, which is 10 stops.
Why does the calculator warn above 30 seconds?
Most cameras cap timed exposures at 30 seconds. Beyond that you need bulb or time mode, a remote release or the self-timer to avoid shake, and you time the exposure yourself, which is what the countdown-style readout is for.
Can I stack ND filters?
Yes, and the stops add: ND8 plus ND64 is 3 + 6 = 9 stops. The tradeoffs are extra glass (more flare and a possible color cast) and vignetting on wide lenses, so one strong filter usually beats three weak ones.