Long exposure and motion

Choose the ND filter, not just the stop count

Start from the shutter reading without a filter. Get the exact reduction, both neighbouring real filters, and the exposure difference each choice leaves behind.

Exposure target Mode Unfiltered shutterEnter a fraction such as 1/125 or seconds such as 0.008.

Target shutterEnter 8 for eight seconds, or 1/2 for half a second.

Filter result

Exact reduction——

Filter factor—mathematical exposure multiplier

Optical density——

Weaker whole-stop choice—brighter than exact when residual is negative

Stronger whole-stop choice—darker than exact when residual is positive

Stacking filtersAdd the stopsand add optical densities; factors multiply

Calculating…

Formula and practical limits

stops = log₂(target time / unfiltered time). The factor is 2^stops; optical density is log₁₀(factor). One stop is therefore OD 0.301, commonly labelled 0.3. NiSi’s published conversion table shows the same stop/factor/density relationship.

Real filter labels round ND512 to ND500 and ND1024 to ND1000. Strong filters can shift colour, and variable ND filters can show an X-pattern near their limit. Meter and make a test frame after fitting the real glass.