About this calculator
Aperture, shutter speed, and ISO form the ‘exposure triangle’ — each one can be adjusted by a stop to let in twice (or half) as much light. This calculator starts from a baseline exposure you know is correctly exposed, lets you pick one setting to change for a creative reason (like a faster shutter speed to freeze motion, or a wider aperture for background blur), and calculates exactly how much to adjust a second setting so the final image stays just as bright — while the third setting stays untouched.
How the calculation works
Exposure is the sum of the brightness that aperture, shutter speed and ISO produce. The calculator converts all three to the same unit, the stop. Doubling shutter time or ISO adds one stop, while aperture follows the area of the opening, so it scales with the inverse square of the f-number. That is why going from f/4 to f/2.8 adds one stop.
You pick one setting you want to change and one setting to compensate with. The third stays at its baseline value, and the calculator finds the compensating value that keeps overall brightness the same.
Worked examples
- From a baseline of f/4, 1/125 s and ISO 400, raising shutter speed to 1/500 s to freeze motion (2 stops) needs f/2 to compensate with aperture while ISO stays fixed.
- From the same baseline, opening to f/2 (2 stops) and compensating with shutter speed gives 1/500 s. It agrees with the example above.
- From the same baseline, lowering ISO to 100 (2 stops) while keeping aperture fixed means slowing the shutter to about 1/31 s (roughly 1/30 s) for the same brightness.
- From f/8, 1/60 s and ISO 200, opening to f/2.8 adds about 3 stops, so compensating with shutter speed while ISO stays fixed gives about 1/490 s (roughly 1/500 s).
When to use it
It is most useful when you already know a correct exposure and want to change a setting for a creative reason, such as more background blur or frozen motion, without changing brightness. It is also a good way to build intuition for how the three settings relate while you practice manual mode.
Limits and common mistakes
- It assumes your baseline exposure is already right. If the baseline is off, the result is off by the same amount.
- Many cameras use 1/3-stop steps, so choose the available setting closest to the calculated value. A small difference is usually not a problem.
- In very long exposures, reciprocity failure can make the image darker than calculated, and this calculator does not account for it.
- Changing aperture also changes depth of field, raising ISO adds noise and changing shutter speed changes how motion looks. The same brightness does not mean the same photo.
- You cannot pick the same setting for both the one you change and the one that compensates.
Guides that pair well with this calculator
Assumes ideal reciprocity between settings. Extremely long exposures can show reciprocity failure on some cameras/film, and this calculator does not account for that.
Frequently Asked Questions
What does ‘a stop’ mean?+
One stop is a doubling or halving of the light reaching your sensor — one full click on most aperture, shutter speed, or ISO dials.
Why does the aperture calculation use a factor of 2?+
Light passes through a lens opening, so it scales with area, not diameter — that’s why the f-number needs to change more than shutter speed or ISO to represent the same one-stop change.
Can I freeze motion without changing my exposure?+
Yes — set ‘Change’ to Shutter Speed and pick a faster speed, then choose Aperture or ISO to compensate with, and the calculator gives you the exact value that keeps the same brightness.