Calculate an ND-filter exposure without mixing up seconds and stops
By Hello Keyboard (AI-assisted)
A camera exposure of 1/80 second is not an input of 80 seconds. That distinction matters when a calculator asks for the base exposure time rather than the denominator shown on a camera display.
Here is a small worked example for a uniform neutral-density filter rated at six stops. It assumes the scene lighting, aperture and ISO stay unchanged. The starting exposure is measured without that ND filter.
Convert the starting time
Suppose the unfiltered exposure is 1/80 second. Write it as a duration in seconds:
1 ÷ 80 = 0.0125 seconds.
Keep both forms in your notes. The fraction matches the original setting; the decimal supplies the seconds value for this calculation. Entering 80 instead would describe an entirely different exposure.
Apply the filter’s stop rating
Each stop of ND reduction requires twice the exposure duration to compensate, with the other exposure settings held constant. NiSi’s guide to ND-filter shutter times explains this relationship.
Six stops therefore give a multiplier of 2⁶ = 64:
0.0125 seconds × 64 = 0.8 seconds.
The multiplier is 64, not six. As an arithmetic check, three stops would give 0.0125 × 8 = 0.1 seconds. Adding another three stops multiplies that by eight again, giving 0.8 seconds.
Match the calculator’s input meanings
The Hello Keyboard ND Filter Calculator documents a base-time field measured in seconds, a filter-strength field measured in stops and a Calculate button.
For this example, enter 0.0125 for the base time and 6 for the strength, then calculate. Compare the displayed answer with the independently calculated reference of 0.8 seconds above. This guide does not claim that the website calculation was interactively tested.
The page explicitly says it does not reinterpret a density label as a stop count. If the filter packaging uses a different notation, look up its stop rating in the manufacturer’s documentation before entering it. Do not paste an unfamiliar number into the stops field and assume the calculator will translate it.
Carry the assumptions into the photograph
Use the exposure without this ND filter as the baseline. Starting from an exposure that already compensates for it would apply the adjustment twice. If the light or other exposure settings change, the original baseline no longer describes the same setup.
Treat 0.8 seconds as a calculated starting point, then inspect the actual photograph and histogram. The arithmetic does not measure the light, assess the filter or guarantee the image you want.
Disclosure: Written by an authorized AI assistant for the Hello Keyboard operator. The linked documentation was read on 11 November 2026, and the example arithmetic was checked locally. No photograph was taken and no website inputs were entered. This is an affiliated educational guide, not an independent product review or a privacy audit.