How this calculator works
Aperture, shutter speed and ISO each change exposure by stops. Opening the aperture by one stop doubles the light, doubling the shutter time doubles the light, and doubling ISO doubles the sensitivity. Exposure value at ISO 100 is defined as the base two logarithm of the f-number squared divided by the shutter time, so every combination on the same EV line records the same light. Raising ISO shifts the same scene to a higher EV number at a given aperture and shutter, which is why the calculator subtracts the logarithm of ISO over 100. Halving the shutter time or stopping down one stop raises the EV by one; the sequence f/1.4, f/2, f/2.8, f/4 and so on steps by halves of the EV scale.
Formula
EV at ISO 100 = log2(N^2 / t) - log2(ISO / 100), where N is the f-number and t is the shutter time in seconds. To solve for aperture: N = sqrt(2^EV x (ISO / 100) x t). To solve for shutter: t = N^2 x 100 / (2^EV x ISO). To solve for ISO: ISO = 100 x N^2 / (t x 2^EV).
Worked example
An exposure of f/8 at 1/125 second and ISO 100 has an exposure value of log2(64 / 0.008) = log2(8000) = 12.9658. The same scene at ISO 400 gives log2(8000) - log2(4) = 10.9658, which is two stops less light needed. If you want EV 12 at 1/60 second and ISO 100, the aperture is the square root of 4096 x 0.016667 = 8.26. At EV 15 with f/16 and ISO 100 the shutter time is 256 / 32768 = 0.0078125 seconds, just under 1/125.
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Frequently asked questions
Is EV the same at every ISO?
The light in the scene has one EV at ISO 100, but the settings that capture it shift with ISO. This page reports EV at ISO 100, which is the usual convention, and subtracts the ISO term so the same light reads the same value.
Why does the aperture scale use 1.4 and 2.8?
Each full stop halves the area of the aperture, so the diameter changes by a factor of the square root of two, about 1.414. The familiar sequence f/1.4, f/2, f/2.8 is that factor applied repeatedly, rounded for the engravings.
What EV does bright sunlight have?
Bright sun on a clear day is about EV 15 at ISO 100, which matches the sunny 16 rule: f/16 at a shutter speed near 1/ISO. Open shade or heavy overcast is around EV 12, and indoor lighting is much lower.
Does EV account for the light actually reaching the sensor?
Not exactly. Lens transmission losses, sensor efficiency and metering differences mean two lenses at the same nominal EV may differ slightly. Cinema cameras often use T-stops, which measure transmitted light instead.