Photography Calculator

A neutral density filter is a grey piece of glass that cuts the light reaching the sensor without changing its colour. Its job is to let you use a long shutter speed in conditions that would otherwise be far too bright — to smooth water into mist, blur clouds into streaks, or erase moving crowds from a scene in daylight. ND filters are rated in stops, and each stop halves the light, so the maths of "what shutter do I get?" is pure powers of two.

The core formula is new shutter = base shutter × 2^stops. Your base shutter is what the scene meters at with no filter; multiply by two raised to the filter's stop count and you have the long exposure. A 10-stop filter multiplies your shutter time by 2¹⁰ = 1024 — so a brisk 1/125s becomes roughly eight seconds. The filter naming can be confusing because two systems coexist: ND1000 means it cuts about 1000× the light, which is 10 stops; ND8 is 3 stops; the calculator's presets map the common names (ND2 through ND32000) straight to stops so you don't have to.

It works just as usefully in reverse. Say you want a specific dreamy shutter — 30 seconds for silky water — and need to know which filter buys it. Then stops = log₂(target ÷ base): meter the scene, set your target, and the calculator tells you how many stops of ND to screw on. That backwards mode is how most long-exposure shots actually get planned, because the look you want dictates the shutter, and the filter is whatever delivers it.

Two field notes. Very dense filters (10 stops and up) are so dark you can't see through them, so compose and focus first, then attach the filter and switch to manual focus. And once exposures run into seconds, the output here is given in minutes and seconds — at those lengths you'll also want a tripod, a remote or self-timer, and on film (not digital) an extra nudge for reciprocity failure.