ND Filter Calculator
Which ND holds the 180-degree shutter open at T2 in full sun. The calculator above takes desired aperture, shutter angle, frame rate and light level and returns required ND, stops reduced and resulting exposure — computed in your browser, with no sign-up and nothing leaving your device.
ND Filter Calculator
- ND density
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- Filter factor
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- Transmission
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Runs entirely in your browser — no sign-up, nothing transmitted.
What the number actually depends on
Direct summer sun at ISO 800, 1/50 s and T2 is roughly six stops over — that is an ND1.8.
Each 0.3 of ND density removes exactly one stop. ND0.9 is three stops, ND1.2 is four.
The gap between a number that goes into a quote and one that holds on the day is rarely the formula — it is whether the constraints were entered honestly. That is why the inputs are spelled out below.
Common mistakes
These come up most often in practice. They rarely break the maths; they routinely break the day:
- Closing the iris instead of adding ND, changing the depth of field mid-scene
- Bringing a variable ND that cross-polarises at the top of its range
In day-to-day production
The 180-degree rule fixes shutter to half the frame rate: at 25 fps that is 1/50 s and nothing else.
The number belongs in production planning — the point where a decision is still free to change. Working it out on the shoot day is no longer calculating; it is finding out.
In TillyGen this is not a separate step: the result sits inside the plan itself, alongside the shot list, schedule and call sheet. Change a constraint and the dependent values move with it — the affected parts of the plan are flagged rather than quietly left behind.
Frequently asked
How accurate is the ND Filter Calculator?
The maths is exact; the inputs rarely are. Treat the result as a solid floor and add your buffer deliberately rather than smuggling it into the inputs — a flattered input hides the buffer from everyone who plans with it later.
Is the calculator free?
No. It runs entirely in your browser, with no sign-up and nothing transmitted. There is no usage limit and no account.
Where does this fit in production planning?
In production planning. There it is a basis for deciding; later it is only a finding.
Can I export the result?
The calculator itself only displays the value. In TillyGen it lands directly in the shot list, schedule and call sheet, and goes out as PDF, CSV or through the API to everyone working from it.
What does the calculator return?
Required ND, stops reduced and resulting exposure. The first value is the one you carry into the plan; the rest show how sensitively it reacts to the inputs.
What should I watch for in the result?
Mainly this: closing the iris instead of adding ND, changing the depth of field mid-scene. That is the mistake that most often turns a correctly calculated number into one that does not hold.