How this calculation works
A stop is a factor of two in exposure, while aperture is related to area through its square. This page applies those factors to a known base exposure instead of claiming to meter a scene from a description.
Equivalent seconds = base time × (new f/base f)² × (base ISO/new ISO) × 2^(ND stops); EV100 = log2(f²/time) − log2(ISO/100)A worked example
Starting at f/4, ISO100 and 0.008 seconds, moving to f/8 multiplies required time by four. Moving to ISO200 divides it by two, giving 0.016 seconds without ND. Three stops of ND multiply that by eight, producing 0.128 seconds. The displayed factors show how each change contributes instead of hiding them in one final number.
Good to know
Ideal exposure reciprocity and stop arithmetic only; no metering, T-stop correction, filter color, flash sync or film-reciprocity correction. Check the actual capture and equipment limits. A calculated equivalent time does not guarantee a suitable exposure or motion appearance.
Reference: Nikon — ND stops and exposure duration (opens in a new tab) ↗
What this calculation answers
A stop is a factor of two in exposure, while aperture is related to area through its square. This page applies those factors to a known base exposure instead of claiming to meter a scene from a description.
Neutral-density stops multiply the required exposure time after accounting for the new aperture and ISO. A filter factor and a stop count are different representations, so enter the number of stops rather than an ND product label without interpreting it.
The Shutter angle and exposure time calculator relates video exposure to frame duration. This page handles equivalent light exposure and ND; it does not ensure the result fits inside a video frame. The Macro bellows extension exposure compensation calculator adds a close-focus extension scenario; it should not be applied twice to a TTL-metered exposure.
Use the right input conventions
Shutter input is seconds. For one over one hundred twenty-five seconds, enter 0.008 rather than 125. Aperture values are f-numbers and both ISO settings use the same conventional numeric scale.
The EV100 output normalizes the base settings for ISO. It is not a scene brightness measurement, calibration certificate or a statement that the chosen base exposure is artistically correct.
Interpret the result
The no-ND time is shown separately so the filter multiplier can be traced. Increasing aperture f-number requires more time, increasing ISO requires less, and each positive ND stop doubles time in the ideal model.
Real filters can alter color or produce variable-ND artifacts. Lens transmission, flash synchronization and film reciprocity at long exposures are separate effects that simple reciprocal arithmetic does not predict.
Work through the example
Starting at f/4, ISO100 and 0.008 seconds, moving to f/8 multiplies required time by four. Moving to ISO200 divides it by two, giving 0.016 seconds without ND.
Three stops of ND multiply that by eight, producing 0.128 seconds. The displayed factors show how each change contributes instead of hiding them in one final number.
Where the model stops
Ideal exposure reciprocity and stop arithmetic only; no metering, T-stop correction, filter color, flash sync or film-reciprocity correction.
Check the actual capture and equipment limits. A calculated equivalent time does not guarantee a suitable exposure or motion appearance.
A few questions, answered
Can I enter an ND factor as stops?
Convert it first: a factor of eight is three stops, not eight stops.
Why is aperture squared?
The light-admitting area changes with the inverse square of f-number under the ideal relation.