The Glass That Slows Down Time: Neutral density filters have one job — reduce light without changing color. But their history is more poetic than that. Early photographers working with glass plates in the 1880s discovered that bright daylight made long exposures practically impossible: you couldn't blur a waterfall or smooth the sea because the shutter had to close so fast. The solution was tinted glass held over the lens — not to add color, but to take light away. Modern ND filters are optically perfect descendants of that improvised darkroom hack, and the math hasn't changed in 140 years: every stop cuts light in half, doubles the time.
Two Different Families of Filter: Neutral density filters and colored filters look similar in your bag but do entirely different things. ND filters subtract light uniformly — they're invisible to the final image except through longer shutter speeds. Colored filters, on the other hand, are the sculptor's chisel of black-and-white film photography. A red filter (#25) blocks blue and green wavelengths: the sky, which is mostly blue, goes nearly black on the film; white clouds pop with extraordinary drama. A green filter does the opposite for foliage. These are tonal tools, not exposure tools — but they still require exposure compensation, which is exactly what the filter factor calculation handles. The stop values here are based on standardized Wratten filter specifications, the same system Kodak published in 1909 and the industry still uses.
The Strange Physics of Film Reciprocity: Here's something digital photographers never have to worry about: film breaks its own rules during long exposures. Normally, halving the light and doubling the time gives the same result — that's the Reciprocity Law. But below about 1 lux (which happens after one second), silver halide crystals in film lose their photochemical efficiency. The longer the exposure, the more you have to overexpose to compensate. Ilford HP5 at 10 seconds needs roughly 14 seconds actual time. At 100 seconds, it might need 200. Every film has a different reciprocity curve, and the only accurate data comes from the manufacturer's technical sheet. The warnings in this calculator give conservative ballpark corrections — always bracket and test your specific film stock.
Why Stacking Works (and When It Doesn't): Filter stops add together because stops are logarithmic. Two ND8 (3-stop) filters equal exactly 6 stops — same as one ND64. This lets you build strong filtration from cheaper, more common pieces. The caution is vignetting: each filter adds a tiny bit of glass at the edge of your lens barrel, and wide-angle lenses see those edges. Beyond two filters on a lens wider than 35mm, you'll start to see dark corners. The other risk is flare: more glass surfaces mean more internal reflections. Shoot with stacked filters in shade, or with a deep lens hood, and you'll minimize both problems.