Use the free Equivalent Exposure Calculator below. Everything runs in your browser, with no sign-up and no upload.
You found the perfect exposure, but the depth of field is wrong. Or the shutter speed is too slow to freeze motion. You do not have to give up the exposure you liked. You only have to trade one setting for another, keeping the total amount of light exactly the same.
What is Equivalent Exposure Calculator?
Equivalent exposure means different combinations of aperture, shutter speed and ISO that all let the same total light reach the sensor. Open the aperture by one stop and you must shorten the shutter speed by one stop. Raise the ISO by one stop and you must close the aperture or shorten the shutter speed by one stop. The picture keeps the same brightness, but its look changes: depth of field, motion blur and noise all shift with the settings you choose.
How it works
The maths behind the tool is simple once you think in stops. Each full stop of aperture halves or doubles the light. Each stop of shutter speed does the same. Each stop of ISO changes the sensor’s sensitivity by the same factor. The calculator works out the new shutter speed from your starting settings and the new aperture and ISO, so the product of light and sensitivity stays constant. It uses the standard f-number scale, where each step multiplies the f-number by about 1.4, and the standard shutter and ISO scales.
How to use it
- Enter the aperture, shutter speed and ISO of the exposure you already like.
- Enter the new aperture you want, for example a wider aperture for a softer background.
- Enter the new ISO, or leave it unchanged if you only want to change the aperture.
- Read the new shutter speed and check it is fast enough for your subject.
Worked example
Suppose you meter f/8 at 1/125 second and ISO 100, but you want the shallow depth of field of f/4. Opening from f/8 to f/4 is two stops more light, so the shutter must get two stops faster: 1/500 second. The brightness is identical, yet the background melts away. The same trade works in reverse: if 1/500 second is too fast for the motion blur you want in a waterfall, close the aperture two stops and you are back at a slow, dreamy shutter speed.
Tips and limits
Watch the edges of the trade. A very wide aperture can make focus unforgiving, especially at close distances. A very slow shutter speed needs a tripod or a steady surface. A very high ISO adds noise, which matters most in large prints and dark shadows. When a trade pushes one setting to an extreme, change two settings a little instead of one setting a lot. And remember that the calculator keeps brightness constant, not the look of the picture: that is the whole point, because you choose the look on purpose.
Please note: photography exposure tools are educational calculators. Sunny 16 and Zone System figures are starting-point rules of thumb, and a meter reading governs in real shooting. Time-lapse figures assume the camera can sustain the interval, and buffer or card limits may intervene.
Frequently asked questions
Does equivalent exposure change the brightness of the photo?
No. That is the definition: the total light stays the same, so the brightness stays the same. What changes is depth of field, motion blur and noise.
Can I trade ISO as well as aperture and shutter speed?
Yes. Raising ISO by one stop is one stop of extra sensitivity, so you must remove one stop of light somewhere else, for example by using a faster shutter speed.
Why does my camera not offer the exact shutter speed shown?
Cameras use rounded steps, usually thirds of a stop. Pick the nearest available speed; a third of a stop is a small difference and will not ruin the exposure.
Try the calculator above with your own numbers, and use the result as a planned starting point for your next shoot. A few minutes of planning before you press the shutter usually saves a whole session of corrections afterwards.
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