How to calculate long exposure times with ND filters

Every stop of ND doubles the exposure. Stacking filters and different labelling systems are where it gets confusing.

A neutral density filter cuts light so you can use a longer shutter speed — for smooth water, blurred cloud, or emptying a street of people.

Step-by-step

1. Meter without the filter and note the shutter speed.
2. Enter that speed, and the filters you are stacking.
3. Read the corrected time.

Three labelling systems for the same thing

This is the real source of confusion. A filter may be sold as ND8, ND 0.9, or "3 stops" — all identical. ND1000, ND 3.0 and 10 stops are likewise the same filter.

The optical density number is the base-10 logarithm; the ND number is the division factor; stops are powers of two. The calculator accepts any of them, which removes an entire class of mistake when stacking filters bought from different manufacturers.

Stacking

Stops add. A 6-stop and a 3-stop together give 9 stops. The ND numbers multiply rather than add — ND64 with ND8 is ND512, not ND72 — which is why working in stops is easier.

Beyond thirty seconds

Most cameras stop at 30 seconds in automatic modes. Past that you need bulb mode and a timer, and at those lengths two other effects appear: sensor noise rises with exposure time, and very long exposures on some cameras trigger long-exposure noise reduction, which takes as long again to process.

Frequently asked questions

Is ND8 the same as a 3-stop filter?

Yes. ND8 divides the light by eight, which is three stops, also written as density 0.9. The three systems describe the same filter.

How do I stack two filters?

Add the stops. A 6-stop plus a 3-stop is 9 stops. The ND numbers multiply instead, which is why stops are the easier unit.

How long is long enough for smooth water?

Around one to two seconds blurs a waterfall pleasantly while keeping some texture. Thirty seconds and beyond turns sea into mist. It is a stylistic choice rather than a correct answer.