If you want to shoot with really long exposure, like maybe you want to make a photograph where the water flowing along in the little spillway up the road looks like flowing smoke instead of broken glass, or you want to make people disappear, you’ll need either a really slow film, a really small aperture, or a filter.
The above options work because you need a longer exposure to get the right exposure on the film, so that’s the story. Long exposure times make fleeting presences go away and moving things blur.
Slow film requires a long exposure, particularly in low light. A small aperture will do the same, require a longer exposure time, but maybe your camera only stops down to f-32. If it’s sunny out and you’ve loaded up some 100 ISO film you might still need to use a shutter speed of 1/500th.
What ND Filters Do
An ND filter will drop the quantity of light that gets to the film in a given period of time, good ones do it without any tint or image degradation–they absorb full spectrum light. Any filter is basically lowering the speed of the film you have in your camera. For color filters they’re slowing down only light of their own color, effectively raising the speed on the opposite wavelength.
You can use your ND filter to make your 100 ISO film work like it’s 50 ISO, 25 ISO, 12.5 ISO, 6.25 ISO or even less! They work across the board, they’re color neutral. ND filters are sold with different descriptions of their power, ND2, 1 Stop, Filter Factor 2, 2X, .5%, 1/2, or .3 are some examples, and those all mean the same thing. Like all things camera, they change in halves up or down the line, that is things move in stops. So if an ND1 lets by all of some quantity of light, an ND2 lets by one full stop less. That’s half the light that an ND1 does, and an ND4 lets by two full stops less or one quarter the light an ND1.
That means using an ND2 filter let’s you double the length of time your shutters open. If you needed 1/60th with no filter you’ll need 1/30th with an ND2 and 1/15th with an ND4. If you want to think about that in terms of film speed an ND2 turns your 400 ISO film into 200 ISO film and ND4 turn’s that 400 ISO film into 100 ISO. Thinking about it in terms of f-stop means you’ll get the same exposure time (but the resulting depth of field characteristics) using no filter and f-16 as you’ll get with an ND2 filter and f-8. So you could shoot wide open on a sunny day and get a nice soft background if you have a heavy enough ND filter in front of the lens.
There’s a big range of ND filters out there, from very minor ones that knock out a third of a stop (or even less) to ones that are almost opaque and cut ten stops or more. No one needs to buy every ND filter, you can get by with a few and just stack those to drop more stops of light as needed. Stacking works the way you’d think, an ND2 which takes one stop stacked with an ND8 which drops light by three stops, all together blocks four stops.
So how that works with math is 2×8=16. 16/2=8, 8/2=4, 4/2=2, 2/2=1. We had to divide by two four times to get to unity, so an ND2 stacked with an ND8 cuts the light by four whole stops. A 2x4x8 stack gives you sixty four which takes six steps of division by two to get to unity–so would cut the light by six stops. Don’t go thinking an ND2 with an ND4 equals an ND6, now you know better.
If you’re going to do any stacking always put your lightest ND filter farthest from the lens and the darkest closest. Darker filters have to block more light, heat up more, and as a result will degrade faster if they’re not great quality. Even if they’re top of the line that heat stress can do some damage so just believe it, alright? I also suspect that particularly with darker filters they’re less truly neutral in density.
What’s Your Exposure?
You got some 100 ISO film loaded up and you’ve stopped your aperture way down to f-32. You’re exposure meter gives you a shutter speed of 1/60th. Drop in an ND2 filter and you can now shoot at half that, 1/30th. Drop in another filter, maybe the next one from your set, ND4, and you can now shoot two stops slower than that, 1/8th. Drop in the next filter from your set so you now have three all in a line, ND2-ND4-ND8 and you get four more stops of exposure time for a 2 second exposure. Easy peasy.