Why the numbers on the box disagree with each other
Filter makers label the same glass three different ways and none of them is the number you need. The ND number is a division factor: ND8 passes an eighth of the light. The density is a logarithm: ND0.9 is the same filter written base 10. What your camera works in isstops, and a stop is a halving — so the only conversion that matters is that eight is three halvings, and three is the number you add to your shutter.
That is also why stacking trips people up. Two filters do not add their ND numbers, they multiply them, which is the same thing as adding their stops. An ND8 in front of an ND64 is nine stops of glass, or ND512 — a far heavier combination than the pair of moderate filters in your bag looks like it should be.
What the answer looks like in the field
- Silky water at midday — metered 1/125 at f/11, fit a 10-stop, and you are at 8.2 seconds. Long enough to smooth moving water, short enough to shoot handheld-adjacent on a solid tripod without a remote.
- The same filter in shade — metered 1/4 instead, and that 10-stop lands at 4 minutes 16 seconds. Same glass, wildly different shoot. This is the case a printed chart quietly drops.
- Empty a busy square — several minutes of exposure erases anyone who kept walking. Solve backwards from the time you want and the instrument tells you how much glass to buy.
Meter first, then fit the glass
A ten-stop filter is close to opaque. Autofocus hunts, the meter guesses, and the composition disappears from the viewfinder — so do the work first: frame, focus, meter, then lock everything to manual and screw the filter on without touching the dials. The number this instrument gives you assumes exactly that order, because it is the only order that produces a repeatable exposure.
Past thirty seconds your camera's own timer runs out and you are into bulb mode with a remote. The readout flags that line as you cross it. If you are planning around the light rather than the water, thegolden hour instrument tells you how long you have, and theexposure calculator handles the trades that do not involve filters at all.
Questions from the bench
How many stops is an ND8 filter?
Three. The ND number is the light-reduction factor, and stops are its log base 2 — ND8 lets through 1/8th of the light, which is three halvings. The same filter is sold as ND0.9 (optical density) and as a "3-stop", so all three labels describe one piece of glass.
What happens when I stack two ND filters?
The stops add and the ND numbers multiply. An ND8 over an ND64 is 3 + 6 = 9 stops, which is ND512 — not ND72. This is the calculation people get wrong most often, and it is why stacking a pair of mid-strength filters gets you further than most photographers expect.
How long is a 10-stop exposure?
It depends entirely on what you metered first. A 10-stop filter multiplies your shutter time by 1,024 — so a bright-day 1/125s becomes 8.2 seconds, but a shaded 1/4s becomes 4 minutes 16 seconds. That is why a printed chart only ever half-answers the question.
When do I need bulb mode?
Past 30 seconds. Almost every camera stops its timed shutter at 30s, so anything longer needs bulb mode and a remote or an intervalometer. This instrument flags the moment your answer crosses that line, because it is the difference between a shot you can take and one you cannot.
Does this account for reciprocity failure?
No — and on a digital sensor it does not need to. Reciprocity failure is a film characteristic where very long exposures under-expose beyond what the math predicts. If you are shooting film, add roughly a stop past a minute and bracket; if you are shooting digital, the number here is the number.
Should I meter before or after fitting the filter?
Before. A 10-stop filter makes the scene too dark for most autofocus and metering systems to read reliably. Meter, focus and lock your settings with the filter off, run the numbers here, then fit the glass and switch to manual.