Filtro ND - Redução em Stops

ND filter: stops of light reduction

A neutral density (ND) filter cuts the light reaching the lens without touching color or contrast. That gives you room for slower shutter speeds or wider apertures even when the scene is bright. The notation runs on a log scale. ND2 is 1 stop and halves the light; ND4 is 2 stops, ND8 is 3, and ND1000 reaches 10. To find the new shutter time, use t_new = t_old · 2^stops. So 1/100s with an ND8 becomes roughly 1/12s. Stack two filters and the stops add up, so ND4 plus ND8 gives you ND32, or 5 stops. Variable ND filters (vND) pair two polarizers, which lets you dial the density up and down continuously.

Applications: long exposure and cinema

Photographers reach for ND filters in a few situations. Long exposure work is the obvious one, where you want silky waterfalls or light trails along a street. They also let you shoot shallow depth of field in bright sun, opening up to f/1.4 outdoors without blowing out the highlights. In cinema they keep you on the 180-degree shutter rule at 24fps with a 1/50s shutter. And for time-lapse they add motion blur between frames, much like the architectural trick of erasing pedestrians through exposures that run several minutes.

FAQ

Which ND filter for waterfalls? Something in the ND8 to ND64 range usually gets you 1–5 second exposures in daylight. Under harsh midday sun you may have to go all the way to an ND1000.

Does ND affect color? A well-made ND stays neutral. Cheaper ones tend to throw a magenta or green cast, and it gets worse as the density climbs, so you fix it in white balance.

Can I stack NDs? You can, and the stops add up rather than multiply. Watch out for vignetting on wide lenses, plus the X-pattern artifact you get when stacked variable NDs are pushed to extreme settings.