Exposure value, or EV, is a single number that stands in for a full combination of aperture, shutter speed and ISO — every combination that lets the same amount of light reach the sensor shares the same EV.
Exposure value gives photographers a way to describe how bright a scene is with one number, rather than reciting three settings that only mean something together. It also makes the arithmetic possible. If a scene sits at EV 12 and you have committed to f/4 for depth of field, there is exactly one shutter-and-ISO pairing that lands the exposure, and a calculator can return it straight away.
The scale is logarithmic, so each whole number is one stop, a doubling or halving of light. That is why the gap between a bright overcast day near EV 13 and a candlelit room near EV 4 is nine stops, not a vague 'much darker'. Meters, flash guide numbers, and the sunny-16 starting point all speak this same language, which is what lets an experienced photographer carry a rough EV chart in their head and get close before the meter even confirms it.
Sunny 16 is really a statement about exposure value: a front-lit scene in full sun sits near EV 15, which at ISO 100 works out to f/16 at 1/125s. Walk that same subject into open shade and the light falls to roughly EV 12, three stops down, so holding the shutter and aperture means ISO has to rise from 100 to 800.
Step indoors near a bright window and the reading drops to about EV 8, another four stops, so even ISO 800 now forces the shutter down to 1/30s or the aperture open to f/4. Each of those moves is close to a single EV step of one stop, and tracking the scene in EV makes the size of every lighting change obvious in advance, instead of discovering it when the histogram piles up against one edge.
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