ND Filter Exposure Calculator

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Setup preset:

Start with a familiar shot, then replace every value with the current meter and target setup.

Exposure job:

Still target Video angle

Choose the exposure value you need to preserve after fitting the external ND.

Metered shutter:

Enter seconds: 1/2000 s is 0.0005, 1/125 s is 0.008, and 2 s is 2.

Metered aperture:

f/{{ formatNumber(current_aperture, 1) }}

This is the aperture used for the unfiltered meter reading.

Metered ISO:

Keep aperture, shutter, and ISO from the same metered exposure.

Target aperture:

f/{{ formatNumber(target_aperture, 1) }}

A wider target aperture usually requires more ND in bright light.

Target ISO:

Changing ISO changes the exact stop difference rather than only the display label.

Target shutter:

Use the shutter time that produces the intended water, cloud, traffic, or subject motion.

Frame rate:

fps

24 fps and a 180° shutter produce approximately 1/48 s per frame.

Shutter angle:

{{ formatNumber(shutter_angle, 0) }}°

Exact°

The target shutter is angle ÷ (360 × frame rate).

Internal ND already in:

stops

Leave at zero unless filtration is already in the target camera light path.

Physical filter increment:

Third- and half-stop settings suit fine control; full stops align with many fixed filter kits.

Exposure bias:

Positive values add filtration; negative values leave more light. The default 0 does not alter the meter match.

stops

Usable variable ND range:

This range changes only the variable-filter fit row; it does not alter the exact exposure calculation.

MinMaxstops

Comfort shutter limit:

The default 30 seconds affects only the planning note in Shutter Ladder.

Metric	Value	Planning use	Copy
{{ row.label }}	{{ row.value }}	{{ row.note }}

Option	Setting	Stop error	Planning note	Copy
{{ row.label }}	{{ row.setting }}	{{ row.error }}	{{ row.note }}

Filter	Stops	Neutral shutter	Error	Planning note	Copy
{{ row.filter }}	{{ row.stops }}	{{ row.shutter }}	{{ row.error }}	{{ row.note }}

Neutral-density (ND) filtration reduces the amount of light reaching a camera so that aperture, shutter time, or ISO can be chosen for the image rather than only for brightness. A photographer may need a longer shutter to blur water, a wider aperture to reduce depth of field in daylight, or a video shutter tied to frame rate. In each case the filter supplies the light reduction that the target exposure settings require.

Stops are the common language for that change. One stop of ND passes half as much light, two stops pass one quarter, and three stops pass one eighth. The filter factor grows as 2 raised to the stop count, so ND8 means an 8× factor and three stops of reduction. Optical-density labels use another convention: about 0.3 optical density per stop.

Metered exposure: A shutter, aperture, and ISO combination that correctly describes the scene before external ND is added.
Target exposure: The shutter, aperture, and ISO combination wanted for motion, depth of field, or video cadence.
External ND: The remaining filtration needed after any internal ND and deliberate exposure bias are included.

All three metered settings must come from the same reading. Mixing the shutter from one meter reading with the aperture or ISO from another invents a scene brightness that never existed. The target settings may differ, but they must represent the intended final shot. For video, a 180° shutter angle corresponds to half of each frame interval, giving about 1/48 s at 24 fps and 1/60 s at 30 fps.

An exact stop result is not always a filter that can be mounted. Fixed filters come in discrete strengths, variable ND filters have declared working ranges, and filter stacks combine two pieces of glass. A practical choice may therefore be slightly brighter or darker than the target, requiring a small shutter, aperture, ISO, or exposure-compensation adjustment.

The calculation models exposure, not optical quality. Real filters can differ from their marked strength and may introduce color shift, flare, vignetting, reflections, or variable-ND artifacts. Confirm the final choice with the camera’s histogram, waveform, false color, or a test exposure under the actual light.

How to Use This Tool:

Keep one valid meter reading as the scene reference, then describe the exposure you want after filtration.

