Stops: The Only Math You Need
Every ND filter is rated in stops, and each stop doubles the exposure time: new time = metered time × 2^stops. A 10-stop ND1000 multiplies by 1,024 — so a metered 1/60 s becomes about 17 seconds. The naming is just powers of two: ND2 = 1 stop, ND4 = 2, ND8 = 3, ND64 = 6, ND1000 ≈ 10. Variable NDs rotate between markings but are harder to calibrate precisely; fixed NDs are honest. Once the stop count is known, the rest is multiplication — which is exactly what the calculator above does, including stacks.
Stacking is where most people stumble: stops add. A 6-stop plus a 3-stop is a 9-stop stack (×512), not "6 then 3 separately." The calculator's chip stack shows the running total so you can mix and match from your filter pouch to hit a target time.
Choosing the Right Strength
Match the filter to the effect, not the fashion. 3-stop (ND8): subtle motion blur in waterfalls while keeping texture, slight cloud streaking. 6-stop (ND64): the versatile sweet spot — silky water in daylight, blurred crowds, 30-second-range exposures without going extreme. 10-stop (ND1000): the dramatic one — glassy lakes, erased people from plazas, streaked skies, minute-plus exposures at midday. 15-stop: specialty territory for solar-ish brightness or extreme minimalism. If you buy one filter, make it the 6-stop; if you buy two, add the 10-stop, and the stack covers everything between.
Stacking Without the Pitfalls
Two filters stack optically as well as mathematically, and the pitfalls multiply too. Vignetting: two thick rings on a wide-angle lens darken the corners — use slim-profile filters or step-up rings, or accept a small crop. Flare: every air-glass surface is a flare opportunity; keep both filters immaculate and shade the front element. Color cast: cheap NDs go green or magenta, worse when stacked — quality glass matters more with every layer. And the iron rule of dark NDs: compose and focus first, then add the filters, because a 10-stop turns your viewfinder (or live view) nearly black.
Reciprocity Failure: Film's Hidden Tax
Digital sensors are linear: twice the time, twice the light, at any duration. Film is not. In very long exposures the emulsion loses efficiency — reciprocity failure — so the metered time underexposes, sometimes dramatically. The rule of thumb is t_corrected ≈ t^1.3 for times over a second: a metered 16 seconds needs roughly 37 seconds of actual exposure. Every stock has its own curve (Fuji Velvia is notoriously hungry; some modern stocks are better), and color balance can shift too, so treat the calculator's compensation as the starting point and bracket a stop each way on important frames.
Bulb Mode and the Timer
Beyond 30 seconds, cameras hand control to you: bulb mode holds the shutter open as long as the release is held. Use a locking remote release or the camera's built-in interval timer — holding the button by hand for 4 minutes introduces shake no tripod can fix. The bulb timer above counts down your computed exposure; start it as you lock the release. Pro tip: most cameras' longest timed setting is 30 seconds, so anything the calculator shows above 30 s means bulb mode plus the timer.
Sharpness at Long Durations
Long exposures magnify every vibration. The checklist: sturdy tripod (hang your bag from the hook for mass), remote release or 2-second self-timer, mirror lock-up on DSLRs, image stabilization off on the tripod (it hunts and adds blur), and viewfinder covered on DSLRs to block light leaks through the eyepiece. Wind is the enemy — shield the tripod with your body, avoid extending the center column, and on gusty days accept that 2-minute exposures may simply not be sharp.
Composing for Time
Long exposure is a compositional tool, not just a technical one. It erases the chaotic and keeps the constant: choppy water becomes glass, crowds vanish from architecture, clouds draw lines across the sky, car headlights paint ribbons. Compose for still anchors — rocks, piers, buildings — against which the motion reads. Watch the edges: a half-blurred branch whipping in the wind becomes an ugly smear, so frame it out or wait for calm. And mind the sky: 3+ minute exposures can blow highlights as clouds thin — check the histogram, not just the preview.
Light Trails, Stars, and Special Cases
Light trails: f/8–f/11, ISO 100, 10–30 seconds after dark — no ND needed. Star trails: stack dozens of 30-second exposures in software rather than one multi-hour shot (cleaner, and a passing cloud ruins one frame instead of the night). Daytime minimalist seascapes: 10-stop + 6-stop stacked for 2–4 minutes at f/11. Sun-in-frame: extreme ND plus careful exposure — never look through the viewfinder at the sun with the filter off during composition on a DSLR; use live view.
Noise and Heat
Long exposures cook the sensor slightly, producing hot pixels and edge glow (amp glow). Countermeasures: shoot at base ISO, enable long-exposure noise reduction (the camera shoots a dark frame of equal length and subtracts it — note this doubles each shot's wall-clock time), keep the camera shaded, and in post, a single dark frame subtracted manually works as well as the in-camera version. Alternatively, stack multiple shorter exposures: ten 30-second frames averaged in software approximate one 5-minute exposure with far less noise.
Scouting and Planning the Shot
Great long exposures are planned, not stumbled upon. Scout at the same time of day you intend to shoot — water levels, cloud direction, and crowd patterns all change by the hour. Tide charts matter for seascapes: an incoming tide over rocks gives explosive foregrounds at 1–2 seconds, while a calm high tide suits 3-minute glass. Check the wind forecast for cloud-streaking days (10–20 mph moves clouds beautifully; dead calm gives you nothing to streak) and for sharpness on gusty days. Apps that predict cloud cover and sun position turn luck into scheduling. Arrive early: the best long-exposure light is often the blue hour after sunset, when ambient levels let you reach minute-plus exposures with modest NDs — and the calculator tells you exactly which filter gets you there from your metered base.
Filters vs. In-Camera Tricks
Modern cameras offer computational alternatives worth knowing. Live ND / composite modes (on some mirrorless bodies) stack multiple short exposures in-camera to simulate ND filters — no glass, no color cast, instant preview, though with some loss of raw flexibility. Multiple-exposure averaging in post achieves the same: twenty 1-second frames averaged equal one 20-second exposure for moving water, with less noise and no 10-stop filter. Phone long-exposure modes do this automatically. These tricks excel for moderate effects, but for true multi-minute ethereal minimalism — glassed-over oceans, vanished crowds — physical ND glass still rules. Use the calculator for the glass; use the tricks when you left the filters at home.
Putting It All Together
The workflow: meter without the filter, choose ND strength for the effect you want (stacking chips as needed), read the computed time, switch to bulb past 30 seconds, lock the release, and start the timer. Check the histogram, adjust, repeat. Long exposure rewards patience twice — once in the taking, once in the result.