Stops vs ND number vs optical density — the conversion chart
Manufacturers label the same filter three different ways, which is where nearly all ND confusion comes from. Stops count halvings of light. The ND number is the light-reduction factor, ≈ 2stops — so ND8 is 3 stops, and "ND1000" is the trade name for the 10-stop filter whose exact factor is 1024. Optical density is the base-10 logarithm of the factor: 0.3 per stop, so a 10-stop filter is OD 3.0. The chart below is computed from those formulas:
| Stops | ND number | Optical density | 1/60 s becomes |
|---|
Stacking ND filters
Stacked filters multiply their factors, which means their stops add: ND8 (3 stops) + ND64 (6 stops) = 9 stops, a factor of 512. Use the second filter selector above and the math is done for you. Two practical cautions: each extra glass surface adds reflections and a chance of vignetting on wide lenses, and cheap filters stack their color casts too — two slightly warm filters make a visibly warm image.
When a long exposure needs a timer
Most cameras time exposures up to 30 seconds; anything longer means bulb mode, where you time the exposure with a remote release. That's what the countdown above is for — it appears automatically whenever the computed exposure exceeds one second, formatted mm:ss so you can watch it while holding the release. For multi-minute exposures, remember reciprocity is not an issue on digital sensors, but long-exposure noise reduction (dark-frame subtraction) will double the total time the camera is busy.
Frequently asked questions
Is ND8 the same as 3 stops?
Yes. ND8 reduces light by a factor of 8 = 2³, which is 3 stops (optical density 0.9). Multiply your metered shutter time by 8: 1/125 s becomes about 1/15 s.
Is ND1000 really 10 stops?
Yes — ND1000 (sometimes sold as ND1024 or "10-stop") has an exact factor of 2¹⁰ = 1024; "1000" is rounding for the label. Optical density 3.0. It's the classic filter for daylight long exposures: 1/125 s becomes roughly 8 seconds.
How do stacked filters combine?
Factors multiply, stops add, densities add. ND8 + ND1000 = 3 + 10 = 13 stops, factor 8192, OD 3.9. The calculator's second filter slot applies this automatically.
Why does my long exposure come out brighter or darker than calculated?
Three usual causes: the light changed during a multi-minute exposure (dusk especially), the filter's true density differs slightly from its label (10-stop filters are often 9.5–10.5 stops in practice), or light leaked through the viewfinder or filter edges. Bracket by ±1 stop of time on your first frame with a new filter and note the correction — it stays consistent for that filter.