Exposure Calculator
Find an equivalent shutter speed when you change aperture or ISO, keeping the same exposure.
About the Exposure Calculator
Enter a known good exposure (aperture, shutter speed and ISO) and the new aperture or ISO you want. The calculator gives the shutter speed that keeps the same exposure, and the exposure value.
Aperture, shutter speed and ISO trade off in stops. The exposure value at ISO 100 is EV = log₂(N² ÷ t). Keeping EV constant while changing one setting gives the others. Wider apertures blur backgrounds; faster shutters freeze motion; higher ISO adds noise.
The formula
EV₁₀₀ = log₂(N² ÷ t) − log₂(ISO ÷ 100)
Worked example
Metered f/8 at 1/125 s, ISO 100; you open up to f/2.8
- f/8 to f/2.8 is three stops more light
- Shutter: about 1/1,000 s (1/1,020 exactly)
- EV₁₀₀ = 12.97
Questions people ask
What's the sunny 16 rule?
In bright sun, at f/16 the shutter speed ≈ 1/ISO.
Why are f-stops 1.4, 2, 2.8…?
Each is √2 times the last, halving the light.
Which shutter speed avoids blur?
Hand-held, about 1 ÷ focal length: 1/50 s or faster with a 50 mm lens.