ND Filter Stop Chart

ND filter names look like they should follow one simple numbering system, but manufacturers actually mix three different conventions — which is exactly why "ND8" and "3 stops" and "0.9 density" all refer to the same filter. This page lays out the full conversion and the complete stop table behind the calculator's dropdown.

Three numbering systems, one physical property

• Filter factor (the "ND number"): the fraction of light transmitted, inverted — ND8 transmits 1/8th of the light, ND1000 transmits 1/1000th. This is the most common consumer-facing name.
• Stops: how many times the light is halved. Since each stop halves light, filter factor and stops relate by stops = log₂(filter factor) — ND8 = log₂(8) = 3 stops exactly.
• Optical density: a photographic/scientific log₁₀ scale, where density = log₁₀(filter factor). ND8's density is log₁₀(8) ≈ 0.9 — which is why you'll sometimes see the same filter labeled "0.9ND."

All three describe the identical physical filter — the calculator's dropdown uses stops directly, since that's what plugs straight into the exposure-doubling formula.

Full stop-equivalent table

Filter name	Stops	Optical density	1/8s becomes
ND2	1	0.3	1/4s
ND4	2	0.6	1/2s
ND8	3	0.9	1s
ND16	4	1.2	2s
ND32	5	1.5	4s
ND64	6	1.8	8s
ND100 (≈ND128)	6.6	2.0	≈9.7s
ND400	8.6	2.6	≈38.8s
ND1000	10	3.0	2 min 8 sec
ND100000 (16.6-stop)	16.6	5.0	≈4 hr 20 min

"1/8s becomes" column computed directly from this calculator's new_shutter = base × 2^stops formula.

Why ND100 and ND400 have non-round stop values

Notice ND100 is 6.6 stops and ND400 is 8.6 stops rather than clean integers — that's because filter manufacturers round their "ND number" marketing names to convenient round figures (100, 400) rather than to the nearest clean power of two. The actual physical transmission of a filter marked "ND100" is often closer to true ND128 (which would be an exact 7 stops) — this calculator's preset list uses the commonly published stop-equivalents from filter manufacturers rather than recomputing from each rounded marketing number, which is why you'll see "ND100 (≈ND128)" noted directly in the dropdown.

Using the raw stops mode for anything not in the table

For a filter strength not listed — a less common ND64+ND8 stack, or a filter labeled only by optical density — switch the calculator to "Raw stops" mode and enter the stop value directly (convert density to stops by dividing by 0.3, or just use stops = log₂(filter factor) from the ND number).

FAQ

Why do two different-looking ND numbers sometimes mean the same filter?

Because "ND100" and "ND128" describe filters close enough in actual transmission that manufacturers market them under whichever round number is more convenient — the underlying stop value is what actually matters for exposure math.

Can I stack two ND filters and add their stops?

Yes — stacked ND filters' stop values add directly (an ND8 + ND4 stack behaves like a 5-stop filter), though stacking introduces more risk of vignetting and color cast than a single strong filter.

What does "optical density" mean on a filter box?

It's a log₁₀ scale of light transmission, common in scientific and some photographic filter labeling — divide a density value by 0.3 to convert it to stops for this calculator's raw-stops mode.

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