Exposure Reciprocity
Aperture, shutter and ISO trade off to keep exposure constant.
Formula
Variables
Example
Halving shutter time? Open one stop or double ISO to compensate.
How it works
This relationship says that, for the same scene brightness, the amount of light reaching the sensor depends on three linked settings: lens aperture, shutter time, and ISO. A larger f-number means a smaller aperture and less light, so you must use a longer exposure time or a higher ISO to keep the image brightness the same. The square on N appears because aperture area changes with the square of the diameter, so f-stops do not trade linearly with light. In practice, doubling the light gathered by the aperture requires opening the lens by one stop, which balances either a one-stop faster shutter or a one-stop lower ISO.
When to use it
Use this when you need to change camera settings without altering exposure, such as freezing motion, reducing noise, or increasing depth of field. It is especially useful in manual mode, when comparing equivalent exposures, or when calculating how to compensate after changing one setting.
Worked example, step by step
Suppose a correct exposure is taken at f/4, ISO 200, and 1/125 s. You want to keep the same exposure but stop down to f/8 for greater depth of field. From f/4 to f/8 is 2 stops less light through the lens. To compensate, you can either: - double the shutter time twice: 1/125 s -> 1/60 s -> 1/30 s, or - raise ISO two stops: 200 -> 400 -> 800. So an equivalent exposure is f/8, ISO 200, 1/30 s, or f/8, ISO 800, 1/125 s.
Common mistakes to avoid
- Treating f-number changes as linear instead of using stop-based changes
- Forgetting that doubling ISO or doubling shutter time each changes exposure by one stop
- Assuming equivalent exposure means identical image quality, when noise and motion blur can still change
Did You Know?
The 'exposure triangle' of aperture, shutter and ISO is really just this reciprocity rule in action.
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More in Photography & Imaging
View allF-Number
BasicAperture f-number is focal length divided by aperture diameter.
Exposure Value (EV)
IntermediateCombines aperture and shutter speed into one exposure figure.
Depth of Field (approx.)
AdvancedRange of distances that appear acceptably sharp.
Hyperfocal Distance
AdvancedFocus distance that keeps everything from H/2 to infinity sharp.
Lens Magnification
IntermediateImage magnification from focal length and subject distance.
Angle of View
IntermediateField of view from sensor size and focal length.