ND filter exposure calculator
Meter without the filter, pick the filter, and get the long-exposure time to set, with stops, optical density and filter factor side by side and a timer for bulb mode.
Shutter time with the filter
8.2 s
1/125 × 2¹⁰
- Stops
- 10
- Optical density
- 3.01
- Filter factor
- 1024×
- Light let through
- 0.098%
Bulb timer
0:08
| Filter | Stops | Shutter time |
|---|---|---|
How to use it
Compose and focus first, then meter the scene without the filter and note the shutter speed. Pick that speed here (or type it), choose your filter and set the camera to the time shown. Up to 30 seconds most cameras can time the exposure themselves; beyond that, use bulb mode and the timer, which can beep when it is time to close the shutter. Add extra stops if you stack a second filter or use a polariser.
Stops, density and factor
Filters are labelled three ways. A filter factor of 8 (ND8) passes 1/8 of the light, which is 3 stops, because each stop halves the light. Optical density d passes 10−d of the light, so 0.3 is close to one stop and 3.0 close to ten. The well-known ND1000 is the 10-stop filter with a density of 3.0. The new exposure time is t × 2stops: a 10-stop filter turns 1/125 s into about 8 seconds.
Practical tips
Cover the viewfinder during long exposures so stray light does not leak in, and expect strong filters to add a slight colour cast. Real filters are seldom exactly their nominal strength, so check the first frame's histogram and adjust.
Sources
- Wikipedia, Neutral-density filter (transmittance 10−d and the table of ND numbers, density and stops).
Questions
Is ND1000 exactly 10 stops?
Almost: a factor of 1000 is 9.97 stops, and filters sold as ND1000 are treated as 10-stop filters with a density of 3.0. The difference is a few hundredths of a second at 8 seconds.
Does the aperture or ISO change?
No. Keep the aperture and ISO you metered with; the filter only changes the shutter time.
Why is my long exposure darker than expected?
Some sensors and films need extra time on very long exposures, and filter strengths vary. Take a test shot and lengthen the time if needed.