F-Stop Time ISO EV

F-stop, shutter, ISO and EV explained

Exposure Value (EV) is photographic shorthand for how much light a given aperture-and-shutter combination puts on the sensor, measured against ISO 100. The defining formula is EV = log₂(N² / t), where N is the f-number and t is the shutter time in seconds. A whole stop either doubles the light or cuts it in half. Move from f/4 to f/2.8 and you gain +1 EV; switch from 1/125 s to 1/250 s and you lose one (−1 EV).

The old Sunny 16 rule is how you calibrate this in the field. Under bright daylight you set f/16 and a shutter equal to 1/ISO, so ISO 100, f/16, 1/100 s lands at roughly EV 15. Sensor sensitivity gets formalized by the Kodak exposure table and ISO 12232, and reciprocity tells you that any equivalent (N, t) pair sitting on the same EV line gives the same exposure once you set aside motion blur and depth of field.

Applications

Where it earns its keep: metering with a handheld light meter, mixing flash with ambient light, choosing shutter speed for action against aperture for depth of field, swapping between exposure pairs while the brightness stays put, and setting manual exposure on film cameras that have no meter of their own.

FAQ

Why is EV referenced to ISO 100? ISO 100 was the historical baseline for daylight films, and it's the native sensitivity of most digital sensors too, so light meters and EV tables end up on the same scale.

How do third-stops fit in? Most modern cameras adjust in 1/3 EV steps (f/2.8, f/3.2, f/3.5, f/4…). Each click is 0.33 EV, which is handy when you want fine exposure compensation rather than a full stop.

Does ISO actually change EV? Strictly speaking, EV only describes the light reaching the sensor. What cameras show you is an “exposure index” that folds ISO in, so doubling ISO acts like +1 EV of brightness in the finished image.