Equivalent Exposure
Any different combination of aperture, shutter speed and ISO that yields identical image brightness.
Formula / rule
N₁²/t₁ = N₂²/t₂
Measured in
stops (EV)
In depth
Equivalent exposures are the working currency of deliberate photography: they let you decide what the image should look like and then find the settings that deliver it. Two equivalents are only equal in brightness — depth of field, motion blur and noise will differ, and that difference is the point.
Examples in practice
f/2.8 at 1/500 s, f/4 at 1/250 s and f/5.6 at 1/125 s are all equivalent, but only the first will throw a background out of focus.