What it's for
The "exposure triangle" (ISO, aperture and shutter speed) lets you reach the same final brightness in different ways — trading one setting for another that exactly offsets the light difference. This calculator takes the original exposure, the field you're about to change, and the field that should compensate for that change, then calculates the new value needed to keep an equivalent exposure.
How the calculation works
Each setting is converted into EV stops relative to its original value: ISO_stops = log₂(new_ISO / original_ISO); aperture_stops = 2 · log₂(new_aperture / original_aperture) (the factor of 2 exists because aperture changes by √2 per stop, not by 2×, since the aperture area — which determines light — is inversely proportional to the square of the f-number); shutter_stops = log₂(new_speed / original_speed). After finding how many stops the changed field gained or lost, we apply the inverse shift to the chosen field, keeping total exposure constant.
Worked example
If you had ISO 100, f/8, 1/100s, and open up to f/4 (opening from f/8 to f/4 gains 2 stops of light), the equivalent shutter speed needs to compensate by losing 2 stops — going from 1/100s to 1/400s.
Why this is useful
It lets you trade depth of field for shutter speed (or vice versa) without recalculating by hand, or simulate how an ISO change would affect a scene's light requirements — essential when adapting a reference photo's settings to different conditions.
Limitations
The calculation assumes a perfect reciprocity relationship between the three parameters, valid for the vast majority of situations — extremely long exposures (minutes) can suffer a small reciprocity deviation on some sensors, not captured here.
Frequently asked questions
Why does aperture use a factor of 2 in the stops calculation?
Because the amount of light passing through the lens is proportional to the aperture's area, which in turn is inversely proportional to the square of the f-number. So each aperture stop corresponds to a √2 change in f-number, not a 2× change.
What does "EV" mean in the result?
EV (Exposure Value) is the unit used to measure the total light shift in stops. An EV of +1 means double the light; -1 means half.
Can I use this to replicate another camera's exposure?
Yes — that's exactly what it's for: keeping the same total amount of light when trading ISO, aperture or shutter speed between different settings or equipment.