Exposure value and stops explained
How exposure value ties aperture, shutter speed and ISO together, why each stop doubles the light, and how to read EV in real scenes.
Where the EV formula comes from
Exposure value collapses two separate controls into one figure so you can compare very different camera settings at a glance. The definition is EV100 = log2(N squared divided by t), where N is the aperture f-number and t is the shutter time in seconds. Squaring the f-number reflects the fact that light passing through the lens scales with the area of the opening, not its diameter. Because the whole expression is a base-2 logarithm, adding one to the EV always means half as much light reaching the sensor.
Why a stop doubles or halves the light
A stop is the fundamental unit of exposure, and one stop is one full EV. Move from f/8 to f/11 and you halve the light, which raises the EV by one. Slow the shutter from 1/250 to 1/125 and you double the light, which lowers the EV by one. This is why many aperture and shutter pairs can produce an identical exposure: f/8 at 1/250 and f/11 at 1/125 both sit at the same EV, trading depth of field for motion blur without changing brightness.
How ISO shifts the reference
EV is defined at ISO 100, so the sensor sensitivity has to be folded in separately. The calculator subtracts log2(ISO / 100) from EV100 to give the EV at your chosen ISO, and reports that gap as the ISO offset in stops. Doubling the ISO from 100 to 200 is one stop, to 400 is two stops, and to 800 is three stops. Raising ISO lets a darker scene reach a correct exposure at the same aperture and shutter, at the cost of more noise.
Reading EV in the field
Photographers use EV as shorthand for scene brightness once they fix an ISO. Direct sun on a bright day meters near EV 15 at ISO 100, the basis of the sunny 16 rule where f/16 pairs with a shutter close to one over the ISO. As light drops, the EV falls: heavy overcast sits around EV 12, indoor rooms around EV 6 to 8, and city streets at night below EV 4. Knowing the target EV lets you choose any aperture and shutter combination that adds up to it.