Equivalent Exposure Calculator
Keep the same exposure when you change aperture, shutter speed or ISO: enter your metered settings, change two of them and get the third, with the nearest standard setting and the stops each change adds.
How it works
equal exposure when t × ISO ÷ N² is constant · new shutter t₂ = t₁ × (N₂ ÷ N₁)² × (ISO₁ ÷ ISO₂) · new aperture N₂ = √(N₁² × t₂ × ISO₂ ÷ (t₁ × ISO₁)) · new ISO = ISO₁ × (N₂ ÷ N₁)² × t₁ ÷ t₂ · EV₁₀₀ = log₂(N² ÷ t) − log₂(ISO ÷ 100)
Aperture, shutter speed and ISO form the exposure triangle: the light reaching the sensor is proportional to the shutter time divided by the square of the f-number, and the brightness of the image also scales with ISO. Opening the aperture one stop (f/8 to f/5.6) doubles the light, so to keep the same exposure you halve the shutter time or halve the ISO. The calculator solves the missing setting exactly and also snaps it to the nearest third-stop value your camera offers. The stops in the table always add up to zero for an equivalent exposure.
Worked example
You metered f/8, 1/125 s at ISO 100 and want a shallower depth of field at f/4. Opening two stops lets in 4× the light, so the shutter must be 4× shorter: 1/125 × (4 ÷ 8)² = 1/500 s at ISO 100. The scene is about EV 12.97.
Sources: Wikipedia — Exposure value · Wikipedia — F-number
Frequently asked questions
What is an equivalent exposure?
Any combination of aperture, shutter speed and ISO that gives the same image brightness. f/8 at 1/125 s, f/5.6 at 1/250 s and f/4 at 1/500 s are all equivalent at the same ISO.
How many stops is f/8 to f/4?
Two stops. Each full stop multiplies the f-number by √2 (about 1.4): f/8 → f/5.6 → f/4. Two stops let in four times the light.
Why change to an equivalent exposure?
To change the look without changing brightness: a wider aperture for background blur, a faster shutter to freeze motion, a slower one for motion blur, or a lower ISO for less noise.
Does reciprocity always hold?
For digital sensors, yes. Film loses sensitivity at exposures longer than about 1 second (reciprocity failure), so long film exposures need extra time — see the film reciprocity calculator.
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This calculator is for educational purposes. Double-check important results before acting on them.