ND Filter Calculator

In short A neutral density filter blocks a fixed fraction of the light, so the new shutter speed is the old one multiplied by two raised to the number of stops. A ten-stop filter multiplies by 1024, turning 1/125 of a second into about 8.2 seconds. Each additional stop doubles it again, which is why long-exposure times climb so fast. The naming is genuinely confusing and all three systems appear on filter boxes. The ND number is the light reduction factor, so ND8 passes one eighth. The decimal is optical density, the base-ten log of that factor, so the same filter is ND0.9. And stops is the base-two log, making it 3 stops. Two of the three are usually printed and rarely the same two. A filter sold as ND1000 is really 1024 times, or exactly ten stops — the thousand is a marketing round number, and the difference is under a twentieth of a stop. The practical points matter more than the arithmetic. Meter, focus and compose BEFORE fitting a strong filter: a ten-stop blocks so much light that autofocus and metering stop working through it. Anything past thirty seconds needs bulb mode and a remote, since that is the longest most cameras offer directly. And cheap strong filters are rarely neutral despite the name, usually pushing the image warm or magenta, so shoot raw and fix the white balance afterwards.