Exposure Time by ISO and Aperture

Exposure time, ISO and aperture

Exposure value comes from EV = log2(N² / t). Here N is the aperture f-number and t is the shutter time in seconds. The reciprocity idea is that ISO · t / N² stays constant when two exposures are equivalent, so if you double the ISO you can either halve t or close the aperture by one stop. There's also the Sunny 16 rule, which says that under bright daylight a good exposure is f/16 at t = 1/ISO seconds (so ISO 100 gives you 1/100 s at f/16).

Applications

Getting a feel for the exposure triangle (ISO, aperture, shutter). It also helps with professional photography in mixed light, broadcast video that follows the 180° shutter rule (t = 1 / (2·fps)), and matching exposure across scenes shot on cameras of different sensitivity.

FAQ

What does "one stop" mean? A stop is a 2× change in light. Going from f/8 to f/11, ISO 200 to 400, or 1/125 s to 1/60 s each doubles or halves the exposure.

Does pushing ISO add noise? It does. A higher ISO amplifies the sensor signal, and the noise floor rides up with it. That said, modern sensors hold up well to around ISO 6400–12800 with only a mild penalty.

Why does video keep the shutter near 1/50 s at 24 fps? That comes from the 180° shutter angle (t = 1/(2·fps)), which gives natural-looking motion blur. Faster shutters tend to look stuttery, while slower ones smear the frame.