Exposure Value Calculator

In short Exposure value is a single number for a combination of aperture and shutter speed: EV equals the base-two logarithm of the f-number squared divided by the shutter time. Every one-EV step is one stop, meaning a doubling or halving of light, and every pair of settings with the same EV is an equivalent exposure. f/8 at 1/125 is EV 12.97, and so are f/5.6 at 1/250 and f/11 at 1/60. ISO is a separate scale on top: doubling it is one stop, and EV is quoted at ISO 100 by convention because that is where every published table is written. The best-known anchor is sunny 16 — on a clear day with distinct shadows, f/16 at one over the ISO gives a correct exposure, which at ISO 100 is f/16 at 1/125. That comes to EV 14.97 here rather than a round 15, and the reason is worth knowing: the numbers printed on your camera are rounded. The true series climbs by the square root of two, so the marked f/5.6 is really 5.657 and the marked 1/125 is really 1/128. At the true values sunny 16 is exactly EV 15. The worst rounding on a standard aperture dial is between f/11 and f/16, where a marked full stop is really 1.081 EV. Cameras meter in true stops and print rounded ones, because 5.657 does not fit on a lens barrel. The full useful range runs from about EV −6 on a moonless night to EV 16 for sun on snow, and since each step is a doubling, that span is a factor of over four hundred thousand.