Photo EV from ISO Aperture Time

EV from ISO, aperture and shutter speed

Exposure Value (EV) rolls the three exposure variables into one number. You get it from EV = log₂(N²/t) − log₂(ISO/100), where N is the f-number, t the shutter time in seconds and ISO the sensor sensitivity. Add +1 EV and you double the light hitting the sensor; −1 EV cuts it in half. Here's a handy anchor. The Sunny 16 rule says ISO 100 + f/16 + 1/100 s gives you EV 15, roughly direct sun at noon. At the other end, EV 0 is ISO 100, f/1.0 and 1 s, the kind of scene you'd shoot under moonlight. Because the stops on aperture (f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16), shutter (1/30, 1/60, 1/125, 1/250…) and ISO (100, 200, 400, 800…) are each 1 EV apart, swapping one for another leaves the exposure where it was.

Applications

Wrapping your head around the exposure triangle, practicing manual-mode shooting, sanity-checking what a handheld light-meter or external app tells you, dialing in exposure compensation for awkward scenes like snow or backlight, showing beginners how to read the in-camera meter, and getting two cameras with different sensors to expose the same way.

FAQ

What does negative EV mean? It's a scene darker than ISO 100 + f/1.0 + 1 s. Shooting the Milky Way puts you around EV −4 to −6, while a city street at night lands closer to EV 3–6.

Why does the formula subtract log₂(ISO/100)? EV uses ISO 100 as its baseline. Double the ISO (say 100 → 200) and you've brightened by a stop, so the equivalent scene EV drops by 1 unit.

Is EV the same as "stops"? When you're talking about a change in exposure, more or less yes; "1 stop overexposed" is the same as +1 EV. The pedantic distinction is that EV is the absolute scale and a stop is the unit you move by.