Stops
Each stop halves transmitted light, so exposure time doubles for every added stop if aperture and ISO stay fixed. A 6-stop filter needs about 64× the base time; 10 stops about 1024×.
Twilight ND Long Exposure
Use solar altitude and a measured base shutter to decide whether ND still belongs on the lens during twilight—and when to remove it as the sky becomes an astrophotography scene.
Interactive ND exposure
Resulting shutter—
Twilight band—
At night, strong ND is usually counterproductive: the scene is already photon-limited.
Why
Each ND stop halves incoming light, so keeping the same exposure doubles shutter time. Twilight classification comes from solar altitude; it does not tell you the correct camera exposure by itself.
Field check
Meter without the filter, calculate the filtered shutter, then verify the histogram and highlight clipping after mounting the filter. Cover the viewfinder if your camera leaks light.
Try it
Start at 1/8 s and add 6 stops. Then move solar altitude from -2° to -14° and consider whether you still need the filter.
Related
What an ND filter actually changes
Changing light
The formula assumes scene brightness is stable. Around sunrise, sunset and twilight, light can change faster than a multi-minute exposure, so the predicted time may already be wrong when the exposure ends.
Filter behavior
Strong ND filters can add color casts, infrared contamination, flare, leakage or vignetting. Stacking filters adds reflection surfaces and can worsen those effects.
Field workflow
Meter without the strong filter, lock focus, calculate filtered time, then verify the histogram after the real exposure. In rapidly changing light, start shorter and recalculate from what the camera actually recorded.