ND filter calculator: how many stops you need
In video, shutter speed is set by the frame rate and the 180° rule, so on a clear day, wide open, there's too much light even at the lowest ISO. The only way to cut it is in front of the lens — an ND filter. The calculator works out how many stops it needs to darken.
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Fill in the fields — the verdict will appear here
A variable ND at its extreme settings causes cross-polarization — a dark X-shaped cross, especially visible on wide-angle lenses. Test your filter on your widest lens and remember its working limit.
How it's calculated
- Shutter speed is set by the 180° rule: it equals half the frame rate, and you can't change it for exposure without losing natural motion blur.
- We calculate how many stops brighter than needed the chosen aperture-plus-ISO pair is at that shutter speed.
- The required filter density is two raised to that number of stops; the nearest one from the standard range is picked.
Example: 25 fps → a 1/50 shutter speed. A clear day (EV 15) at f/2.8 and ISO 100 gives 6.4 stops of excess, i.e. a density of 84 — the nearest standard is ND64, leaving 0.4 stops to make up.
Shutter speed by the 180° rule = 1/(2·frames per second). Stops over = EV₁₀₀ + log₂(ISO/100) − log₂(N²/t); required density = 2^(stops over).
This is an estimate
Verify final values against current standards, the project design, and equipment nameplates.