Free Film & Video Tool · Calculator
ND Filter Calculator
Convert ND filter strength between stops, filter factor (ND8, ND64…) and optical density (0.3, 0.9…), and work out the shutter, aperture or ISO change that keeps your exposure.
Deterministic maths that runs entirely in your browser — no AI model, no uploads, no account. Formulas and assumptions are explained below the calculator.
Inputs
Results update as you type.
Results
Stops
3
Filter factor
ND8
12.5% of the light gets through
Optical density
0.9
With the filter on and nothing else changed
The image is 3 stops darker.
To get the same exposure back, change ONE of these (others held constant):
- Shutter: 1/50 s → 1/6.3 s (long-exposure stills; changes motion blur)
- Aperture: f/8 → f/2.83 (nearest marked f/2.8) (shallower depth of field)
- ISO: 800 → 6400 (more noise; may exceed your camera's range)
Filmmakers usually do the opposite: keep the shutter (e.g. 180°) and the chosen aperture, and use ND to cut bright light so the image isn't overexposed.
How it works
ND (neutral density) filters reduce the light reaching the sensor without changing its colour. They're labelled in three ways — stops, filter factor (ND2, ND8…) and optical density (0.3, 0.9…) — and this calculator converts between them and shows the exposure change needed to compensate.
The relationships
- Filter factor = 2stops — each stop halves the light
- Stops = log₂(filter factor)
- Optical density = log₁₀(filter factor) ≈ 0.301 × stops, so 0.3 ≈ 1 stop
Common filters
| Marking | Stops | Factor | Density |
|---|---|---|---|
| ND2 | 1 | 2× | 0.3 |
| ND4 | 2 | 4× | 0.6 |
| ND8 | 3 | 8× | 0.9 |
| ND16 | 4 | 16× | 1.2 |
| ND32 | 5 | 32× | 1.51 |
| ND64 | 6 | 64× | 1.81 |
| ND1000 | 9.97 | 1000× | 3 |
| ND 0.3 | 1 | 2× | 0.3 |
| ND 0.6 | 1.99 | 3.98× | 0.6 |
| ND 0.9 | 2.99 | 7.94× | 0.9 |
| ND 1.2 | 3.99 | 15.85× | 1.2 |
| ND 1.8 | 5.98 | 63.1× | 1.8 |
ND1000 is often called “10 stops”; mathematically it is 9.97 stops. Density-marked filters are similarly about 0.3% off a whole stop each.
Which settings are held constant?
The compensation figures change one setting at a time and hold the other two fixed: a longer shutter (long-exposure photography, more motion blur), a wider aperture (shallower depth of field) or a higher ISO (more noise). For video, the usual workflow runs the other way: lock the shutter — see our Shutter Angle Calculator — and the aperture, then add ND until the exposure is right.
Assumptions and limitations
- Assumes an ideal filter; real filters can be slightly stronger or weaker than marked and may add a colour cast.
- Very long exposures can suffer reciprocity effects (film) or extra noise (digital).
- Stacking filters adds stops (and multiplies factors), but also adds reflections and potential vignetting.
FAQ
- How many stops is an ND8 filter?
- Three stops. The number is the filter factor: ND8 lets through 1/8 of the light, and 8 = 2 × 2 × 2, so it's 3 stops. ND64 is 6 stops.
- What does ND 0.9 mean?
- 0.9 is the optical density, the notation many cinema filters use. Each 0.3 of density is about one stop, so ND 0.9 is about 3 stops (a factor of roughly 8).
- Why do filmmakers use ND filters?
- Video usually runs at a fixed shutter (often around 180°) and a chosen aperture for depth of field. In bright light the only way to avoid overexposure without changing those creative choices is to cut the light reaching the sensor — which is what ND does.
- Are variable ND markings accurate?
- Treat them as approximate. Variable NDs can vary between units and across their range, and strong settings can show colour casts or cross-polarisation artefacts. Check exposure with your camera's tools.
- Is it free to use?
- Yes. There's no signup or account, and the calculations run entirely in your browser — nothing you type is sent to a server or an AI model.
- Can I share or save my results?
- Yes. Your inputs are saved in the page address as you type, so “Copy link” gives you a URL that reopens the calculator with the same settings. “Copy results” copies a plain-text summary.
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