Choose a setup preset or replace every preset value. Enter one shutter, aperture, and ISO combination that represents scene brightness before the planned external ND; record any internal ND separately.
Select Still target and enter the intended shutter, aperture, and ISO, or select Video angle and enter frame rate, shutter angle, target aperture, and target ISO. Subtract internal ND and add exposure bias only when those adjustments are deliberate.
Choose the physical filter increment and enter the declared variable-ND range. Compare the exact result with the rounded pick, fixed filter, stack, and Shutter Ladder.

Interpreting Results:

Exact external ND is the mathematical requirement after internal ND and bias. Practical pick rounds that value to the selected full-, half-, or third-stop increment; its signed error shows how far the physical choice falls from the target.

A positive error means the filter is darker than the exact target. A negative error means it is brighter.
A negative raw requirement means the target already admits less light than the metered setup. Adding external ND would move it farther away, so revise the exposure settings.
A variable ND is in range when the exact requirement is equal to either endpoint or lies between them. Outside that range, use a different filter plan or change exposure settings.
A shutter marked beyond the comfort limit is a planning warning about the entered limit, not a camera or subject capability test.

Technical Details:

Exposure value (EV) provides one scale for shutter time, f-number, and ISO. Two setting combinations with the same EV represent the same modeled scene exposure. The stop difference between the metered and target EV values is the filtration requirement before internal ND and bias adjustments.

Formula Core

For aperture N, shutter time t in seconds, and ISO sensitivity S, the ISO-adjusted exposure value is:

EV = \log_{2} (\frac{N^{2}}{t}) - \log_{2} (\frac{S}{100})

In video-angle mode, frame rate f and shutter angle θ set the target shutter time directly.

t_{video} = \frac{θ}{360 f}

Let I be internal ND stops and b the signed exposure bias. The raw external requirement is preserved for the too-dark warning, while the usable exact result is limited to 0 through 30 stops.

s_{raw} = {EV}_{meter} - {EV}_{target} - I + b s = \min (30, \max (0, s_{raw}))

A physical recommendation rounds s to the nearest selected increment Δ. Filter factor, estimated transmission, and optical density follow from the unrounded exact requirement.

s_{pick} = Δ round (\frac{s}{Δ}) factor = 2^{s} transmission = \frac{100}{2^{s}} % OD = 0.3 s

Rule Core

Rules for practical ND filter options and shutter-ladder labels
Result	Selection rule
Practical pick	Nearest full, half, or one-third stop, according to the selected physical increment.
Single fixed filter	Common fixed-filter strength with the smallest absolute stop error.
Two-filter stack	Pair from the positive common-filter set up to 10 stops each with the smallest combined stop error.
Variable ND fit	Below range when exact stops < minimum, above range when exact stops > maximum, otherwise in range.
Shutter ladder	Within one-third stop is close unless the resulting shutter exceeds the entered comfort limit; otherwise the row is labeled brighter or darker by the sign of filter error.

Accuracy Notes:

Use a stable meter reading from the same scene and light. Changing clouds, artificial-light flicker, or reframing can invalidate the reference.
Check the camera manufacturer’s shutter conventions. A 180° angle is a common video guideline, not a mandatory creative rule.
Marked filter strength may differ from measured transmission. Test demanding color or exposure work with a histogram, waveform, or calibrated meter.
Stacking filters can add vignetting, flare, reflections, and color shift. Variable ND filters can become uneven near their limits.
Very long exposures also depend on sensor noise, subject motion, tripod stability, changing light, and the camera’s actual shutter limit.

Worked Examples:

One-eighth second to one second

A scene meters correctly at f/8, 1/8 s, and ISO 100. Keeping f/8 and ISO 100 while targeting a 1 s shutter lowers the target EV by three stops. With no internal ND or bias, the exact requirement is 3 stops. The matching fixed filter is ND8, its filter factor is 8×, optical density is 0.9, and estimated transmission is 12.5%.

References:

The Joy of Long Exposure Photography, Nikon USA.
0.3 Neutral-Density Filter Specification, The Tiffen Company.
Movie-recording basics, Sony.