Calculate shutter speed with ND filters (1-15 stops) for long exposure photography.
An ND filter calculator derives the shutter speed that preserves a metered exposure once a neutral density filter cuts the incoming light. You enter the shutter speed the camera meters with no filter attached and the filter's strength in stops, and the tool doubles the time once for every stop, because each stop halves the light and so halves the shutter speed needed to compensate. The common surprise is that the number printed on the filter is not the stop count: an ND8 filter cuts light by a factor of 8, which is 3 stops, not 8.
A concrete input and expected output from the current implementation.
Input
Base shutter speed 1/125 s, ND filter 10 stops
Expected output
8.2 s
A 10-stop filter multiplies the exposure time by 2^10 = 1024, so 1/125 s becomes 1024/125 = 8.192 s, rounded to 8.2 s.
Each stop halves the light reaching the sensor, so the exposure time must double to keep brightness constant. N stops multiply the base shutter speed by 2^N: a 6-stop filter multiplies by 64, a 10-stop by 1024. Filter labels use three conventions: ND numbers (ND8, ND64) state the reduction factor, optical density (0.3, 0.6, 0.9) marks 1, 2, and 3 stops, and stop counts state the power of two directly. The calculator takes stops, so translate the label first.
The result is the shutter speed that reproduces the same exposure at the same aperture and ISO. The calculation assumes the sensor responds linearly, which digital sensors do across practical exposure lengths. Times beyond 30 seconds exceed what most cameras set in their normal modes, so bulb mode plus a remote release is required. A fractional result such as 8.2 s can be rounded to the nearest one-third stop the camera accepts; a few tenths of a second either way are invisible in a long exposure.
ND1000 transmits 1/1000 of the incoming light, and stops are log2(1000) = 9.97. Manufacturers round that to 10, and the 0.03-stop difference is about 3 percent of exposure, far below what filter tolerances guarantee. Enter 10 and treat the result as approximate unless you have measured your specific filter.
Yes. The filter simply removes a fixed number of stops of light, and any combination of shutter speed, aperture, and ISO that restores that many stops gives the same exposure. The calculator only shows the shutter-speed answer, which is what long-exposure photography needs; a handheld shot could absorb the stops with aperture or ISO instead.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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