Exposure Value and Equivalent Exposures
Exposure Value (EV) is a single number representing the combined effect of aperture and shutter speed (at a given ISO) on exposure. EV 0 is defined as f/1 at 1 second at ISO 100. Each one-stop increase in EV halves the amount of light reaching the sensor. The critical concept is equivalent exposures: any combination of aperture and shutter speed that produces the same EV captures the same total light. f/4 at 1/250s = f/5.6 at 1/125s = f/8 at 1/60s — all three are EV 12 at ISO 100 and produce identical overall brightness. The photographer chooses among equivalent combinations based on creative priorities: f/4 at 1/250s freezes moderate motion with moderate depth of field; f/8 at 1/60s blurs moderate motion and increases depth of field. This is the creative heart of manual exposure. ISO modifies EV proportionally: doubling ISO allows the same exposure at one stop less light (half the aperture area or twice the shutter speed). Stops are additive: going from ISO 100 to ISO 400 is +2 stops of sensor sensitivity; compensating by reducing exposure 2 stops (e.g., f/5.6 at 1/500s → f/11 at 1/500s) restores the original brightness. Metered EV assumes a neutral scene — this is where the camera's meter fails on non-neutral subjects and why exposure compensation exists.