How this calculator works
Every scene has an exposure value at ISO 100, and the settings must add up to it. The shutter time that balances a given aperture and ISO is the f-number squared times 100, divided by two raised to the exposure value and multiplied by the ISO. Dark scenes have low or negative exposure values, so the shutter grows quickly: each stop of darkness doubles the time. A fast aperture and high ISO buy back stops, but they also narrow depth of field and add noise. The table of lighting conditions uses the traditional exposure value scale for reflected light, which assumes an average scene reflectance of about 18 percent.
Formula
shutter time t = N^2 x 100 / (2^EV x ISO), where N is the f-number, EV is the exposure value at ISO 100 and ISO is the sensor sensitivity. The number of stops above one second is log2(t).
Worked example
Bright sun has an exposure value of about 15. At f/16 and ISO 100 the shutter time is 256 x 100 / (32768 x 100) = 0.0078125 seconds, or 1/128, which is why the sunny 16 rule suggests 1/125. Under starlight, EV is about -6. At f/2.8 and ISO 3200 the time is 7.84 x 100 / (0.015625 x 3200) = 15.68 seconds. The same scene at ISO 100 would need 501.76 seconds, showing why high ISO and a fast lens matter at night.
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Frequently asked questions
How do I know the exposure value of my scene?
Use the closest lighting condition in the list, or meter the scene and convert. A camera meter normally reports a combination of aperture and shutter, and this page can turn that into an EV if you switch to the exposure calculator.
Why is my night photo blurry at these settings?
Long shutter times record anything that moves, including stars, branches and camera shake. Use a tripod, a remote release and the two second timer, and consider raising ISO to shorten the exposure if the subject moves.
Should I use the reciprocal rule for handheld shots?
The old rule is that the handheld shutter time should be no longer than 1 divided by the focal length, adjusted for crop factor and stabilisation. The values here often exceed that, so use a support when the shutter is long.
Does the moon need a long exposure?
Not a very long one. The moon is sunlit and surprisingly bright, around EV 14 with a long lens, so a shutter near 1/125 second at f/8 and ISO 100 is often enough. Long exposures just make it a white blob.