ND filter chart: stops, density and exposure times

ND number, stops and optical density

For a filter that reduces light by a factor x (the "ND number"): stops = log₂(x), optical density = log₁₀(x) and the light transmitted is 1 ÷ x. Stops are what matter for exposure; density is how many glass makers label filters (0.9, 1.8, 3.0).

Filters sold in whole stops use rounded names: a 10-stop filter is 2¹⁰ = 1,024×, but it is called ND1000 and labelled 3.0. Filters such as ND100 and ND400 are not whole stops at all — 6.64 and 8.64 stops — which is why the "nearest ⅓ stop" column helps when you set exposure on a camera that moves in thirds.

Exposure times with light and medium filters

Meter without the filter, find that shutter speed on the left and read across. Times use the exact factor of each filter (time × factor).

Exposure times with strong filters

Anything over 30 s needs Bulb mode and a remote release; the ND filter calculator has a countdown timer for it.

Why ND1000 is not exactly 10 stops

A true 10-stop filter cuts light by 1,024×, an ND1000 by 1,000×. From a base of 1/60 s, the first gives 17.1 s and the second 16.7 s — a difference of 0.03 stops, far smaller than the variation between real filters of the same nominal strength. In practice, take a test exposure, check the histogram and adjust: a strong filter that is "10 stops" on the box is often a third of a stop off.

Stacking filters

When you stack filters, the factors multiply and both stops and optical densities add:

Every extra glass surface adds a little flare and colour cast, and two square filters in a holder can vignette on wide lenses, so a single strong filter usually gives cleaner results than a stack of the same total strength.

Which filter do I need for a target time?

The stops you need are log₂(target time ÷ metered time). Rounded to the nearest common filter:

A metered 1/125 s is typical for f/8–f/11 at ISO 100 in daylight. At 1/1000 s — bright sun at a wide aperture — even a 10-stop filter only reaches about one second, which is why 15-stop filters exist.

FAQ

How many stops is an ND1000 filter?

9.97 stops — log₂(1000). It is sold as a 10-stop filter with an optical density of 3.0; the exact 10-stop factor would be 1,024×, a difference of only 0.03 stops.

How do I convert optical density to stops?

Divide the density by log₁₀(2) ≈ 0.301. A 0.9 filter is 2.99 stops (ND8), 1.8 is 5.98 (ND64) and 3.0 is 9.97 (ND1000). Each 0.3 of density is almost exactly one stop.

How long is 1/125 s with an ND1000?

1/125 × 1000 = 8 s. With a 6-stop ND64 it is 0.5 s, and with a 15-stop ND32000 it is 4 min 16 s.

What does an ND100000 filter do?

It blocks 16.6 stops (optical density 5.0), passing 0.001% of the light — the strength of the solar filters used to photograph the sun. Only point a camera at the sun through a filter sold and certified for solar use.

Technique, focusing and colour-cast tips are in the ND filter long exposure guide; for any base speed and stacked filters, use the ND filter calculator.