Photography Calculator

Exposure value is a single number that describes how much light a given combination of aperture, shutter speed, and ISO lets reach the sensor. The math is EV = log₂(N² ÷ t) at ISO 100, where N is the f-number and t is the shutter time in seconds; raising the ISO above 100 subtracts from that number, one EV per doubling. The point of collapsing three settings into one figure is that it makes them comparable — f/8 at 1/250s and f/4 at 1/1000s are the same exposure because they land on the same EV.

Every full stop is a factor of two in light, and that's the thread running through all three controls. Open the aperture one stop (say f/8 to f/5.6) and you double the light; halve the shutter time (1/250s to 1/125s) and you double it again; bump ISO from 100 to 200 and you double the signal. Because they all move in stops, you can trade one for another freely: give up a stop of shutter to gain a stop of aperture and the picture stays exactly as bright. That's why this solver works in every direction — fix any three of {aperture, shutter, ISO, EV} and the fourth is determined.

The shutter field accepts the three ways photographers actually write shutter speeds: a fraction like "1/125", a decimal like "0.5", or a whole number of seconds like "30". Internally they're all converted to seconds before the log₂ runs, so "1/125" and "0.008" give the same EV. Type whichever form is on your dial.

The calculator also names the scene the computed EV corresponds to, following the same chart photographers memorize as the "sunny 16" rule. EV 15 is bright sun with hard shadows, EV 12 is overcast, EV 8 is a neon sign or a spotlit subject, EV 4 is a candlelit close-up. That label is a sanity check: if you meter a cloudy afternoon and the math says EV 15, something in your settings is off.