Exposure Value & Equivalent Exposure
Exposure value from aperture, shutter speed and ISO, and the equivalent setting when you change two of the three; shutter as 1/250 or in seconds.
The EV at ISO 100 for the settings you metered, the third setting that keeps the same exposure when you change the other two, and how many stops each control moved.
Example: f/16 at 1/125 and ISO 100 is EV 15; opening to f/4 at the same ISO needs 1/2000, four stops faster, for the same exposure.
Stops add,
stops cancel.
What exposure value counts, why aperture, shutter and ISO trade in stops, how the third setting is found, and what EV does not tell you.
EV
Exposure value is log₂(N² ÷ t), shifted by log₂(ISO ÷ 100) to refer to ISO 100: each whole number is one stop, a doubling or halving of light. The sunny-16 rule — f/16 at 1/ISO in bright sun — is about EV 15.
Equivalent exposure
Two settings give the same exposure when N² ÷ (t · ISO) is the same. Fix any two of aperture, shutter and ISO and the third follows; opening two stops on the aperture is cancelled by a shutter two stops faster or an ISO two stops lower.
Reading shutter speeds
Enter shutter speeds as a camera shows them — 1/250, 1/8 — or in seconds (2, 0.5). Results are shown the same way, rounded as the camera would display them; the exact seconds are kept for the arithmetic.
What the number does not settle
EV describes a setting, not a scene: the right exposure for a subject is the meter’s and your judgement. Motion blur, depth of field and noise change with each control even when the exposure does not. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.