Equivalent Exposure Calculator (ISO / Aperture / Shutter)
Lock two of ISO, aperture, and shutter speed and solve the third for an identical exposure, using the exposure-triangle relationship. Runs in your browser.
1/100 s
1/25 s
The exposure triangle keeps total light constant: exposure ∝ N² ÷ (shutter × ISO), where N is the f-number. Open the aperture, lengthen the shutter, or raise ISO and you must compensate elsewhere by the same number of stops to keep the same brightness. This solves the locked third setting exactly. Each setting still affects the look — aperture changes depth of field, shutter changes motion blur, ISO changes noise — even when exposure is equivalent. Everything runs in your browser.
About this tool
Every photograph's brightness is set by three controls — ISO (sensor sensitivity), aperture (the f-number, how wide the lens opens), and shutter speed (how long the sensor is exposed) — known together as the exposure triangle. The key insight is that many different combinations of the three produce exactly the same exposure: open the aperture one stop (twice the light) and you can halve the shutter time or halve the ISO to compensate, and the image brightness stays identical. This calculator does that compensation math precisely. You enter a reference exposure you know is correct, then change two of the three settings on the 'new' exposure and let the tool solve the third so the total light is unchanged. It works from the physical relationship that exposure is proportional to N² ÷ (shutter × ISO), where N is the f-number, and reports the difference in 'stops' (0 means an exactly equivalent exposure). The reason you would deliberately trade one setting for another is that each control changes the look of the image as well as its brightness: a wider aperture (smaller f-number) blurs the background with shallow depth of field; a longer shutter blurs motion (or, shortened, freezes it); and a higher ISO brightens the image at the cost of more digital noise. So a photographer might lock in a wide aperture for a creamy background and a fast shutter to freeze action, then solve for the ISO that makes the exposure correct — exactly the kind of question this tool answers. It assumes a linear relationship and ignores reciprocity failure (relevant only for very long film exposures) and lens light-transmission differences (T-stops). Everything runs in your browser; nothing is uploaded.
How to use it
- Enter a reference exposure (ISO, aperture, shutter in seconds) you know is correct.
- Choose which new setting to solve for — ISO, aperture, or shutter.
- Enter the two settings you want to fix on the new exposure.
- Read the solved third value that keeps the exposure identical, and the stops difference (should be 0).
Frequently asked questions
- What is the exposure triangle?
- The three controls that determine exposure: ISO (sensitivity), aperture (f-number), and shutter speed (duration). Changing one shifts brightness; compensating with another by the same number of stops keeps the exposure the same.
- How is equivalent exposure calculated?
- Exposure is proportional to N² ÷ (shutter × ISO), where N is the f-number. The tool holds that quantity constant from your reference and solves the locked setting, so the total light reaching the sensor is unchanged.
- What is a "stop"?
- A stop is a doubling or halving of light. One stop more exposure = twice the light. Each setting moves in stops: aperture (f/2.8→f/4 is one stop), shutter (1/100→1/50 is one stop), ISO (100→200 is one stop).
- If exposures are equivalent, why choose one over another?
- Because each setting changes the image differently: aperture controls depth of field (background blur), shutter controls motion blur, and ISO controls noise. You pick the combination that gives the look you want, then balance exposure with the third.
- How do I enter shutter speed?
- Enter it in seconds — 0.01 for 1/100s, 0.5 for half a second, 2 for two seconds. The tool shows the conventional 1/x form alongside the decimal.
- What does this leave out?
- It assumes an ideal linear response. It ignores reciprocity failure (only matters for very long film exposures) and the difference between f-stops and T-stops (actual light transmission). For digital photography in normal ranges it is exact.