ND Filter Calculator
Calculate nd filter from the published formulas and tables for the craft.
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What this tool does
Calculate nd filter from the published formulas and tables for the craft. Each stop halves the light, so the exposure time doubles per stop. ND1000 filters are sold as 10-stop filters although their nominal factor is 1000 rather than exactly 1024 — expect about 4 s from a 1/250 s base (exactly 4.096 s by 2^10).
How to use the ND Filter Calculator
- Enter or select base shutter speed (seconds, e.g. 0.004 = 1/250).
- Enter or select nd filter strength (stops).
- Read the calculated result; change any measurement to compare alternatives.
Formula
new shutter speed = base shutter speed × 2^stops; filter factor = 2^stops (an ND1000 is nominally 10 stops, factor ≈1000)
- base
- Base shutter speed (seconds, e.g. 0.004 = 1/250)
- stops
- ND filter strength (stops)
Each stop halves the light, so the exposure time doubles per stop. ND1000 filters are sold as 10-stop filters although their nominal factor is 1000 rather than exactly 1024 — expect about 4 s from a 1/250 s base (exactly 4.096 s by 2^10).
Worked example
For nd filter calculator, the following measurements illustrate the exact method: Base shutter speed (seconds, e.g. 0.004 = 1/250): 0.004; ND filter strength (stops): 10.
Inputs
- Base shutter speed (seconds, e.g. 0.004 = 1/250)0.004
- ND filter strength (stops)10
Result
- New shutter speed (seconds)4.1
- New shutter speed4.096 s
- Filter factor (×)1,024
- Equivalent ND number (approx.)1,024
Results explained
- New shutter speed (seconds)
- New shutter speed (seconds) from the formula above. Each stop halves the light, so the exposure time doubles per stop. ND1000 filters are sold as 10-stop filters although their nominal factor is 1000 rather than exactly 1024 — expect about 4 s from a 1/250 s base (exactly 4.096 s by 2^10).
- New shutter speed
- New shutter speed from the formula above. Each stop halves the light, so the exposure time doubles per stop. ND1000 filters are sold as 10-stop filters although their nominal factor is 1000 rather than exactly 1024 — expect about 4 s from a 1/250 s base (exactly 4.096 s by 2^10).
- Filter factor (×)
- Filter factor (×) from the formula above. Each stop halves the light, so the exposure time doubles per stop. ND1000 filters are sold as 10-stop filters although their nominal factor is 1000 rather than exactly 1024 — expect about 4 s from a 1/250 s base (exactly 4.096 s by 2^10).
- Equivalent ND number (approx.)
- Equivalent ND number (approx.) from the formula above. Each stop halves the light, so the exposure time doubles per stop. ND1000 filters are sold as 10-stop filters although their nominal factor is 1000 rather than exactly 1024 — expect about 4 s from a 1/250 s base (exactly 4.096 s by 2^10).
Frequently asked questions
Multiply your metered (no-filter) shutter speed by 2 raised to the filter's stops: new time = base × 2^stops. A 1/250 s base with a 10-stop ND1000 gives 0.004 × 1024 ≈ 4.1 s — the familiar 4-second result on ND charts.
ND8 is 3 stops (factor 8), ND64 is 6 stops (factor 64) and ND1000 is 10 stops (nominal factor 1000, 2^10 = 1024). In optical-density terms each 0.3 of density is one stop: ND 0.9 = 3 stops, ND 1.8 = 6 stops, ND 3.0 = 10 stops.
The table doubles your entered base shutter speed stop by stop from 1 to 10 stops, formatted the way a camera displays it (fractions below one second, seconds and minutes above), so you can read off any filter you own without re-entering values.
Real ND filters vary a little from their rating, strong filters add a colour cast, and reciprocity and meter error creep in at multi-minute exposures. Bracket around the calculated time, and for bulb exposures beyond a minute or two treat the result as a starting point.
No — this calculator holds aperture and ISO fixed and compensates with shutter speed alone, which is how long-exposure work is normally planned. If you also change aperture or ISO, meter the new base exposure first and then apply the stops.