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Exposure Value (EV) & Equivalent Exposure Calculator

Compute a photo's Exposure Value from aperture, shutter speed, and ISO, and find the shutter speed needed at a different aperture/ISO to keep the same exposure.

Exposure Value (EV, at ISO 100)
10.94
Equivalent shutter speed at the target settings
1/30.6 s

How it works

Exposure Value at the ISO 100 reference is defined as EV = log2(N² ÷ t), where N is the f-number and t the shutter time in seconds — each whole-number increase in EV corresponds to one 'stop' less light. Shooting at a different ISO shifts how much actual light that same EV number represents, via the standard relation EV(ISO) = EV(100) + log2(ISO ÷ 100), so this calculator reports EV(100) = log2(N² ÷ t) − log2(ISO ÷ 100) from whatever aperture, shutter speed, and ISO you enter.

Exposure (the actual amount of light reaching the sensor, adjusted for sensitivity) is proportional to (t × ISO) ÷ N². Holding that quantity constant while switching to a target aperture and target ISO and solving for the new shutter time gives t₂ = t₁ × (N₂ ÷ N₁)² × (ISO₁ ÷ ISO₂) — the standard 'equivalent exposure' reciprocity relationship for trading one setting against another (e.g. stopping down for more depth of field, then lengthening the shutter to compensate) without changing how bright the final image looks.

FAQ

Why does the equivalent shutter speed need the aperture squared, not just the aperture?

Because an f-number describes a ratio of focal length to the lens's physical aperture diameter, and the light-gathering area of a circular aperture scales with the square of its diameter. Halving the f-number (e.g. f/8 to f/4) doubles the diameter but quadruples the area, so exposure scales with 1/N², not 1/N.

What does leaving the target aperture and ISO the same as the originals do?

It just echoes your original shutter speed back as the 'equivalent' one, since nothing changed — useful mainly as a sanity check that the calculator is set up the way you expect before changing the target fields to explore a real trade-off.

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