What this calculator does
Aperture, shutter speed and ISO each control exposure in units of stops, where one full stop always doubles or halves the amount of light reaching the sensor. Because all three share that same unit, a change in one can always be offset by an opposite change in another, leaving the overall exposure unchanged.
This is what "equivalent exposure" means: different combinations of the three settings that all let the same total amount of light contribute to the image. Photographers use it constantly, whether to trade shutter speed for a wider aperture to blur a background, or to trade ISO for a faster shutter speed to freeze motion, without the image coming out brighter or darker.
The formula
Convert the change you made in one setting into stops: aperture stops are 2×log2(old f-stop ÷ new f-stop), shutter stops are log2(new time ÷ old time), and ISO stops are log2(new ISO ÷ old ISO). For equivalent exposure, the stop changes across all three settings must sum to zero, so the setting you are solving for needs the opposite number of stops to the one that changed, and this calculator converts that stop change back into an actual new value.
| Term | Meaning |
|---|---|
| Stop | A doubling or halving of light. Each of aperture, shutter speed and ISO is measured in the same stop units. |
| Equivalent exposure | Different combinations of aperture, shutter speed and ISO that let through the same total light. |
| Stop change | How many stops brighter (positive) or darker (negative) a setting change made, before it is offset elsewhere. |
The inputs explained
| Field | What to enter |
|---|---|
| Original aperture (f-stop) | The aperture (f-stop) before the change. |
| Original shutter speed (seconds, e.g. 0.01 for 1/100s) | The shutter speed before the change, in seconds (for example, 0.01 for 1/100s). |
| Original ISO | The ISO before the change. |
| Setting that changed | Which of the three settings you are changing. |
| New value of the changed setting | The new value for the setting you are changing. |
| Setting to solve for | Which of the remaining two settings should be adjusted to compensate. |
When to use it
Opening the aperture for background blur
Switching to a wider aperture for a shallower depth of field adds light, and the shutter speed or ISO needs to come down by the same number of stops to avoid overexposing the shot.
Freezing motion with a faster shutter
A faster shutter speed to freeze action removes light, which typically needs to be made up with a wider aperture or a higher ISO to keep the exposure the same.
Lowering ISO for image quality
Dropping to a lower ISO to reduce noise removes light-gathering sensitivity, and that shortfall needs to be recovered through a wider aperture or a slower shutter speed.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the compensating shutter speed changes as the new aperture widens
A fixed starting point, across a range of new aperture values.
| New aperture (f-stop) | New shutter speed | Stop change from the setting you changed |
|---|---|---|
| f/11 | 0.0189 s | -0.92 stops |
| f/8 | 0.0100 s | 0.00 stops |
| f/5.6 | 0.0049 s | 1.03 stops |
| f/4 | 0.0025 s | 2.00 stops |
| f/2.8 | 0.0012 s | 3.03 stops |
| f/2 | 0.000625 s | 4.00 stops |
Questions
Why does aperture use 2×log2 instead of just log2?
A full stop in aperture corresponds to the f-number changing by a factor of the square root of two, not a factor of two, because aperture controls the area of the opening, which scales with the square of that ratio. The factor of two in the formula corrects for that so the result comes out in true stops of light.
Do all three settings have to be involved in every exposure change?
No, this calculator solves for one compensating setting at a time, holding the third one fixed. In practice a photographer might split the compensation across two settings instead of putting it all on one, which is a matter of creative choice rather than a fixed rule.
Is a stop the same size regardless of which setting it is on?
Yes, that is the entire point of the stop system: one stop of aperture, one stop of shutter speed and one stop of ISO all represent the same doubling or halving of light, which is what allows them to be traded against each other directly.
Why would I deliberately choose an equivalent exposure over the original settings?
The total light stays the same, but each setting has side effects beyond exposure: aperture affects depth of field, shutter speed affects motion blur, and ISO affects image noise. Moving between equivalent exposures lets you pick which of those side effects matters most for a given shot.
For how a chosen aperture and subject distance translate into actual sharpness range, see the depth of field calculator.